List (FSharp.Core)

List Module

Namespace: FSharp.Collections
Assembly: FSharp.Core.dll

Contains operations for working with values of type list.

Functions and values

Function or value Description


                
                  
                    
                      List.allPairs list1 list2

Full Usage: List.allPairs list1 list2

Parameters:

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: ('T1 * 'T2) list The resulting list of pairs.

Returns a new list that contains all pairings of elements from two lists.

list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: ('T1 * 'T2) list: The resulting list of pairs.

Example

 let people = [ "Kirk"; "Spock"; "McCoy" ]
 let numbers = [ 1; 2 ]

 people |> List.allPairs numbers

val people: string list

val numbers: int list

Multiple items
module List from Microsoft.FSharp.Collections

--------------------
type List<'T> = | op_Nil | op_ColonColon of Head: 'T * Tail: 'T list interface IReadOnlyList<'T> interface IReadOnlyCollection<'T> interface IEnumerable interface IEnumerable<'T> member GetReverseIndex: rank: int * offset: int -> int member GetSlice: startIndex: int option * endIndex: int option -> 'T list static member Cons: head: 'T * tail: 'T list -> 'T list member Head: 'T member IsEmpty: bool member Item: index: int -> 'T with get ...

val allPairs: list1: 'T1 list -> list2: 'T2 list -> ('T1 * 'T2) list

Evaluates to

 [ (1, "Kirk"); (1, "Spock"); (1, "McCoy"); (2, "Kirk"); (2, "Spock"); (2, "McCoy") ]


                
                  
                    
                      List.append list1 list2

Full Usage: List.append list1 list2

Parameters:

list1 : 'T list - The first input list.

list2 : 'T list - The second input list.

Returns: 'T list The resulting list.

Returns a new list that contains the elements of the first list followed by elements of the second list.

list1 : 'T list: The first input list.
list2 : 'T list: The second input list.

Returns: 'T list: The resulting list.

Example

 List.append [ 1..3 ] [ 4..7 ]
 
 [ 4..7 ] |> List.append [ 1..3 ]

val append: list1: 'T list -> list2: 'T list -> 'T list

Evaluates to

 [ 1; 2; 3; 4; 5; 6; 7 ]


                
                  
                    
                      List.average list

Full Usage: List.average list

Parameters:

list : ^T list - The input list.

Returns: ^T The resulting average.
Modifiers: inline
Type parameters: ^T

Returns the average of the values in a non-empty list.

list : ^T list: The input list.

Returns: ^T: The resulting average.

ArgumentException

Thrown when the input list is empty.

Example

 [1.0 .. 9.0] |> List.average

val average: list: 'T list -> 'T (requires member (+) and member DivideByInt and member Zero)

Evaluates to

5.0


                
                  
                    
                      List.averageBy projection list

Full Usage: List.averageBy projection list

Parameters:

projection : 'T -> ^U - The function to transform the list elements into the values to be averaged.

list : 'T list - The input list.

Returns: ^U The resulting average.
Modifiers: inline
Type parameters: 'T, ^U

Returns the average of values in a list generated by applying a function to each element of the list.

projection : 'T -> ^U: The function to transform the list elements into the values to be averaged.
list : 'T list: The input list.

Returns: ^U: The resulting average.

ArgumentException

Thrown when the list is empty.

Example

Calculate average age of persons by extracting their age from a record type.

 type People = { Name: string; Age: int }

 let getAgeAsFloat person = float person.Age

 let people = 
     [ { Name = "Kirk"; Age = 26 }
       { Name = "Spock"; Age = 90 }
       { Name = "McCoy"; Age = 37 } ]

 people |> List.averageBy getAgeAsFloat

type People = { Name: string Age: int }

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val getAgeAsFloat: person: People -> float

val person: People

Multiple items
val float: value: 'T -> float (requires member op_Explicit)

--------------------
type float = System.Double

--------------------
type float<'Measure> = float

People.Age: int

val people: People list

val averageBy: projection: ('T -> 'U) -> list: 'T list -> 'U (requires member (+) and member DivideByInt and member Zero)

Evaluates to

 51.0


                
                  
                    
                      List.choose chooser list

Full Usage: List.choose chooser list

Parameters:

chooser : 'T -> 'U option - The function to be applied to the list elements.

list : 'T list - The input list.

Returns: 'U list The resulting list comprising the values v where the chooser function returned Some(x).

Applies a function to each element in a list and then returns a list of values v where the applied function returned Some(v). Returns an empty list when the input list is empty or when the applied chooser function returns None for all elements.

chooser : 'T -> 'U option: The function to be applied to the list elements.
list : 'T list: The input list.

Returns: 'U list: The resulting list comprising the values v where the chooser function returned Some(x).

Example

Using the identity function id (is defined like fun x -> x):

 
 let input1 = [ Some 1; None; Some 3; None ]

 input1 |> List.choose id

val input1: int option list

union case Option.Some: Value: 'T -> Option<'T>

union case Option.None: Option<'T>

val choose: chooser: ('T -> 'U option) -> list: 'T list -> 'U list

val id: x: 'T -> 'T

Evaluates to

 [ 1; 3 ]

Example

 type Happiness =
     | AlwaysHappy
     | MostOfTheTimeGrumpy

 type People = { Name: string; Happiness: Happiness }

 let takeJustHappyPersons person =
     match person.Happiness with
     | AlwaysHappy -> Some person.Name
     | MostOfTheTimeGrumpy -> None

 let candidatesForTheTrip = 
     [ { Name = "SpongeBob"
         Happiness = AlwaysHappy }
       { Name = "Patrick"
         Happiness = AlwaysHappy }
       { Name = "Squidward"
         Happiness = MostOfTheTimeGrumpy } ]

 candidatesForTheTrip
 |> List.choose takeJustHappyPersons

type Happiness = | AlwaysHappy | MostOfTheTimeGrumpy

type People = { Name: string Happiness: Happiness }

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

val takeJustHappyPersons: person: People -> string option

val person: People

People.Happiness: Happiness

union case Happiness.AlwaysHappy: Happiness

union case Option.Some: Value: 'T -> Option<'T>

People.Name: string

union case Happiness.MostOfTheTimeGrumpy: Happiness

union case Option.None: Option<'T>

val candidatesForTheTrip: People list

val choose: chooser: ('T -> 'U option) -> list: 'T list -> 'U list

Evaluates to

 [ "SpongeBob"; "Patrick" ]

Example

 let input3: int option list = [] 

 input3 |> List.choose id
 Evaluates to:
 empty list

val input3: int option list

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

type 'T option = Option<'T>

type 'T list = List<'T>

val choose: chooser: ('T -> 'U option) -> list: 'T list -> 'U list

val id: x: 'T -> 'T

Example

 let input4: string option list = [None; None]

 input4 |> List.choose id
 Evaluates to
 empty list

val input4: string option list

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

type 'T option = Option<'T>

type 'T list = List<'T>

union case Option.None: Option<'T>

val choose: chooser: ('T -> 'U option) -> list: 'T list -> 'U list

val id: x: 'T -> 'T

Example

Using the identity function id (is defined like fun x -> x):

 
 let input5 = [ Some 1; None; Some 3; None ]

 input5 |> List.choose id  // evaluates [1; 3]

val input5: int option list

union case Option.Some: Value: 'T -> Option<'T>

union case Option.None: Option<'T>

val choose: chooser: ('T -> 'U option) -> list: 'T list -> 'U list

val id: x: 'T -> 'T


                
                  
                    
                      List.chunkBySize chunkSize list

Full Usage: List.chunkBySize chunkSize list

Parameters:

chunkSize : int - The maximum size of each chunk.

list : 'T list - The input list.

Returns: 'T list list The list divided into chunks.

Divides the input list into lists (chunks) of size at most chunkSize. Returns a new list containing the generated lists (chunks) as its elements. Returns an empty list when the input list is empty.

chunkSize : int: The maximum size of each chunk.
list : 'T list: The input list.

Returns: 'T list list: The list divided into chunks.

ArgumentException Thrown when chunkSize is not positive.

Example

 [ 1..10 ] |> List.chunkBySize 3

val chunkBySize: chunkSize: int -> list: 'T list -> 'T list list

Evaluates to

 [ [ 1; 2; 3 ]
   [ 4; 5; 6 ]
   [ 7; 8; 9 ]
   [ 10 ] ]

Example

 [ 1..5 ] |> List.chunkBySize 10

val chunkBySize: chunkSize: int -> list: 'T list -> 'T list list

Evaluates to

 [ [ 1; 2; 3; 4; 5 ] ]


                
                  
                    
                      List.collect mapping list

Full Usage: List.collect mapping list

Parameters:

mapping : 'T -> 'U list - The function to transform each input element into a sublist to be concatenated.

list : 'T list - The input list.

Returns: 'U list The concatenation of the transformed sublists.

For each element of the list, applies the given function. Concatenates all the results and returns the combined list.

mapping : 'T -> 'U list: The function to transform each input element into a sublist to be concatenated.
list : 'T list: The input list.

Returns: 'U list: The concatenation of the transformed sublists.

Example

For each positive number in the array we are generating all the previous positive numbers

 [1..4] |> List.collect (fun x -> [1..x])

val collect: mapping: ('T -> 'U list) -> list: 'T list -> 'U list

val x: int

The sample evaluates to [1; 1; 2; 1; 2; 3; 1; 2; 3; 4] (added extra spaces for easy reading)


                
                  
                    
                      List.compareWith comparer list1 list2

Full Usage: List.compareWith comparer list1 list2

Parameters:

comparer : 'T -> 'T -> int - A function that takes an element from each list and returns an int. If it evaluates to a non-zero value iteration is stopped and that value is returned.

list1 : 'T list - The first input list.

list2 : 'T list - The second input list.

Returns: int Returns the first non-zero result from the comparison function. If the first list has a larger element, the return value is always positive. If the second list has a larger element, the return value is always negative. When the elements are equal in the two lists, 1 is returned if the first list is longer, 0 is returned if they are equal in length, and -1 is returned when the second list is longer.
Modifiers: inline
Type parameters: 'T

Compares two lists using the given comparison function, element by element.

comparer : 'T -> 'T -> int: A function that takes an element from each list and returns an int. If it evaluates to a non-zero value iteration is stopped and that value is returned.
list1 : 'T list: The first input list.
list2 : 'T list: The second input list.

Returns: int: Returns the first non-zero result from the comparison function. If the first list has a larger element, the return value is always positive. If the second list has a larger element, the return value is always negative. When the elements are equal in the two lists, 1 is returned if the first list is longer, 0 is returned if they are equal in length, and -1 is returned when the second list is longer.

Example

 let closerToNextDozen a b =
   (a % 12).CompareTo(b % 12)

 let input1 = [1; 10]
 let input2 = [1; 10]

 (input1, input2) ||> List.compareWith closerToNextDozen

val closerToNextDozen: a: int -> b: int -> int

val a: int

val b: int

val input1: int list

val input2: int list

val compareWith: comparer: ('T -> 'T -> int) -> list1: 'T list -> list2: 'T list -> int

Evaluates to 0

Example

 let closerToNextDozen a b =
   (a % 12).CompareTo(b % 12)

 let input1 = [1; 5]
 let input2 = [1; 8]

 (input1, input2) ||> List.compareWith closerToNextDozen

val closerToNextDozen: a: int -> b: int -> int

val a: int

val b: int

val input1: int list

val input2: int list

val compareWith: comparer: ('T -> 'T -> int) -> list1: 'T list -> list2: 'T list -> int

Evaluates to -1

Example

 let closerToNextDozen a b =
   (a % 12).CompareTo(b % 12)

 let input1 = [1; 11]
 let input2 = [1; 13]

 (input1, input2) ||> List.compareWith closerToNextDozen

val closerToNextDozen: a: int -> b: int -> int

val a: int

val b: int

val input1: int list

val input2: int list

val compareWith: comparer: ('T -> 'T -> int) -> list1: 'T list -> list2: 'T list -> int

Evaluates to 1

Example

 let closerToNextDozen a b =
   (a % 12).CompareTo(b % 12)

 let input1 = [1; 2]
 let input2 = [1]

 (input1, input2) ||> List.compareWith closerToNextDozen

val closerToNextDozen: a: int -> b: int -> int

val a: int

val b: int

val input1: int list

val input2: int list

val compareWith: comparer: ('T -> 'T -> int) -> list1: 'T list -> list2: 'T list -> int

Evaluates to 1

Example

 let closerToNextDozen a b =
   (a % 12).CompareTo(b % 12)

 let input1 = [1]
 let input2 = [1; 2]

 (input1, input2) ||> List.compareWith closerToNextDozen

val closerToNextDozen: a: int -> b: int -> int

val a: int

val b: int

val input1: int list

val input2: int list

val compareWith: comparer: ('T -> 'T -> int) -> list1: 'T list -> list2: 'T list -> int

Evaluates to -1


                
                  
                    
                      List.concat lists

Full Usage: List.concat lists

Parameters:

lists : 'T list seq - The input sequence of lists.

Returns: 'T list The resulting concatenated list.

Returns a new list that contains the elements of each of the lists in order.

lists : 'T list seq: The input sequence of lists.

Returns: 'T list: The resulting concatenated list.

Example

 let input = [ [1;2]
               [3;4;5]
               [6;7;8;9] ]
 input |> List.concat  // evaluates [1; 2; 3; 4; 5; 6; 7; 8; 9]

val input: int list list

val concat: lists: 'T list seq -> 'T list


                
                  
                    
                      List.contains value source

Full Usage: List.contains value source

Parameters:

value : 'T - The value to locate in the input list.

source : 'T list - The input list.

Returns: bool True if the input list contains the specified element; false otherwise.
Modifiers: inline
Type parameters: 'T

Tests if the list contains the specified element.

value : 'T: The value to locate in the input list.
source : 'T list: The input list.

Returns: bool: True if the input list contains the specified element; false otherwise.

Example

 [1..9] |> List.contains 0

val contains: value: 'T -> source: 'T list -> bool (requires equality)

Evaluates to false.

Example

 [1..9] |> List.contains 3

val contains: value: 'T -> source: 'T list -> bool (requires equality)

Evaluates to true.

Example

 let input = [1, "SpongeBob"; 2, "Patrick"; 3, "Squidward"; 4, "Mr. Krabs"]

 input |> List.contains (2, "Patrick")

val input: (int * string) list

val contains: value: 'T -> source: 'T list -> bool (requires equality)

Evaluates to true.

Example

 let input = [1, "SpongeBob"; 2, "Patrick"; 3, "Squidward"; 4, "Mr. Krabs"]

 input |> List.contains (22, "Patrick")

val input: (int * string) list

val contains: value: 'T -> source: 'T list -> bool (requires equality)

Evaluates to false.


                
                  
                    
                      List.countBy projection list

Full Usage: List.countBy projection list

Parameters:

projection : 'T -> 'Key - A function transforming each item of the input list into a key to be compared against the others.

list : 'T list - The input list.

Returns: ('Key * int) list The result list.

Applies a key-generating function to each element of a list and returns a list yielding unique keys and their number of occurrences in the original list.

projection : 'T -> 'Key: A function transforming each item of the input list into a key to be compared against the others.
list : 'T list: The input list.

Returns: ('Key * int) list: The result list.

Example

Counting the number of occurrences of chars

 let input = ['H'; 'a'; 'p'; 'p'; 'y']

 input |> List.countBy id

val input: char list

val countBy: projection: ('T -> 'Key) -> list: 'T list -> ('Key * int) list (requires equality)

val id: x: 'T -> 'T

Evaluates [('H', 1); ('a', 1); ('p', 2); ('y', 1)]


                
                  
                    
                      List.distinct list

Full Usage: List.distinct list

Parameters:

list : 'T list - The input list.

Returns: 'T list The result list.

Returns a list that contains no duplicate entries according to generic hash and equality comparisons on the entries. If an element occurs multiple times in the list then the later occurrences are discarded.

list : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let input = [6;1;2;3;1;4;5;5]

 input |> List.distinct

val input: int list

val distinct: list: 'T list -> 'T list (requires equality)

Evaluates to [6; 1; 2; 3; 4; 5].


                
                  
                    
                      List.distinctBy projection list

Full Usage: List.distinctBy projection list

Parameters:

projection : 'T -> 'Key - A function transforming the list items into comparable keys.

list : 'T list - The input list.

Returns: 'T list The result list.

Returns a list that contains no duplicate entries according to the generic hash and equality comparisons on the keys returned by the given key-generating function. If an element occurs multiple times in the list then the later occurrences are discarded.

projection : 'T -> 'Key: A function transforming the list items into comparable keys.
list : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let isEven x = 0 = x % 2

 let input = [6;1;2;3;1;4;5;5]

 input |> List.distinctBy isEven  // evaluates [6; 1]

val isEven: x: int -> bool

val x: int

val input: int list

val distinctBy: projection: ('T -> 'Key) -> list: 'T list -> 'T list (requires equality)

Full Usage: List.empty

Returns: 'T list

Returns an empty list of the given type.

Returns: 'T list


                
                  
                    
                      List.exactlyOne list

Full Usage: List.exactlyOne list

Parameters:

list : 'T list - The input list.

Returns: 'T The only element of the list.

Returns the only element of the list.

list : 'T list: The input list.

Returns: 'T: The only element of the list.

ArgumentException

Thrown when the input does not have precisely one element.

Example

 ["the chosen one"] |> List.exactlyOne // evaluates "the chosen one"

val exactlyOne: list: 'T list -> 'T

Example

 let input : string list = []

 input |> List.exactlyOne

val input: string list

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

type 'T list = List<'T>

val exactlyOne: list: 'T list -> 'T

Will throw the exception: System.ArgumentException: The input sequence was empty

Example

 [1..5] |> List.exactlyOne

val exactlyOne: list: 'T list -> 'T

Will throw the exception: System.ArgumentException: The input sequence contains more than one element


                
                  
                    
                      List.except itemsToExclude list

Full Usage: List.except itemsToExclude list

Parameters:

itemsToExclude : 'T seq - A sequence whose elements that also occur in the input list will cause those elements to be removed from the result.

list : 'T list - A list whose elements that are not also in itemsToExclude will be returned.

Returns: 'T list A list that contains the distinct elements of list that do not appear in itemsToExclude.

Returns a new list with the distinct elements of the input list which do not appear in the itemsToExclude sequence, using generic hash and equality comparisons to compare values.

itemsToExclude : 'T seq: A sequence whose elements that also occur in the input list will cause those elements to be removed from the result.
list : 'T list: A list whose elements that are not also in itemsToExclude will be returned.

Returns: 'T list: A list that contains the distinct elements of list that do not appear in itemsToExclude.

ArgumentNullException

Thrown when itemsToExclude is null.

Example

 let input = [1, "Kirk"; 2, "Spock"; 3, "Kenobi"] 

 input |> List.except [3, "Kenobi"]

val input: (int * string) list

val except: itemsToExclude: 'T seq -> list: 'T list -> 'T list (requires equality)

Evaluates to [(1, "Kirk"); (2, "Spock")].

Example

 [0..10] |> List.except [1..5]  // evaluates [0; 6; 7; 8; 9; 10]

val except: itemsToExclude: 'T seq -> list: 'T list -> 'T list (requires equality)

Example

 [1..5] |> List.except [0..10]  // evaluates []

val except: itemsToExclude: 'T seq -> list: 'T list -> 'T list (requires equality)


                
                  
                    
                      List.exists predicate list

Full Usage: List.exists predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: bool True if any element satisfies the predicate.

Tests if any element of the list satisfies the given predicate.

The predicate is applied to the elements of the input list. If any application returns true then the overall result is true and no further elements are tested. Otherwise, false is returned.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: bool: True if any element satisfies the predicate.

Example

 let input = [1, "Kirk"; 2, "Spock"; 3, "Kenobi"] 
 
 input |> List.exists (fun x -> x = (3, "Kenobi"))  // evaluates true
 
 input |> List.exists (fun (n, name) -> n > 5)      // evaluates false

val input: (int * string) list

val exists: predicate: ('T -> bool) -> list: 'T list -> bool

val x: int * string

val n: int

val name: string


                
                  
                    
                      List.exists2 predicate list1 list2

Full Usage: List.exists2 predicate list1 list2

Parameters:

predicate : 'T1 -> 'T2 -> bool - The function to test the input elements.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: bool True if any pair of elements satisfy the predicate.

Tests if any pair of corresponding elements of the lists satisfies the given predicate.

The predicate is applied to matching elements in the two collections up to the lesser of the two lengths of the collections. If any application returns true then the overall result is true and no further elements are tested. Otherwise, if one collections is longer than the other then the ArgumentException exception is raised. Otherwise, false is returned.

predicate : 'T1 -> 'T2 -> bool: The function to test the input elements.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: bool: True if any pair of elements satisfy the predicate.

ArgumentException

Thrown when the input lists differ in length.

Example

Check if the sum of pairs (from 2 different lists) have at least one even number

 let anEvenSum a b  = 0 = (a + b) % 2
 
 ([1..4], [2..5]) 
 ||> List.exists2 anEvenSum     // evaluates false
 
 ([1..4], [2;4;5;6])
 ||> List.exists2 anEvenSum   // evaluates true

val anEvenSum: a: int -> b: int -> bool

val a: int

val b: int

val exists2: predicate: ('T1 -> 'T2 -> bool) -> list1: 'T1 list -> list2: 'T2 list -> bool


                
                  
                    
                      List.filter predicate list

Full Usage: List.filter predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T list A list containing only the elements that satisfy the predicate.

Returns a new collection containing only the elements of the collection for which the given predicate returns "true"

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T list: A list containing only the elements that satisfy the predicate.

Example

 let input = [1, "Luke"; 2, "Kirk"; 3, "Kenobi"; 4, "Spock"]

 let isEven x = 0 = x % 2

 let isComingFromStarTrek (x,_) = isEven x
 
 input |> List.filter isComingFromStarTrek

val input: (int * string) list

val isEven: x: int -> bool

val x: int

val isComingFromStarTrek: x: int * 'a -> bool

val filter: predicate: ('T -> bool) -> list: 'T list -> 'T list

Evaluates to [(2, "Kirk"); (4, "Spock")]


                
                  
                    
                      List.find predicate list

Full Usage: List.find predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T The first element that satisfies the predicate.

Returns the first element for which the given function returns True. Raises KeyNotFoundException if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T: The first element that satisfies the predicate.

KeyNotFoundException

Thrown if the predicate evaluates to false for all the elements of the list.

Example

 let isEven x  = 0 = x % 2

 let isGreaterThan x y = y > x

 let input = [1, "Luke"; 2, "Kirk"; 3, "Spock"; 4, "Kenobi"]
 
 input |> List.find (fun (x,_) -> isEven x)              // evaluates (2, "Kirk")
 input |> List.find (fun (x,_) -> x |> isGreaterThan 6)  // raises an exception

val isEven: x: int -> bool

val x: int

val isGreaterThan: x: 'a -> y: 'a -> bool (requires comparison)

val x: 'a (requires comparison)

val y: 'a (requires comparison)

val input: (int * string) list

val find: predicate: ('T -> bool) -> list: 'T list -> 'T


                
                  
                    
                      List.findBack predicate list

Full Usage: List.findBack predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T The last element that satisfies the predicate.

Returns the last element for which the given function returns True. Raises KeyNotFoundException if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T: The last element that satisfies the predicate.

KeyNotFoundException

Thrown if the predicate evaluates to false for all the elements of the list.

Example

 let isEven x  = 0 = x % 2

 let isGreaterThan x y = y > x

 let input = [1, "Luke"; 2, "Kirk"; 3, "Spock"; 4, "Kenobi"]
 
 input |> List.findBack (fun (x,_) -> isEven x)              // evaluates (4, "Kenobi")
 input |> List.findBack (fun (x,_) -> x |> isGreaterThan 6)  // raises an exception

val isEven: x: int -> bool

val x: int

val isGreaterThan: x: 'a -> y: 'a -> bool (requires comparison)

val x: 'a (requires comparison)

val y: 'a (requires comparison)

val input: (int * string) list

val findBack: predicate: ('T -> bool) -> list: 'T list -> 'T


                
                  
                    
                      List.findIndex predicate list

Full Usage: List.findIndex predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: int The index of the first element that satisfies the predicate.

Returns the index of the first element in the list that satisfies the given predicate. Raises KeyNotFoundException if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: int: The index of the first element that satisfies the predicate.

ArgumentException

Thrown if the predicate evaluates to false for all the elements of the list.

Example

 let isEven x  = 0 = x % 2

 let isGreaterThan x y = y > x

 let input = [1, "Luke"; 2, "Kirk"; 3, "Spock"; 4, "Kenobi"]
 
 input |> List.findIndex (fun (x,_) -> isEven x)              // evaluates 1
 input |> List.findIndex (fun (x,_) -> x |> isGreaterThan 6)  // raises an exception

val isEven: x: int -> bool

val x: int

val isGreaterThan: x: 'a -> y: 'a -> bool (requires comparison)

val x: 'a (requires comparison)

val y: 'a (requires comparison)

val input: (int * string) list

val findIndex: predicate: ('T -> bool) -> list: 'T list -> int


                
                  
                    
                      List.findIndexBack predicate list

Full Usage: List.findIndexBack predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: int The index of the last element that satisfies the predicate.

Returns the index of the last element in the list that satisfies the given predicate. Raises KeyNotFoundException if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: int: The index of the last element that satisfies the predicate.

ArgumentException

Thrown if the predicate evaluates to false for all the elements of the list.

Example

 let isEven x  = 0 = x % 2

 let isGreaterThan x y = y > x

 let input = [1, "Luke"; 2, "Kirk"; 3, "Spock"; 4, "Kenobi"]
 
 input |> List.findIndexBack (fun (x,_) -> isEven x)              // evaluates 3
 input |> List.findIndexBack (fun (x,_) -> x |> isGreaterThan 6)  // raises an exception

val isEven: x: int -> bool

val x: int

val isGreaterThan: x: 'a -> y: 'a -> bool (requires comparison)

val x: 'a (requires comparison)

val y: 'a (requires comparison)

val input: (int * string) list

val findIndexBack: predicate: ('T -> bool) -> list: 'T list -> int


                
                  
                    
                      List.fold folder state list

Full Usage: List.fold folder state list

Parameters:

folder : 'State -> 'T -> 'State - The function to update the state given the input elements.

state : 'State - The initial state.

list : 'T list - The input list.

Returns: 'State The final state value.

Applies a function to each element of the collection, threading an accumulator argument through the computation. Take the second argument, and apply the function to it and the first element of the list. Then feed this result into the function along with the second element and so on. Return the final result. If the input function is f and the elements are i0...iN then computes f (... (f s i0) i1 ...) iN.

folder : 'State -> 'T -> 'State: The function to update the state given the input elements.
state : 'State: The initial state.
list : 'T list: The input list.

Returns: 'State: The final state value.

Example

Making the sum of squares for the first 5 natural numbers

 (0, [1..5]) ||> List.fold (fun s v -> s + v * v)  // evaluates 55

val fold<'T,'State> : folder: ('State -> 'T -> 'State) -> state: 'State -> list: 'T list -> 'State

val s: int

val v: int

Example

Shopping for fruits hungry, you tend to take more of each as the hunger grows

 type Fruit = Apple | Pear | Orange

 type BagItem = { fruit: Fruit; quantity: int }

 let takeMore (previous: BagItem list) fruit = 
     let toTakeThisTime = 
         match previous with 
         | bagItem :: otherBagItems -> bagItem.quantity + 1 
         | [] -> 1 
     { fruit = fruit; quantity = toTakeThisTime } :: previous

 let inputs = [ Apple; Pear; Orange ]
 
 ([], inputs) ||> List.fold takeMore

type Fruit = | Apple | Pear | Orange

type BagItem = { fruit: Fruit quantity: int }

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val takeMore: previous: BagItem list -> fruit: Fruit -> BagItem list

val previous: BagItem list

type 'T list = List<'T>

val fruit: Fruit

val toTakeThisTime: int

val bagItem: BagItem

val otherBagItems: BagItem list

BagItem.quantity: int

val inputs: Fruit list

union case Fruit.Apple: Fruit

union case Fruit.Pear: Fruit

union case Fruit.Orange: Fruit

val fold<'T,'State> : folder: ('State -> 'T -> 'State) -> state: 'State -> list: 'T list -> 'State

Evaluates to

  [{ fruit = Orange; quantity = 3 }
   { fruit = Pear; quantity = 2 }
   { fruit = Apple; quantity = 1 }]


                
                  
                    
                      List.fold2 folder state list1 list2

Full Usage: List.fold2 folder state list1 list2

Parameters:

folder : 'State -> 'T1 -> 'T2 -> 'State - The function to update the state given the input elements.

state : 'State - The initial state.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: 'State The final state value.

Applies a function to corresponding elements of two collections, threading an accumulator argument through the computation. The collections must have identical sizes. If the input function is f and the elements are i0...iN and j0...jN then computes f (... (f s i0 j0)...) iN jN.

folder : 'State -> 'T1 -> 'T2 -> 'State: The function to update the state given the input elements.
state : 'State: The initial state.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: 'State: The final state value.

Example

Count the number of times the coins match:
 type CoinToss = Head | Tails

 let inputs1 = [Tails; Head; Tails]
 let inputs2 = [Tails; Head; Head]

 (0, inputs1, inputs2) |||> List.fold2 (fun acc input1 input2 ->
     match (input1, input2) with
     | Head, Head -> acc + 1
     | Tails, Tails -> acc + 1
     | _ -> acc)

val fold2<'T1,'T2,'State> : folder: ('State -> 'T1 -> 'T2 -> 'State) -> state: 'State -> list1: 'T1 list -> list2: 'T2 list -> 'State

Evaluates to 2. Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.foldBack folder list state

Full Usage: List.foldBack folder list state

Parameters:

folder : 'T -> 'State -> 'State - The function to update the state given the input elements.

list : 'T list - The input list.

state : 'State - The initial state.

Returns: 'State The state object after the folding function is applied to each element of the list.

Applies a function to each element of the collection, starting from the end, threading an accumulator argument through the computation. If the input function is f and the elements are i0...iN then computes f i0 (...(f iN s)).

folder : 'T -> 'State -> 'State: The function to update the state given the input elements.
list : 'T list: The input list.
state : 'State: The initial state.

Returns: 'State: The state object after the folding function is applied to each element of the list.

Example

Making the sum of squares for the first 5 natural numbers

 ([1..5], 0) ||> List.foldBack (fun v acc -> acc + v * v)  // evaluates 55

val foldBack: folder: ('T -> 'State -> 'State) -> list: 'T list -> state: 'State -> 'State

val v: int

val acc: int

Note acc is a commonly used abbreviation for "accumulator".

Example

Shopping for fruits hungry, you tend to take more of each as the hunger grows

 type Fruit = Apple | Pear | Orange

 type BagItem = { fruit: Fruit; quantity: int }

 let takeMore fruit (previous: BagItem list) =
     let toTakeThisTime = 
         match previous with 
         | bagItem :: otherBagItems -> bagItem.quantity + 1 
         | [] -> 1 
     { fruit = fruit; quantity = toTakeThisTime } :: previous

 let input = [ Apple; Pear; Orange ]
 
 (input, []) ||> List.foldBack takeMore

type Fruit = | Apple | Pear | Orange

type BagItem = { fruit: Fruit quantity: int }

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val takeMore: fruit: Fruit -> previous: BagItem list -> BagItem list

val fruit: Fruit

val previous: BagItem list

type 'T list = List<'T>

val toTakeThisTime: int

val bagItem: BagItem

val otherBagItems: BagItem list

BagItem.quantity: int

val input: Fruit list

union case Fruit.Apple: Fruit

union case Fruit.Pear: Fruit

union case Fruit.Orange: Fruit

val foldBack: folder: ('T -> 'State -> 'State) -> list: 'T list -> state: 'State -> 'State

Evaluates to

  [{ fruit = Apple; quantity = 3 }
   { fruit = Pear; quantity = 2 }
   { fruit = Orange; quantity = 1 }]


                
                  
                    
                      List.foldBack2 folder list1 list2 state

Full Usage: List.foldBack2 folder list1 list2 state

Parameters:

folder : 'T1 -> 'T2 -> 'State -> 'State - The function to update the state given the input elements.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

state : 'State - The initial state.

Returns: 'State The final state value.

folder : 'T1 -> 'T2 -> 'State -> 'State: The function to update the state given the input elements.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.
state : 'State: The initial state.

Returns: 'State: The final state value.

Example

Count the positives, negatives and accumulate some text from back to front:
 type Count =
   { Positive: int
     Negative: int
     Text: string }

 let inputs1 = [ -1; -2; -3 ]
 let inputs2 = [ 3; 2; 1 ]
 let initialState = {Positive = 0; Negative = 0; Text = ""}

 (inputs1, inputs2, initialState) |||> List.foldBack2 (fun a b acc  ->
     let text = acc.Text + "(" + string a + "," + string b + ") "
     if a + b >= 0 then
         { acc with
             Positive = acc.Positive + 1
             Text = text }
     else
         { acc with
             Negative = acc.Negative + 1
             Text = text }
 )

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

namespace Microsoft.FSharp.Text

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

val foldBack2: folder: ('T1 -> 'T2 -> 'State -> 'State) -> list1: 'T1 list -> list2: 'T2 list -> state: 'State -> 'State

Evaluates to

 { Positive = 2
   Negative = 1
   Text = "(-3,1) (-2,2) (-1,3) " }

namespace Microsoft.FSharp.Text

Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.forall predicate list

Full Usage: List.forall predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: bool True if all of the elements satisfy the predicate.

Tests if all elements of the collection satisfy the given predicate.

The predicate is applied to the elements of the input list. If any application returns false then the overall result is false and no further elements are tested. Otherwise, true is returned.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: bool: True if all of the elements satisfy the predicate.

Example

 let isEven a = a % 2 = 0

 [2; 42] |> List.forall isEven // evaluates to true

 [1; 2] |> List.forall isEven // evaluates to false

val isEven: a: int -> bool

val a: int

val forall: predicate: ('T -> bool) -> list: 'T list -> bool


                
                  
                    
                      List.forall2 predicate list1 list2

Full Usage: List.forall2 predicate list1 list2

Parameters:

predicate : 'T1 -> 'T2 -> bool - The function to test the input elements.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: bool True if all of the pairs of elements satisfy the predicate.

Tests if all corresponding elements of the collection satisfy the given predicate pairwise.

The predicate is applied to matching elements in the two collections up to the lesser of the two lengths of the collections. If any application returns false then the overall result is false and no further elements are tested. Otherwise, if one collection is longer than the other then the ArgumentException exception is raised. Otherwise, true is returned.

predicate : 'T1 -> 'T2 -> bool: The function to test the input elements.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: bool: True if all of the pairs of elements satisfy the predicate.

ArgumentException

Thrown when the input lists differ in length.

Example

 let inputs1 = [1; 2; 3]
 let inputs2 = [1; 2; 3]

 (inputs1, inputs2) ||> List.forall2 (=)

val inputs1: int list

val inputs2: int list

val forall2: predicate: ('T1 -> 'T2 -> bool) -> list1: 'T1 list -> list2: 'T2 list -> bool

Evaluates to true.

Example

 let items1 = [2017; 1; 1]
 let items2 = [2019; 19; 8]

 (items1, items2) ||> List.forall2 (=)

val items1: int list

val items2: int list

val forall2: predicate: ('T1 -> 'T2 -> bool) -> list1: 'T1 list -> list2: 'T2 list -> bool

Evaluates to false.

Example

 let items1 = [1; 2; 3]
 let items2 = [1; 2]

 (items1, items2) ||> List.forall2 (=)

val items1: int list

val items2: int list

val forall2: predicate: ('T1 -> 'T2 -> bool) -> list1: 'T1 list -> list2: 'T2 list -> bool

Throws ArgumentException.


                
                  
                    
                      List.groupBy projection list

Full Usage: List.groupBy projection list

Parameters:

projection : 'T -> 'Key - A function that transforms an element of the list into a comparable key.

list : 'T list - The input list.

Returns: ('Key * 'T list) list The result list.

Applies a key-generating function to each element of a list and yields a list of unique keys. Each unique key contains a list of all elements that match to this key.

projection : 'T -> 'Key: A function that transforms an element of the list into a comparable key.
list : 'T list: The input list.

Returns: ('Key * 'T list) list: The result list.

Example

 let inputs = [1; 2; 3; 4; 5]

 inputs |> List.groupBy (fun n -> n % 2)

val inputs: int list

val groupBy: projection: ('T -> 'Key) -> list: 'T list -> ('Key * 'T list) list (requires equality)

val n: int

Evaluates to [(1, [1; 3; 5]); (0, [2; 4])]


                
                  
                    
                      List.head list

Full Usage: List.head list

Parameters:

list : 'T list - The input list.

Returns: 'T The first element of the list.

Returns the first element of the list.

list : 'T list: The input list.

Returns: 'T: The first element of the list.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = ["banana"; "pear"]

 inputs |> List.head

val inputs: string list

val head: list: 'T list -> 'T

Evaluates to banana

Example

 [] |> List.head

val head: list: 'T list -> 'T

Throws ArgumentException


                
                  
                    
                      List.indexed list

Full Usage: List.indexed list

Parameters:

list : 'T list - The input list.

Returns: (int * 'T) list The list of indexed elements.

Returns a new list whose elements are the corresponding elements of the input list paired with the index (from 0) of each element.

list : 'T list: The input list.

Returns: (int * 'T) list: The list of indexed elements.

Example

 let inputs = ["a"; "b"; "c"]

 inputs |> List.indexed

val inputs: string list

val indexed: list: 'T list -> (int * 'T) list

Evaluates to [(0, "a"); (1, "b"); (2, "c")]


                
                  
                    
                      List.init length initializer

Full Usage: List.init length initializer

Parameters:

length : int - The length of the list to generate.

initializer : int -> 'T - The function to generate an element from an index.

Returns: 'T list The list of generated elements.

Creates a list by calling the given generator on each index.

length : int: The length of the list to generate.
initializer : int -> 'T: The function to generate an element from an index.

Returns: 'T list: The list of generated elements.

ArgumentException

Thrown when the input length is negative.

Example

 List.init 4 (fun v -> v + 5)

val init: length: int -> initializer: (int -> 'T) -> 'T list

val v: int

Evaluates to [5; 6; 7; 8]

Example

 List.init -5 (fun v -> v + 5)

val init: length: int -> initializer: (int -> 'T) -> 'T list

val v: int

Throws ArgumentException


                
                  
                    
                      List.insertAt index value source

Full Usage: List.insertAt index value source

Parameters:

index : int - The index where the item should be inserted.

value : 'T - The value to insert.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list with a new item inserted before the given index.

index : int: The index where the item should be inserted.
value : 'T: The value to insert.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException

Thrown when index is below 0 or greater than source.Length.

Example

 let inputs = [ 0; 1; 2 ]

 inputs |> List.insertAt 1 9

val inputs: int list

val insertAt: index: int -> value: 'T -> source: 'T list -> 'T list

Evaluates to [ 0; 9; 1; 2 ].


                
                  
                    
                      List.insertManyAt index values source

Full Usage: List.insertManyAt index values source

Parameters:

index : int - The index where the items should be inserted.

values : 'T seq - The values to insert.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list with new items inserted before the given index.

index : int: The index where the items should be inserted.
values : 'T seq: The values to insert.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException

Thrown when index is below 0 or greater than source.Length.

Example

 let inputs = [ 0; 1; 2 ]

 inputs |> List.insertManyAt 1 [ 8; 9 ]
 Evaluates to [ 0; 8; 9; 1; 2 ].

val inputs: int list

val insertManyAt: index: int -> values: 'T seq -> source: 'T list -> 'T list


                
                  
                    
                      List.isEmpty list

Full Usage: List.isEmpty list

Parameters:

list : 'T list - The input list.

Returns: bool True if the list is empty.

Returns true if the list contains no elements, false otherwise.

list : 'T list: The input list.

Returns: bool: True if the list is empty.

Example

 [ ] |> List.isEmpty

val isEmpty: list: 'T list -> bool

Evaluates to true

Example

 [ "pear"; "banana" ] |> List.isEmpty

val isEmpty: list: 'T list -> bool

Evaluates to false


                
                  
                    
                      List.item index list

Full Usage: List.item index list

Parameters:

index : int - The index to retrieve.

list : 'T list - The input list.

Returns: 'T The value at the given index.

Indexes into the list. The first element has index 0.

index : int: The index to retrieve.
list : 'T list: The input list.

Returns: 'T: The value at the given index.

ArgumentException

Thrown when the index is negative or the input list does not contain enough elements.

Example

 let inputs = [ "a"; "b"; "c" ]

 inputs |> List.item 1

val inputs: string list

val item: index: int -> list: 'T list -> 'T

Evaluates to "b"

Example

 let inputs = [ "a"; "b"; "c" ]

 inputs |> List.item 4

val inputs: string list

val item: index: int -> list: 'T list -> 'T

Throws ArgumentException


                
                  
                    
                      List.iter action list

Full Usage: List.iter action list

Parameters:

action : 'T -> unit - The function to apply to elements from the input list.

list : 'T list - The input list.

Modifiers: inline
Type parameters: 'T

Applies the given function to each element of the collection.

action : 'T -> unit: The function to apply to elements from the input list.
list : 'T list: The input list.

Example

 let inputs = [ "a"; "b"; "c" ]

 inputs |> List.iter (printfn "%s")

val inputs: string list

val iter: action: ('T -> unit) -> list: 'T list -> unit

val printfn: format: Printf.TextWriterFormat<'T> -> 'T

Evaluates to unit and prints

 a
 b
 c

in the console.


                
                  
                    
                      List.iter2 action list1 list2

Full Usage: List.iter2 action list1 list2

Parameters:

action : 'T1 -> 'T2 -> unit - The function to apply to pairs of elements from the input lists.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Applies the given function to two collections simultaneously. The collections must have identical sizes.

action : 'T1 -> 'T2 -> unit: The function to apply to pairs of elements from the input lists.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Example

 let inputs1 = [ "a"; "b"; "c" ]
 let inputs2 = [ 1; 2; 3 ]

 (inputs1, inputs2) ||> List.iter2 (printfn "%s: %i")

val inputs1: string list

val inputs2: int list

val iter2: action: ('T1 -> 'T2 -> unit) -> list1: 'T1 list -> list2: 'T2 list -> unit

val printfn: format: Printf.TextWriterFormat<'T> -> 'T

Evaluates to unit and prints

 a: 1
 b: 2
 c: 3

in the console.


                
                  
                    
                      List.iteri action list

Full Usage: List.iteri action list

Parameters:

action : int -> 'T -> unit - The function to apply to the elements of the list along with their index.

list : 'T list - The input list.

Modifiers: inline
Type parameters: 'T

Applies the given function to each element of the collection. The integer passed to the function indicates the index of the element.

action : int -> 'T -> unit: The function to apply to the elements of the list along with their index.
list : 'T list: The input list.

Example

 let inputs = [ "a"; "b"; "c" ]

 inputs |> List.iteri (fun i v -> printfn "{i}: {v}")

val inputs: string list

val iteri: action: (int -> 'T -> unit) -> list: 'T list -> unit

val i: int

val v: string

val printfn: format: Printf.TextWriterFormat<'T> -> 'T

Evaluates to unit and prints

 0: a
 1: b
 2: c

in the console.


                
                  
                    
                      List.iteri2 action list1 list2

Full Usage: List.iteri2 action list1 list2

Parameters:

action : int -> 'T1 -> 'T2 -> unit - The function to apply to a pair of elements from the input lists along with their index.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Applies the given function to two collections simultaneously. The collections must have identical sizes. The integer passed to the function indicates the index of the element.

action : int -> 'T1 -> 'T2 -> unit: The function to apply to a pair of elements from the input lists along with their index.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Example

 let inputs1 = [ "a"; "b"; "c" ]
 let inputs2 = [ "banana"; "pear"; "apple" ]

 (inputs1, inputs2) ||> List.iteri2 (fun i s1 s2 -> printfn "Index %d: %s - %s" i s1 s2)

val inputs1: string list

val inputs2: string list

val iteri2: action: (int -> 'T1 -> 'T2 -> unit) -> list1: 'T1 list -> list2: 'T2 list -> unit

val i: int

val s1: string

val s2: string

val printfn: format: Printf.TextWriterFormat<'T> -> 'T

Evaluates to unit and prints

 Index 0: a - banana
 Index 1: b - pear
 Index 2: c - apple

in the console.


                
                  
                    
                      List.last list

Full Usage: List.last list

Parameters:

list : 'T list - The input list.

Returns: 'T The last element of the list.

Returns the last element of the list.

list : 'T list: The input list.

Returns: 'T: The last element of the list.

ArgumentException

Thrown when the input does not have any elements.

Example

 [ "pear"; "banana" ] |> List.last

val last: list: 'T list -> 'T

Evaluates to banana

Example

 [ ] |> List.last

val last: list: 'T list -> 'T

Throws ArgumentException


                
                  
                    
                      List.length list

Full Usage: List.length list

Parameters:

list : 'T list - The input list.

Returns: int The length of the list.

Returns the length of the list.

The notation array.Length is preferred.

list : 'T list: The input list.

Returns: int: The length of the list.

Example

 let inputs = [ "a"; "b"; "c" ]

 inputs |> List.length

val inputs: string list

val length: list: 'T list -> int

Evaluates to 3


                
                  
                    
                      List.map mapping list

Full Usage: List.map mapping list

Parameters:

mapping : 'T -> 'U - The function to transform elements from the input list.

list : 'T list - The input list.

Returns: 'U list The list of transformed elements.

Builds a new collection whose elements are the results of applying the given function to each of the elements of the collection.

mapping : 'T -> 'U: The function to transform elements from the input list.
list : 'T list: The input list.

Returns: 'U list: The list of transformed elements.

Example

 let inputs = [ "a"; "bbb"; "cc" ]

 inputs |> List.map (fun x -> x.Length)

val inputs: string list

val map: mapping: ('T -> 'U) -> list: 'T list -> 'U list

val x: string

property System.String.Length: int with get

Evaluates to [ 1; 3; 2 ]


                
                  
                    
                      List.map2 mapping list1 list2

Full Usage: List.map2 mapping list1 list2

Parameters:

mapping : 'T1 -> 'T2 -> 'U - The function to transform pairs of elements from the input lists.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: 'U list The list of transformed elements.

Builds a new collection whose elements are the results of applying the given function to the corresponding elements of the two collections pairwise.

mapping : 'T1 -> 'T2 -> 'U: The function to transform pairs of elements from the input lists.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: 'U list: The list of transformed elements.

Example

 let inputs1 = ["a"; "bad"; "good"]
 let inputs2 = [0; 2; 1]

 (inputs1, inputs2) ||> List.map2 (fun x y -> x.[y])

val inputs1: string list

val inputs2: int list

val map2: mapping: ('T1 -> 'T2 -> 'U) -> list1: 'T1 list -> list2: 'T2 list -> 'U list

val x: string

val y: int

Evaluates to seq ['a'; 'd'; 'o']


                
                  
                    
                      List.map3 mapping list1 list2 list3

Full Usage: List.map3 mapping list1 list2 list3

Parameters:

mapping : 'T1 -> 'T2 -> 'T3 -> 'U - The function to transform triples of elements from the input lists.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

list3 : 'T3 list - The third input list.

Returns: 'U list The list of transformed elements.

Builds a new collection whose elements are the results of applying the given function to the corresponding elements of the three collections simultaneously.

mapping : 'T1 -> 'T2 -> 'T3 -> 'U: The function to transform triples of elements from the input lists.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.
list3 : 'T3 list: The third input list.

Returns: 'U list: The list of transformed elements.

Example

 let inputs1 = [ "a"; "t"; "ti" ]
 let inputs2 = [ "l"; "h"; "m" ]
 let inputs3 = [ "l"; "e"; "e" ]

 (inputs1, inputs2, inputs3) |||> List.map3 (fun x y z -> x + y + z)

val inputs1: string list

val inputs2: string list

val inputs3: string list

val map3: mapping: ('T1 -> 'T2 -> 'T3 -> 'U) -> list1: 'T1 list -> list2: 'T2 list -> list3: 'T3 list -> 'U list

val x: string

val y: string

val z: string

Evaluates to [ "all"; "the"; "time" ]


                
                  
                    
                      List.mapFold mapping state list

Full Usage: List.mapFold mapping state list

Parameters:

mapping : 'State -> 'T -> 'Result * 'State - The function to transform elements from the input list and accumulate the final value.

state : 'State - The initial state.

list : 'T list - The input list.

Returns: 'Result list * 'State The list of transformed elements, and the final accumulated value.

Combines map and fold. Builds a new list whose elements are the results of applying the given function to each of the elements of the input list. The function is also used to accumulate a final value.

mapping : 'State -> 'T -> 'Result * 'State: The function to transform elements from the input list and accumulate the final value.
state : 'State: The initial state.
list : 'T list: The input list.

Returns: 'Result list * 'State: The list of transformed elements, and the final accumulated value.

Example

Accumulate the charges, and double them as well

 type Charge =
     | In of int
     | Out of int

 let inputs = [ In 1; Out 2; In 3 ]

 let newCharges, balance =
     (0, inputs) ||> List.mapFold (fun acc charge ->
         match charge with
         | In i -> In (i*2), acc + i
         | Out o -> Out (o*2), acc - o)

type Charge = | In of int | Out of int

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val inputs: Charge list

union case Charge.In: int -> Charge

union case Charge.Out: int -> Charge

val newCharges: Charge list

val balance: int

val mapFold<'T,'State,'Result> : mapping: ('State -> 'T -> 'Result * 'State) -> state: 'State -> list: 'T list -> 'Result list * 'State

val acc: int

val charge: Charge

val i: int

val o: int

Evaluates newCharges to [In 2; Out 4; In 6] and balance to 2. Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.mapFoldBack mapping list state

Full Usage: List.mapFoldBack mapping list state

Parameters:

mapping : 'T -> 'State -> 'Result * 'State - The function to transform elements from the input list and accumulate the final value.

list : 'T list - The input list.

state : 'State - The initial state.

Returns: 'Result list * 'State The list of transformed elements, and the final accumulated value.

Combines map and foldBack. Builds a new list whose elements are the results of applying the given function to each of the elements of the input list. The function is also used to accumulate a final value.

mapping : 'T -> 'State -> 'Result * 'State: The function to transform elements from the input list and accumulate the final value.
list : 'T list: The input list.
state : 'State: The initial state.

Returns: 'Result list * 'State: The list of transformed elements, and the final accumulated value.

Example

Accumulate the charges from back to front, and double them as well

 type Charge =
     | In of int
     | Out of int

 let charges = [ In 1; Out 2; In 3 ]

 let newCharges, balance =
     (charges, 0) ||> List.mapFoldBack (fun charge acc ->
         match charge with
         | In i -> In (i*2), acc + i
         | Out o -> Out (o*2), acc - o)

type Charge = | In of int | Out of int

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val charges: Charge list

union case Charge.In: int -> Charge

union case Charge.Out: int -> Charge

val newCharges: Charge list

val balance: int

val mapFoldBack: mapping: ('T -> 'State -> 'Result * 'State) -> list: 'T list -> state: 'State -> 'Result list * 'State

val charge: Charge

val acc: int

val i: int

val o: int

Evaluates newCharges to [In 2; Out 4; In 6] and balance to 2. Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.mapi mapping list

Full Usage: List.mapi mapping list

Parameters:

mapping : int -> 'T -> 'U - The function to transform elements and their indices.

list : 'T list - The input list.

Returns: 'U list The list of transformed elements.

Builds a new collection whose elements are the results of applying the given function to each of the elements of the collection. The integer index passed to the function indicates the index (from 0) of the element being transformed.

mapping : int -> 'T -> 'U: The function to transform elements and their indices.
list : 'T list: The input list.

Returns: 'U list: The list of transformed elements.

Example

 let inputs = [ 10; 10; 10 ]

 inputs |> List.mapi (fun i x -> i + x)

val inputs: int list

val mapi: mapping: (int -> 'T -> 'U) -> list: 'T list -> 'U list

val i: int

val x: int

Evaluates to [ 10; 11; 12 ]


                
                  
                    
                      List.mapi2 mapping list1 list2

Full Usage: List.mapi2 mapping list1 list2

Parameters:

mapping : int -> 'T1 -> 'T2 -> 'U - The function to transform pairs of elements from the two lists and their index.

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: 'U list The list of transformed elements.

Like mapi, but mapping corresponding elements from two lists of equal length.

mapping : int -> 'T1 -> 'T2 -> 'U: The function to transform pairs of elements from the two lists and their index.
list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: 'U list: The list of transformed elements.

Example

 let inputs1 = ["a"; "bad"; "good"]
 let inputs2 = [0; 2; 1]

 (inputs1, inputs2) ||> List.mapi2 (fun i x y -> i, x[y])

val inputs1: string list

val inputs2: int list

val mapi2: mapping: (int -> 'T1 -> 'T2 -> 'U) -> list1: 'T1 list -> list2: 'T2 list -> 'U list

val i: int

val x: string

val y: int

Evaluates to [(0, 'a'); (1, 'd'); (2, 'o')]


                
                  
                    
                      List.max list

Full Usage: List.max list

Parameters:

list : 'T list - The input list.

Returns: 'T The maximum element.
Modifiers: inline
Type parameters: 'T

Return the greatest of all elements of the list, compared via Operators.max.

Raises ArgumentException if list is empty

list : 'T list: The input list.

Returns: 'T: The maximum element.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = [ 10; 12; 11 ]

 inputs |> List.max

val inputs: int list

val max: list: 'T list -> 'T (requires comparison)

Evaluates to 12

Example

 let inputs = [ ]

 inputs |> List.max

val inputs: 'a list

val max: list: 'T list -> 'T (requires comparison)

Throws System.ArgumentException.


                
                  
                    
                      List.maxBy projection list

Full Usage: List.maxBy projection list

Parameters:

projection : 'T -> 'U - The function to transform the list elements into the type to be compared.

list : 'T list - The input list.

Returns: 'T The maximum element.
Modifiers: inline
Type parameters: 'T, 'U

Returns the greatest of all elements of the list, compared via Operators.max on the function result.

Raises ArgumentException if list is empty.

projection : 'T -> 'U: The function to transform the list elements into the type to be compared.
list : 'T list: The input list.

Returns: 'T: The maximum element.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = ["aaa"; "b"; "cccc"]

 inputs |> List.maxBy (fun s -> s.Length)

val inputs: string list

val maxBy: projection: ('T -> 'U) -> list: 'T list -> 'T (requires comparison)

val s: string

property System.String.Length: int with get

Evaluates to "cccc"

Example

 let inputs = []

 inputs |> List.maxBy (fun (s: string) -> s.Length)

val inputs: 'a list

val maxBy: projection: ('T -> 'U) -> list: 'T list -> 'T (requires comparison)

val s: string

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

property System.String.Length: int with get

Throws System.ArgumentException.


                
                  
                    
                      List.min list

Full Usage: List.min list

Parameters:

list : 'T list - The input list.

Returns: 'T The minimum value.
Modifiers: inline
Type parameters: 'T

Returns the lowest of all elements of the list, compared via Operators.min.

Raises ArgumentException if list is empty

list : 'T list: The input list.

Returns: 'T: The minimum value.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = [10; 12; 11]

 inputs |> List.min

val inputs: int list

val min: list: 'T list -> 'T (requires comparison)

Evaluates to 10

Example

 let inputs = []

 inputs |> List.min

val inputs: 'a list

val min: list: 'T list -> 'T (requires comparison)

Throws System.ArgumentException.


                
                  
                    
                      List.minBy projection list

Full Usage: List.minBy projection list

Parameters:

projection : 'T -> 'U - The function to transform list elements into the type to be compared.

list : 'T list - The input list.

Returns: 'T The minimum value.
Modifiers: inline
Type parameters: 'T, 'U

Returns the lowest of all elements of the list, compared via Operators.min on the function result

Raises ArgumentException if list is empty.

projection : 'T -> 'U: The function to transform list elements into the type to be compared.
list : 'T list: The input list.

Returns: 'T: The minimum value.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = ["aaa"; "b"; "cccc"]

 inputs |> List.minBy (fun s -> s.Length)

val inputs: string list

val minBy: projection: ('T -> 'U) -> list: 'T list -> 'T (requires comparison)

val s: string

property System.String.Length: int with get

Evaluates to "b"

Example

 let inputs = []

 inputs |> List.minBy (fun (s: string) -> s.Length)

val inputs: 'a list

val minBy: projection: ('T -> 'U) -> list: 'T list -> 'T (requires comparison)

val s: string

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

property System.String.Length: int with get

Throws System.ArgumentException.


                
                  
                    
                      List.ofArray array

Full Usage: List.ofArray array

Parameters:

array : 'T array - The input array.

Returns: 'T list The list of elements from the array.

Builds a list from the given array.

array : 'T array: The input array.

Returns: 'T list: The list of elements from the array.

Example

 let inputs = [| 1; 2; 5 |]

 inputs |> List.ofArray

val inputs: int array

val ofArray: array: 'T array -> 'T list

Evaluates to [ 1; 2; 5 ].


                
                  
                    
                      List.ofSeq source

Full Usage: List.ofSeq source

Parameters:

source : 'T seq - The input sequence.

Returns: 'T list The list of elements from the sequence.

Builds a new list from the given enumerable object.

source : 'T seq: The input sequence.

Returns: 'T list: The list of elements from the sequence.

Example

 let inputs = seq { 1; 2; 5 }

 inputs |> List.ofSeq

val inputs: int seq

Multiple items
val seq: sequence: 'T seq -> 'T seq

--------------------
type 'T seq = System.Collections.Generic.IEnumerable<'T>

val ofSeq: source: 'T seq -> 'T list

Evaluates to [ 1; 2; 5 ].


                
                  
                    
                      List.pairwise list

Full Usage: List.pairwise list

Parameters:

list : 'T list - The input list.

Returns: ('T * 'T) list The result list.

Returns a list of each element in the input list paired with its predecessor, with the exception of the first element which is only returned as the predecessor of the second element. The predecessor comes first in the returned pairs.

list : 'T list: The input list.

Returns: ('T * 'T) list: The result list.

Example

 let inputs = [1; 2; 3; 4]

 inputs |> List.pairwise

val inputs: int list

val pairwise: list: 'T list -> ('T * 'T) list

Evaluates to [(1, 2); (2, 3); (3, 4)].


                
                  
                    
                      List.partition predicate list

Full Usage: List.partition predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T list * 'T list A list containing the elements for which the predicate evaluated to true and a list containing the elements for which the predicate evaluated to false.

Splits the collection into two collections, containing the elements for which the given predicate returns True and False respectively. Element order is preserved in both of the created lists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T list * 'T list: A list containing the elements for which the predicate evaluated to true and a list containing the elements for which the predicate evaluated to false.

Example

 let inputs = [1; 2; 3; 4]

 let evens, odds = inputs |> List.partition (fun x -> x % 2 = 0)

val inputs: int list

val evens: int list

val odds: int list

val partition: predicate: ('T -> bool) -> list: 'T list -> 'T list * 'T list

val x: int

Evaluates evens to [2; 4] and odds to [1; 3].


                
                  
                    
                      List.permute indexMap list

Full Usage: List.permute indexMap list

Parameters:

indexMap : int -> int - The function to map input indices to output indices.

list : 'T list - The input list.

Returns: 'T list The permuted list.

Returns a list with all elements permuted according to the specified permutation.

indexMap : int -> int: The function to map input indices to output indices.
list : 'T list: The input list.

Returns: 'T list: The permuted list.

ArgumentException

Thrown when indexMap does not produce a valid permutation.

Example

 let inputs = [1; 2; 3; 4]

 inputs |> List.permute (fun x -> (x + 1) % 4)

val inputs: int list

val permute: indexMap: (int -> int) -> list: 'T list -> 'T list

val x: int

Evaluates to [4; 1; 2; 3].


                
                  
                    
                      List.pick chooser list

Full Usage: List.pick chooser list

Parameters:

chooser : 'T -> 'U option - The function to generate options from the elements.

list : 'T list - The input list.

Returns: 'U The first resulting value.

Applies the given function to successive elements, returning the first result where function returns Some(x) for some x. If no such element exists then raise KeyNotFoundException

chooser : 'T -> 'U option: The function to generate options from the elements.
list : 'T list: The input list.

Returns: 'U: The first resulting value.

KeyNotFoundException

Thrown when the list is empty.

Example

 let input = [1; 2; 3]

 input |> List.pick (fun n -> if n % 2 = 0 then Some (string n) else None)

val input: int list

val pick: chooser: ('T -> 'U option) -> list: 'T list -> 'U

val n: int

union case Option.Some: Value: 'T -> Option<'T>

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

union case Option.None: Option<'T>

Evaluates to "2".

Example

 let input = [1; 2; 3]

 input |> List.pick (fun n -> if n > 3 then Some (string n) else None)

val input: int list

val pick: chooser: ('T -> 'U option) -> list: 'T list -> 'U

val n: int

union case Option.Some: Value: 'T -> Option<'T>

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

union case Option.None: Option<'T>

Throws KeyNotFoundException.


                
                  
                    
                      List.randomChoice source

Full Usage: List.randomChoice source

Parameters:

source : 'T list - The input list.

Returns: 'T A randomly selected element from the input list.

Returns a random element from the given list.

source : 'T list: The input list.

Returns: 'T: A randomly selected element from the input list.

ArgumentException

Thrown when the input list is empty.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomChoice

val inputs: int list

Can evaluate to 3.


                
                  
                    
                      List.randomChoiceBy randomizer source

Full Usage: List.randomChoiceBy randomizer source

Parameters:

randomizer : unit -> float - The randomizer function, must return a float number from [0.0..1.0) range.

source : 'T list - The input list.

Returns: 'T A randomly selected element from the input list.

Returns a random element from the given list using the specified randomizer function.

randomizer : unit -> float: The randomizer function, must return a float number from [0.0..1.0) range.
source : 'T list: The input list.

Returns: 'T: A randomly selected element from the input list.

ArgumentException	Thrown when the input list is empty.
ArgumentOutOfRangeException	Thrown when the randomizer function returns a value outside the range [0, 1).

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomChoiceBy Random.Shared.NextDouble

val inputs: int list

Can evaluate to 3.


                
                  
                    
                      List.randomChoiceWith random source

Full Usage: List.randomChoiceWith random source

Parameters:

random : Random - The Random instance.

source : 'T list - The input list.

Returns: 'T A randomly selected element from the input list.

Returns a random element from the given list with the specified Random instance, each element can be selected multiple times.

random : Random: The Random instance.
source : 'T list: The input list.

Returns: 'T: A randomly selected element from the input list.

ArgumentNullException	Thrown when the random argument is null.
ArgumentException	Thrown when the input list is empty.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomChoiceWith Random.Shared

val inputs: int list

Can evaluate to 3.


                
                  
                    
                      List.randomChoices count source

Full Usage: List.randomChoices count source

Parameters:

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a list of random elements from the given list.

count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomChoices 3

val inputs: int list

Can evaluate to [ 3; 1; 3 ].


                
                  
                    
                      List.randomChoicesBy randomizer count source

Full Usage: List.randomChoicesBy randomizer count source

Parameters:

randomizer : unit -> float - The randomizer function, must return a float number from [0.0..1.0) range.

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a list of random elements from the given list using the specified randomizer function.

randomizer : unit -> float: The randomizer function, must return a float number from [0.0..1.0) range.
count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentException	Thrown when the input list is empty.
ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.
ArgumentOutOfRangeException	Thrown when the randomizer function returns a value outside the range [0, 1).

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomChoicesBy Random.Shared.NextDouble 3

val inputs: int list

Can evaluate to [ 3; 1; 3 ].


                
                  
                    
                      List.randomChoicesWith random count source

Full Usage: List.randomChoicesWith random count source

Parameters:

random : Random - The Random instance.

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a list of random elements from the given list with the specified Random instance, each element can be selected multiple times.

random : Random: The Random instance.
count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentNullException	Thrown when the random argument is null.
ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> Array.randomChoicesWith Random.Shared 3

val inputs: int list

module Array from Microsoft.FSharp.Collections

Can evaluate to [ 3; 1; 3 ].


                
                  
                    
                      List.randomSample count source

Full Usage: List.randomSample count source

Parameters:

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a random sample of elements from the given list, each element can be selected only once.

count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.
ArgumentException	Thrown when count is greater than the length of the input list.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomSample 3

val inputs: int list

Can evaluate to [ 3; 1; 2 ].


                
                  
                    
                      List.randomSampleBy randomizer count source

Full Usage: List.randomSampleBy randomizer count source

Parameters:

randomizer : unit -> float - The randomizer function, must return a float number from [0.0..1.0) range.

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a random sample of elements from the given list using the specified randomizer function, each element can be selected only once.

randomizer : unit -> float: The randomizer function, must return a float number from [0.0..1.0) range.
count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.
ArgumentException	Thrown when count is greater than the length of the input list.
ArgumentOutOfRangeException	Thrown when the randomizer function returns a value outside the range [0, 1).

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomSampleBy Random.Shared.NextDouble 3

val inputs: int list

Can evaluate to [ 3; 1; 2 ].


                
                  
                    
                      List.randomSampleWith random count source

Full Usage: List.randomSampleWith random count source

Parameters:

random : Random - The Random instance.

count : int - The number of elements to return.

source : 'T list - The input list.

Returns: 'T list A list of randomly selected elements from the input list.

Returns a random sample of elements from the given list with the specified Random instance, each element can be selected only once.

random : Random: The Random instance.
count : int: The number of elements to return.
source : 'T list: The input list.

Returns: 'T list: A list of randomly selected elements from the input list.

ArgumentNullException	Thrown when the random argument is null.
ArgumentException	Thrown when count is more than 0 and the input list is empty.
ArgumentException	Thrown when count is less than 0.
ArgumentException	Thrown when count is greater than the length of the input list.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomSampleWith Random.Shared 3

val inputs: int list

Can evaluate to [ 3; 1; 2 ].


                
                  
                    
                      List.randomShuffle source

Full Usage: List.randomShuffle source

Parameters:

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list shuffled in a random order.

source : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomShuffle
 Can evaluate to [ 0; 2; 4; 3; 1 ].

val inputs: int list


                
                  
                    
                      List.randomShuffleBy randomizer source

Full Usage: List.randomShuffleBy randomizer source

Parameters:

randomizer : unit -> float - The randomizer function, must return a float number from [0.0..1.0) range.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list shuffled in a random order using the specified randomizer function.

randomizer : unit -> float: The randomizer function, must return a float number from [0.0..1.0) range.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentOutOfRangeException

Thrown when the randomizer function returns a value outside the range [0, 1).

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomShuffleBy Random.Shared.NextDouble

val inputs: int list

Can evaluate to [ 0; 2; 4; 3; 1 ].


                
                  
                    
                      List.randomShuffleWith random source

Full Usage: List.randomShuffleWith random source

Parameters:

random : Random - The Random instance.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list shuffled in a random order with the specified Random instance.

random : Random: The Random instance.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentNullException

Thrown when the random argument is null.

Example

 let inputs = [ 0; 1; 2; 3; 4 ]

 inputs |> List.randomShuffleWith Random.Shared

val inputs: int list

Can evaluate to [ 0; 2; 4; 3; 1 ].


                
                  
                    
                      List.reduce reduction list

Full Usage: List.reduce reduction list

Parameters:

reduction : 'T -> 'T -> 'T - The function to reduce two list elements to a single element.

list : 'T list - The input list.

Returns: 'T The final reduced value.

Apply a function to each element of the collection, threading an accumulator argument through the computation. Apply the function to the first two elements of the list. Then feed this result into the function along with the third element and so on. Return the final result. If the input function is f and the elements are i0...iN then computes f (... (f i0 i1) i2 ...) iN.

Raises ArgumentException if list is empty

reduction : 'T -> 'T -> 'T: The function to reduce two list elements to a single element.
list : 'T list: The input list.

Returns: 'T: The final reduced value.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = [1; 3; 4; 2]

 inputs |> List.reduce (fun a b -> a * 10 + b)

val inputs: int list

val reduce: reduction: ('T -> 'T -> 'T) -> list: 'T list -> 'T

val a: int

val b: int

Evaluates to 1342, by computing ((1 * 10 + 3) * 10 + 4) * 10 + 2


                
                  
                    
                      List.reduceBack reduction list

Full Usage: List.reduceBack reduction list

Parameters:

reduction : 'T -> 'T -> 'T - A function that takes in the next-to-last element of the list and the current accumulated result to produce the next accumulated result.

list : 'T list - The input list.

Returns: 'T The final result of the reductions.

reduction : 'T -> 'T -> 'T: A function that takes in the next-to-last element of the list and the current accumulated result to produce the next accumulated result.
list : 'T list: The input list.

Returns: 'T: The final result of the reductions.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = [1; 3; 4; 2]

 inputs |> List.reduceBack (fun a b -> a + b * 10)

val inputs: int list

val reduceBack: reduction: ('T -> 'T -> 'T) -> list: 'T list -> 'T

val a: int

val b: int

Evaluates to 2431, by computing 1 + (3 + (4 + 2 * 10) * 10) * 10


                
                  
                    
                      List.removeAt index source

Full Usage: List.removeAt index source

Parameters:

index : int - The index of the item to be removed.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list with the item at a given index removed.

index : int: The index of the item to be removed.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException

Thrown when index is outside 0..source.Length - 1

Example

 let inputs = [ 0; 1; 2 ]

 inputs |> List.removeAt 1

val inputs: int list

val removeAt: index: int -> source: 'T list -> 'T list

let inputs = [ 0; 2 ]


                
                  
                    
                      List.removeManyAt index count source

Full Usage: List.removeManyAt index count source

Parameters:

index : int - The index of the item to be removed.

count : int - The number of items to remove.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list with the number of items starting at a given index removed.

index : int: The index of the item to be removed.
count : int: The number of items to remove.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException

Thrown when index is outside 0..source.Length - count

Example

 let inputs = [ 0; 1; 2; 3 ]

 inputs |> List.removeManyAt 1 2

val inputs: int list

val removeManyAt: index: int -> count: int -> source: 'T list -> 'T list

Evaluates to [ 0; 3 ].


                
                  
                    
                      List.replicate count initial

Full Usage: List.replicate count initial

Parameters:

count : int - The number of elements to replicate.

initial : 'T - The value to replicate

Returns: 'T list The generated list.

Creates a list by replicating the given initial value.

count : int: The number of elements to replicate.
initial : 'T: The value to replicate

Returns: 'T list: The generated list.

Example

 List.replicate 3 "a"

val replicate: count: int -> initial: 'T -> 'T list

Evaluates to [ "a"; "a"; "a" ].


                
                  
                    
                      List.rev list

Full Usage: List.rev list

Parameters:

list : 'T list - The input list.

Returns: 'T list The reversed list.

Returns a new list with the elements in reverse order.

list : 'T list: The input list.

Returns: 'T list: The reversed list.

Example

 let inputs = [ 0; 1; 2 ]

 inputs |> List.rev

val inputs: int list

val rev: list: 'T list -> 'T list

Evaluates to [ 2; 1; 0 ].


                
                  
                    
                      List.scan folder state list

Full Usage: List.scan folder state list

Parameters:

folder : 'State -> 'T -> 'State - The function to update the state given the input elements.

state : 'State - The initial state.

list : 'T list - The input list.

Returns: 'State list The list of states.

Applies a function to each element of the collection, threading an accumulator argument through the computation. Take the second argument, and apply the function to it and the first element of the list. Then feed this result into the function along with the second element and so on. Returns the list of intermediate results and the final result.

folder : 'State -> 'T -> 'State: The function to update the state given the input elements.
state : 'State: The initial state.
list : 'T list: The input list.

Returns: 'State list: The list of states.

Example

Apply a list charges and collect the running balances as each is applied:

 type Charge =
     | In of int
     | Out of int

 let inputs = [In 1; Out 2; In 3]

 (0, inputs) ||> List.scan (fun acc charge ->
     match charge with
     | In i -> acc + i
     | Out o -> acc - o)

type Charge = | In of int | Out of int

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val inputs: Charge list

union case Charge.In: int -> Charge

union case Charge.Out: int -> Charge

val scan<'T,'State> : folder: ('State -> 'T -> 'State) -> state: 'State -> list: 'T list -> 'State list

val acc: int

val charge: Charge

val i: int

val o: int

Evaluates to [0; 1; -1; 2]. Note 0 is the initial state, 1 the next state, -1 the next state, and 2 the final state. Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.scanBack folder list state

Full Usage: List.scanBack folder list state

Parameters:

folder : 'T -> 'State -> 'State - The function to update the state given the input elements.

list : 'T list - The input list.

state : 'State - The initial state.

Returns: 'State list The list of states.

Like foldBack, but returns both the intermediary and final results

folder : 'T -> 'State -> 'State: The function to update the state given the input elements.
list : 'T list: The input list.
state : 'State: The initial state.

Returns: 'State list: The list of states.

Example

Apply a list charges from back to front, and collect the running balances as each is applied:

 type Charge =
     | In of int
     | Out of int

 let inputs = [ In 1; Out 2; In 3 ]

 (inputs, 0) ||> List.scanBack (fun charge acc ->
     match charge with
     | In i -> acc + i
     | Out o -> acc - o)

type Charge = | In of int | Out of int

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val inputs: Charge list

union case Charge.In: int -> Charge

union case Charge.Out: int -> Charge

val scanBack: folder: ('T -> 'State -> 'State) -> list: 'T list -> state: 'State -> 'State list

val charge: Charge

val acc: int

val i: int

val o: int

Evaluates to [2; 1; 3; 0] by processing each input from back to front. Note 0 is the initial state, 3 the next state, 1 the next state, and 2 the final state, and the states are produced from back to front. Note acc is a commonly used abbreviation for "accumulator".


                
                  
                    
                      List.singleton value

Full Usage: List.singleton value

Parameters:

value : 'T - The input item.

Returns: 'T list The result list of one item.
Modifiers: inline
Type parameters: 'T

Returns a list that contains one item only.

value : 'T: The input item.

Returns: 'T list: The result list of one item.

Example

 List.singleton 7

val singleton: value: 'T -> 'T list

Evaluates to [ 7 ].


                
                  
                    
                      List.skip count list

Full Usage: List.skip count list

Parameters:

count : int - The number of elements to skip. If the number is 0 or negative the input list is returned.

list : 'T list - The input list.

Returns: 'T list The list after removing the first N elements.

Returns the list after removing the first N elements.

count : int: The number of elements to skip. If the number is 0 or negative the input list is returned.
list : 'T list: The input list.

Returns: 'T list: The list after removing the first N elements.

ArgumentException

Thrown when count exceeds the number of elements in the list.

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.skip 2

val inputs: string list

val skip: count: int -> list: 'T list -> 'T list

Evaluates to ["c"; "d"]

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.skip 5

val inputs: string list

val skip: count: int -> list: 'T list -> 'T list

Throws ArgumentException.

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.skip -1

val inputs: string list

val skip: count: int -> list: 'T list -> 'T list

Evaluates to ["a"; "b"; "c"; "d"].


                
                  
                    
                      List.skipWhile predicate list

Full Usage: List.skipWhile predicate list

Parameters:

predicate : 'T -> bool - A function that evaluates an element of the list to a boolean value.

list : 'T list - The input list.

Returns: 'T list The result list.

Bypasses elements in a list while the given predicate returns True, and then returns the remaining elements of the list.

predicate : 'T -> bool: A function that evaluates an element of the list to a boolean value.
list : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let inputs = ["a"; "bbb"; "cc"; "d"]

 inputs |> List.skipWhile (fun x -> x.Length < 3)

val inputs: string list

val skipWhile: predicate: ('T -> bool) -> list: 'T list -> 'T list

val x: string

property System.String.Length: int with get

Evaluates to ["bbb"; "cc"; "d"]


                
                  
                    
                      List.sort list

Full Usage: List.sort list

Parameters:

list : 'T list - The input list.

Returns: 'T list The sorted list.

Sorts the given list using Operators.compare.

This is a stable sort, i.e. the original order of equal elements is preserved.

list : 'T list: The input list.

Returns: 'T list: The sorted list.

Example

 let input = [8; 4; 3; 1; 6; 1]

 List.sort input

val input: int list

val sort: list: 'T list -> 'T list (requires comparison)

Evaluates to [1; 1 3; 4; 6; 8].


                
                  
                    
                      List.sortBy projection list

Full Usage: List.sortBy projection list

Parameters:

projection : 'T -> 'Key - The function to transform the list elements into the type to be compared.

list : 'T list - The input list.

Returns: 'T list The sorted list.

Sorts the given list using keys given by the given projection. Keys are compared using Operators.compare.

This is a stable sort, i.e. the original order of equal elements is preserved.

projection : 'T -> 'Key: The function to transform the list elements into the type to be compared.
list : 'T list: The input list.

Returns: 'T list: The sorted list.

Example

 let input = [ "a"; "bbb"; "cccc"; "dd" ]

 input |> List.sortBy (fun s -> s.Length)

val input: string list

val sortBy: projection: ('T -> 'Key) -> list: 'T list -> 'T list (requires comparison)

val s: string

property System.String.Length: int with get

Evaluates to ["a"; "dd"; "bbb"; "cccc"].


                
                  
                    
                      List.sortByDescending projection list

Full Usage: List.sortByDescending projection list

Parameters:

projection : 'T -> 'Key - The function to transform the list elements into the type to be compared.

list : 'T list - The input list.

Returns: 'T list The sorted list.
Modifiers: inline
Type parameters: 'T, 'Key

Sorts the given list in descending order using keys given by the given projection. Keys are compared using Operators.compare.

This is a stable sort, i.e. the original order of equal elements is preserved.

projection : 'T -> 'Key: The function to transform the list elements into the type to be compared.
list : 'T list: The input list.

Returns: 'T list: The sorted list.

Example

 let input = ["a"; "bbb"; "cccc"; "dd"]

 input |> List.sortByDescending (fun s -> s.Length)

val input: string list

val sortByDescending: projection: ('T -> 'Key) -> list: 'T list -> 'T list (requires comparison)

val s: string

property System.String.Length: int with get

Evaluates to ["cccc"; "bbb"; "dd"; "a"].


                
                  
                    
                      List.sortDescending list

Full Usage: List.sortDescending list

Parameters:

list : 'T list - The input list.

Returns: 'T list The sorted list.
Modifiers: inline
Type parameters: 'T

Sorts the given list in descending order using Operators.compare.

This is a stable sort, i.e. the original order of equal elements is preserved.

list : 'T list: The input list.

Returns: 'T list: The sorted list.

Example

 let input = [8; 4; 3; 1; 6; 1]

 input |> List.sortDescending

val input: int list

val sortDescending: list: 'T list -> 'T list (requires comparison)

Evaluates to [8; 6; 4; 3; 1; 1].


                
                  
                    
                      List.sortWith comparer list

Full Usage: List.sortWith comparer list

Parameters:

comparer : 'T -> 'T -> int - The function to compare the list elements.

list : 'T list - The input list.

Returns: 'T list The sorted list.

Sorts the given list using the given comparison function.

This is a stable sort, i.e. the original order of equal elements is preserved.

comparer : 'T -> 'T -> int: The function to compare the list elements.
list : 'T list: The input list.

Returns: 'T list: The sorted list.

Example

Sort a list of pairs using a comparison function that compares string lengths then index numbers:

 let compareEntries (n1: int, s1: string) (n2: int, s2: string) =
     let c = compare s1.Length s2.Length
     if c <> 0 then c else
     compare n1 n2

 let input = [ (0,"aa"); (1,"bbb"); (2,"cc"); (3,"dd") ]

 input |> List.sortWith compareEntries

val compareEntries: n1: int * s1: string -> n2: int * s2: string -> int

val n1: int

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

val s1: string

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

val n2: int

val s2: string

val c: int

val compare: e1: 'T -> e2: 'T -> int (requires comparison)

property System.String.Length: int with get

val input: (int * string) list

val sortWith: comparer: ('T -> 'T -> int) -> list: 'T list -> 'T list

Evaluates to [(0, "aa"); (2, "cc"); (3, "dd"); (1, "bbb")].


                
                  
                    
                      List.splitAt index list

Full Usage: List.splitAt index list

Parameters:

index : int - The index at which the list is split.

list : 'T list - The input list.

Returns: 'T list * 'T list The two split lists.

Splits a list into two lists, at the given index.

index : int: The index at which the list is split.
list : 'T list: The input list.

Returns: 'T list * 'T list: The two split lists.

InvalidOperationException

Thrown when split index exceeds the number of elements in the list.

Example

 let input = [8; 4; 3; 1; 6; 1]

 let front, back = input |> List.splitAt 3

val input: int list

val front: int list

val back: int list

val splitAt: index: int -> list: 'T list -> 'T list * 'T list

Evaluates front to [8; 4; 3] and back to [1; 6; 1].


                
                  
                    
                      List.splitInto count list

Full Usage: List.splitInto count list

Parameters:

count : int - The maximum number of chunks.

list : 'T list - The input list.

Returns: 'T list list The list split into chunks.

Splits the input list into at most count chunks.

count : int: The maximum number of chunks.
list : 'T list: The input list.

Returns: 'T list list: The list split into chunks.

ArgumentException Thrown when count is not positive.

Example

 [1..10] |> List.splitInto 2

val splitInto: count: int -> list: 'T list -> 'T list list

Evaluates to [[1; 2; 3; 4; 5]; [6; 7; 8; 9; 10]].

Example

 [1..10] |> List.splitInto 4

val splitInto: count: int -> list: 'T list -> 'T list list

Evaluates to [[1; 2; 3]; [4; 5; 6]; [7; 8]; [9; 10]].


                
                  
                    
                      List.sum list

Full Usage: List.sum list

Parameters:

list : ^T list - The input list.

Returns: ^T The resulting sum.
Modifiers: inline
Type parameters: ^T

Returns the sum of the elements in the list.

list : ^T list: The input list.

Returns: ^T: The resulting sum.

Example

 let input = [ 1; 5; 3; 2 ]

 input |> List.sum

val input: int list

val sum: list: 'T list -> 'T (requires member (+) and member Zero)

Evaluates to 11.


                
                  
                    
                      List.sumBy projection list

Full Usage: List.sumBy projection list

Parameters:

projection : 'T -> ^U - The function to transform the list elements into the type to be summed.

list : 'T list - The input list.

Returns: ^U The resulting sum.
Modifiers: inline
Type parameters: 'T, ^U

Returns the sum of the results generated by applying the function to each element of the list.

projection : 'T -> ^U: The function to transform the list elements into the type to be summed.
list : 'T list: The input list.

Returns: ^U: The resulting sum.

Example

 let input = [ "aa"; "bbb"; "cc" ]

 input |> List.sumBy (fun s -> s.Length)

val input: string list

val sumBy: projection: ('T -> 'U) -> list: 'T list -> 'U (requires member (+) and member Zero)

val s: string

property System.String.Length: int with get

Evaluates to 7.


                
                  
                    
                      List.tail list

Full Usage: List.tail list

Parameters:

list : 'T list - The input list.

Returns: 'T list The list after removing the first element.

Returns the list after removing the first element.

list : 'T list: The input list.

Returns: 'T list: The list after removing the first element.

ArgumentException

Thrown when the list is empty.

Example

 let inputs = ["a"; "bb"; "ccc"]

 inputs |> List.tail

val inputs: string list

val tail: list: 'T list -> 'T list

Evaluates to ["bb"; "ccc"]


                
                  
                    
                      List.take count list

Full Usage: List.take count list

Parameters:

count : int - The number of items to take.

list : 'T list - The input list.

Returns: 'T list The result list.

Returns the first N elements of the list.

Throws InvalidOperationException if the count exceeds the number of elements in the list. List.truncate returns as many items as the list contains instead of throwing an exception.

count : int: The number of items to take.
list : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException	Thrown when the input list is empty.
InvalidOperationException	Thrown when count exceeds the number of elements in the list.

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.take 2

val inputs: string list

val take: count: int -> list: 'T list -> 'T list

Evaluates to ["a"; "b"]

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.take 6

val inputs: string list

val take: count: int -> list: 'T list -> 'T list

Throws InvalidOperationException.

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.take 0

val inputs: string list

val take: count: int -> list: 'T list -> 'T list

Evaluates to the empty list.


                
                  
                    
                      List.takeWhile predicate list

Full Usage: List.takeWhile predicate list

Parameters:

predicate : 'T -> bool - A function that evaluates to false when no more items should be returned.

list : 'T list - The input list.

Returns: 'T list The result list.

Returns a list that contains all elements of the original list while the given predicate returns True, and then returns no further elements.

predicate : 'T -> bool: A function that evaluates to false when no more items should be returned.
list : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let inputs = ["a"; "bb"; "ccc"; "d"]

 inputs |> List.takeWhile (fun x -> x.Length < 3)

val inputs: string list

val takeWhile: predicate: ('T -> bool) -> list: 'T list -> 'T list

val x: string

property System.String.Length: int with get

Evaluates to ["a"; "bb"]


                
                  
                    
                      List.toArray list

Full Usage: List.toArray list

Parameters:

list : 'T list - The input list.

Returns: 'T array The array containing the elements of the list.

Builds an array from the given list.

list : 'T list: The input list.

Returns: 'T array: The array containing the elements of the list.

Example

 let inputs = [ 1; 2; 5 ]

 inputs |> List.toArray

val inputs: int list

val toArray: list: 'T list -> 'T array

Evaluates to [| 1; 2; 5 |].


                
                  
                    
                      List.toSeq list

Full Usage: List.toSeq list

Parameters:

list : 'T list - The input list.

Returns: 'T seq The sequence of elements in the list.

Views the given list as a sequence.

list : 'T list: The input list.

Returns: 'T seq: The sequence of elements in the list.

Example

 let inputs = [ 1; 2; 5 ]

 inputs |> List.toSeq

val inputs: int list

val toSeq: list: 'T list -> 'T seq

Evaluates to seq { 1; 2; 5 }.


                
                  
                    
                      List.transpose lists

Full Usage: List.transpose lists

Parameters:

lists : 'T list seq - The input sequence of list.

Returns: 'T list list The transposed list.

Returns the transpose of the given sequence of lists.

lists : 'T list seq: The input sequence of list.

Returns: 'T list list: The transposed list.

ArgumentNullException	Thrown when the input sequence is null.
ArgumentException	Thrown when the input lists differ in length.

Example

 let inputs =
     [ [ 10; 20; 30 ]
       [ 11; 21; 31 ] ]

 inputs |> List.transpose

val inputs: int list list

val transpose: lists: 'T list seq -> 'T list list

Evaluates to [ [10; 11]; [20; 21]; [30; 31] ].


                
                  
                    
                      List.truncate count list

Full Usage: List.truncate count list

Parameters:

count : int - The maximum number of items to return.

list : 'T list - The input list.

Returns: 'T list The result list.

Returns at most N elements in a new list.

count : int: The maximum number of items to return.
list : 'T list: The input list.

Returns: 'T list: The result list.

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.truncate 2

val inputs: string list

val truncate: count: int -> list: 'T list -> 'T list

Evaluates to ["a"; "b"]

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.truncate 6

val inputs: string list

val truncate: count: int -> list: 'T list -> 'T list

Evaluates to ["a"; "b"; "c"; "d"]

Example

 let inputs = ["a"; "b"; "c"; "d"]

 inputs |> List.truncate 0

val inputs: string list

val truncate: count: int -> list: 'T list -> 'T list

Evaluates to the empty list.


                
                  
                    
                      List.tryExactlyOne list

Full Usage: List.tryExactlyOne list

Parameters:

list : 'T list - The input list.

Returns: 'T option The only element of the list or None.

Returns the only element of the list or None if it is empty or contains more than one element.

list : 'T list: The input list.

Returns: 'T option: The only element of the list or None.

Example

 [1] |> List.tryExactlyOne               // evaluates Some 1
 [1;2] |> List.tryExactlyOne             // evaluates None
 ([] : int list) |> List.tryExactlyOne   // evaluates None

val tryExactlyOne: list: 'T list -> 'T option

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

type 'T list = List<'T>


                
                  
                    
                      List.tryFind predicate list

Full Usage: List.tryFind predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T option The first element for which the predicate returns true, or None if every element evaluates to false.

Returns the first element for which the given function returns True. Return None if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T option: The first element for which the predicate returns true, or None if every element evaluates to false.

Example

Try to find the first even number:

 let inputs = [1; 2; 3]

 inputs |> List.tryFind (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFind: predicate: ('T -> bool) -> list: 'T list -> 'T option

val elm: int

Evaluates to Some 2

Example

Try to find the first even number:

 let inputs = [1; 5; 3]

 inputs |> List.tryFind (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFind: predicate: ('T -> bool) -> list: 'T list -> 'T option

val elm: int

Evaluates to None


                
                  
                    
                      List.tryFindBack predicate list

Full Usage: List.tryFindBack predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T option The last element for which the predicate returns true, or None if every element evaluates to false.

Returns the last element for which the given function returns True. Return None if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T option: The last element for which the predicate returns true, or None if every element evaluates to false.

Example

Try to find the first even number from the back:

 let inputs = [1; 2; 3; 4; 5]

 inputs |> List.tryFindBack (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindBack: predicate: ('T -> bool) -> list: 'T list -> 'T option

val elm: int

Evaluates to Some 4

Example

Try to find the first even number from the back:

 let inputs = [1; 5; 3]

 inputs |> List.tryFindBack (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindBack: predicate: ('T -> bool) -> list: 'T list -> 'T option

val elm: int

Evaluates to None


                
                  
                    
                      List.tryFindIndex predicate list

Full Usage: List.tryFindIndex predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: int option The index of the first element for which the predicate returns true, or None if every element evaluates to false.

Returns the index of the first element in the list that satisfies the given predicate. Return None if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: int option: The index of the first element for which the predicate returns true, or None if every element evaluates to false.

Example

Try to find the index of the first even number:

 let inputs = [1; 2; 3; 4; 5]

 inputs |> List.tryFindIndex (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindIndex: predicate: ('T -> bool) -> list: 'T list -> int option

val elm: int

Evaluates to Some 1

Example

Try to find the index of the first even number:

 let inputs = [1; 3; 5; 7]

 inputs |> List.tryFindIndex (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindIndex: predicate: ('T -> bool) -> list: 'T list -> int option

val elm: int

Evaluates to None


                
                  
                    
                      List.tryFindIndexBack predicate list

Full Usage: List.tryFindIndexBack predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: int option The index of the last element for which the predicate returns true, or None if every element evaluates to false.

Returns the index of the last element in the list that satisfies the given predicate. Return None if no such element exists.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: int option: The index of the last element for which the predicate returns true, or None if every element evaluates to false.

Example

Try to find the index of the first even number from the back:

 let inputs = [1; 2; 3; 4; 5]

 inputs |> List.tryFindIndexBack (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindIndexBack: predicate: ('T -> bool) -> list: 'T list -> int option

val elm: int

Evaluates to Some 3

Example

Try to find the index of the first even number from the back:

 let inputs = [1; 3; 5; 7]

 inputs |> List.tryFindIndexBack (fun elm -> elm % 2 = 0)

val inputs: int list

val tryFindIndexBack: predicate: ('T -> bool) -> list: 'T list -> int option

val elm: int

Evaluates to None


                
                  
                    
                      List.tryHead list

Full Usage: List.tryHead list

Parameters:

list : 'T list - The input list.

Returns: 'T option The first element of the list or None.

Returns the first element of the list, or None if the list is empty.

list : 'T list: The input list.

Returns: 'T option: The first element of the list or None.

Example

 let inputs = [ "banana"; "pear" ]

 inputs |> List.tryHead

val inputs: string list

val tryHead: list: 'T list -> 'T option

Evaluates to Some "banana"

Example

 let inputs : int list = []

 inputs |> List.tryHead

val inputs: int list

Multiple items
val int: value: 'T -> int (requires member op_Explicit)

--------------------
type int = int32

--------------------
type int<'Measure> = int

type 'T list = List<'T>

val tryHead: list: 'T list -> 'T option

Evaluates to None


                
                  
                    
                      List.tryItem index list

Full Usage: List.tryItem index list

Parameters:

index : int - The index to retrieve.

list : 'T list - The input list.

Returns: 'T option The value at the given index or None.

Tries to find the nth element in the list. Returns None if index is negative or the list does not contain enough elements.

index : int: The index to retrieve.
list : 'T list: The input list.

Returns: 'T option: The value at the given index or None.

Example

 let inputs = ["a"; "b"; "c"]

 inputs |> List.tryItem 1

val inputs: string list

val tryItem: index: int -> list: 'T list -> 'T option

Evaluates to Some "b".

Example

 let inputs = ["a"; "b"; "c"]

 inputs |> List.tryItem 4

val inputs: string list

val tryItem: index: int -> list: 'T list -> 'T option

Evaluates to None.


                
                  
                    
                      List.tryLast list

Full Usage: List.tryLast list

Parameters:

list : 'T list - The input list.

Returns: 'T option The last element of the list or None.

Returns the last element of the list. Return None if no such element exists.

list : 'T list: The input list.

Returns: 'T option: The last element of the list or None.

Example

 [ "pear"; "banana" ] |> List.tryLast

val tryLast: list: 'T list -> 'T option

Evaluates to Some "banana"

Example

 [ ] |> List.tryLast

val tryLast: list: 'T list -> 'T option

Evaluates to None


                
                  
                    
                      List.tryPick chooser list

Full Usage: List.tryPick chooser list

Parameters:

chooser : 'T -> 'U option - The function to generate options from the elements.

list : 'T list - The input list.

Returns: 'U option The first resulting value or None.

Applies the given function to successive elements, returning Some(x) the first result where function returns Some(x) for some x. If no such element exists then return None.

chooser : 'T -> 'U option: The function to generate options from the elements.
list : 'T list: The input list.

Returns: 'U option: The first resulting value or None.

Example

 let input = [1; 2; 3]

 input |> List.tryPick (fun n -> if n % 2 = 0 then Some (string n) else None)

val input: int list

val tryPick: chooser: ('T -> 'U option) -> list: 'T list -> 'U option

val n: int

union case Option.Some: Value: 'T -> Option<'T>

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

union case Option.None: Option<'T>

Evaluates to Some "2".

Example

 let input = [1; 2; 3]

 input |> List.tryPick (fun n -> if n > 3 then Some (string n) else None)

val input: int list

val tryPick: chooser: ('T -> 'U option) -> list: 'T list -> 'U option

val n: int

union case Option.Some: Value: 'T -> Option<'T>

Multiple items
val string: value: 'T -> string

--------------------
type string = System.String

union case Option.None: Option<'T>

Evaluates to None.


                
                  
                    
                      List.unfold generator state

Full Usage: List.unfold generator state

Parameters:

generator : 'State -> ('T * 'State) option - A function that takes in the current state and returns an option tuple of the next element of the list and the next state value.

state : 'State - The initial state value.

Returns: 'T list The result list.

Returns a list that contains the elements generated by the given computation. The generator is repeatedly called to build the list until it returns None. The given initial state argument is passed to the element generator.

generator : 'State -> ('T * 'State) option: A function that takes in the current state and returns an option tuple of the next element of the list and the next state value.
state : 'State: The initial state value.

Returns: 'T list: The result list.

Example

 1 |> List.unfold (fun state -> if state > 100 then None else Some (state, state * 2))

val unfold<'T,'State> : generator: ('State -> ('T * 'State) option) -> state: 'State -> 'T list

val state: int

union case Option.None: Option<'T>

union case Option.Some: Value: 'T -> Option<'T>

Evaluates to [1; 2; 4; 8; 16; 32; 64]


                
                  
                    
                      List.unzip list

Full Usage: List.unzip list

Parameters:

list : ('T1 * 'T2) list - The input list.

Returns: 'T1 list * 'T2 list Two lists of split elements.

Splits a list of pairs into two lists.

list : ('T1 * 'T2) list: The input list.

Returns: 'T1 list * 'T2 list: Two lists of split elements.

Example

 let inputs = [(1, "one"); (2, "two")]

 let numbers, names = inputs |> List.unzip

val inputs: (int * string) list

val numbers: int list

val names: string list

val unzip: list: ('T1 * 'T2) list -> 'T1 list * 'T2 list

Evaluates numbers to [1; 2] and names to ["one"; "two"].


                
                  
                    
                      List.unzip3 list

Full Usage: List.unzip3 list

Parameters:

list : ('T1 * 'T2 * 'T3) list - The input list.

Returns: 'T1 list * 'T2 list * 'T3 list Three lists of split elements.

Splits a list of triples into three lists.

list : ('T1 * 'T2 * 'T3) list: The input list.

Returns: 'T1 list * 'T2 list * 'T3 list: Three lists of split elements.

Example

 let inputs = [(1, "one", "I"); (2, "two", "II")]

 let numbers, names, roman = inputs |> List.unzip3

val inputs: (int * string * string) list

val numbers: int list

val names: string list

val roman: string list

val unzip3: list: ('T1 * 'T2 * 'T3) list -> 'T1 list * 'T2 list * 'T3 list

Evaluates numbers to [1; 2], names to ["one"; "two"] and roman to ["I"; "II"].


                
                  
                    
                      List.updateAt index value source

Full Usage: List.updateAt index value source

Parameters:

index : int - The index of the item to be replaced.

value : 'T - The new value.

source : 'T list - The input list.

Returns: 'T list The result list.

Return a new list with the item at a given index set to the new value.

index : int: The index of the item to be replaced.
value : 'T: The new value.
source : 'T list: The input list.

Returns: 'T list: The result list.

ArgumentException

Thrown when index is outside 0..source.Length - 1

Example

 let inputs = [ 0; 1; 2 ]

 inputs |> List.updateAt 1 9

val inputs: int list

val updateAt: index: int -> value: 'T -> source: 'T list -> 'T list

Evaluates to [ 0; 9; 2 ].


                
                  
                    
                      List.where predicate list

Full Usage: List.where predicate list

Parameters:

predicate : 'T -> bool - The function to test the input elements.

list : 'T list - The input list.

Returns: 'T list A list containing only the elements that satisfy the predicate.

Returns a new list containing only the elements of the list for which the given predicate returns "true"

This is identical to List.filter.

predicate : 'T -> bool: The function to test the input elements.
list : 'T list: The input list.

Returns: 'T list: A list containing only the elements that satisfy the predicate.

Example

Select only the even numbers:

 let inputs = [1; 2; 3; 4]

 inputs |> List.where (fun elm -> elm % 2 = 0)

val inputs: int list

val where: predicate: ('T -> bool) -> list: 'T list -> 'T list

val elm: int

Evaluates to [2; 4]


                
                  
                    
                      List.windowed windowSize list

Full Usage: List.windowed windowSize list

Parameters:

windowSize : int - The number of elements in each window.

list : 'T list - The input list.

Returns: 'T list list The result list.

Returns a list of sliding windows containing elements drawn from the input list. Each window is returned as a fresh list.

windowSize : int: The number of elements in each window.
list : 'T list: The input list.

Returns: 'T list list: The result list.

ArgumentException

Thrown when windowSize is not positive.

Example

 let inputs = [1; 2; 3; 4; 5]

 inputs |> List.windowed 3

val inputs: int list

val windowed: windowSize: int -> list: 'T list -> 'T list list

Evaluates to [[1; 2; 3]; [2; 3; 4]; [3; 4; 5]]


                
                  
                    
                      List.zip list1 list2

Full Usage: List.zip list1 list2

Parameters:

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

Returns: ('T1 * 'T2) list A single list containing pairs of matching elements from the input lists.

Combines the two lists into a list of pairs. The two lists must have equal lengths.

list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.

Returns: ('T1 * 'T2) list: A single list containing pairs of matching elements from the input lists.

Example

 let numbers = [1; 2]
 let names = ["one"; "two"]

 List.zip numbers names

val numbers: int list

val names: string list

val zip: list1: 'T1 list -> list2: 'T2 list -> ('T1 * 'T2) list

Evaluates to [(1, "one"); (2, "two")].


                
                  
                    
                      List.zip3 list1 list2 list3

Full Usage: List.zip3 list1 list2 list3

Parameters:

list1 : 'T1 list - The first input list.

list2 : 'T2 list - The second input list.

list3 : 'T3 list - The third input list.

Returns: ('T1 * 'T2 * 'T3) list A single list containing triples of matching elements from the input lists.

Combines the three lists into a list of triples. The lists must have equal lengths.

list1 : 'T1 list: The first input list.
list2 : 'T2 list: The second input list.
list3 : 'T3 list: The third input list.

Returns: ('T1 * 'T2 * 'T3) list: A single list containing triples of matching elements from the input lists.

Example

 let numbers = [1; 2]
 let names = ["one"; "two"]
 let roman = ["I"; "II"]

 List.zip3 numbers names roman

val numbers: int list

val names: string list

val roman: string list

val zip3: list1: 'T1 list -> list2: 'T2 list -> list3: 'T3 list -> ('T1 * 'T2 * 'T3) list

Evaluates to [(1, "one", "I"); (2, "two", "II")].