F# Compiler Guide

Interactive Service: Embedding F# Interactive

This tutorial demonstrates how to embed F# interactive in your application. F# interactive is an interactive scripting environment that compiles F# code into highly efficient IL code and executes it on the fly. The F# interactive service allows you to embed F# evaluation in your application.

NOTE: There is a number of options for embedding F# Interactive. The easiest one is to use the fsi.exe process and communicate with it using standard input and standard output. In this tutorial, we look at calling F# Interactive directly through .NET API. However, if you have no control over the input, it is a good idea to run F# interactive in a separate process. One reason is that there is no way to handle StackOverflowException and so a poorly written script can terminate the host process. Remember that while calling F# Interactive through .NET API, --shadowcopyreferences option will be ignored. For detailed discussion, please take a look at this thread. NOTE: If FsiEvaluationSession.Create fails with an error saying that FSharp.Core.dll cannot be found, add the FSharp.Core.sigdata and FSharp.Core.optdata files. More info here.

However, the F# interactive service is still useful, because you might want to wrap it in your own executable that is then executed (and communicates with the rest of your application), or if you only need to execute a limited subset of F# code (e.g. generated by your own DSL).

Starting the F# interactive

First, we need to reference the libraries that contain the F# interactive service:

#r "FSharp.Compiler.Service.dll"

open FSharp.Compiler.Interactive.Shell
open FSharp.Compiler.Tokenization

To communicate with F# interactive, we need to create streams that represent input and output. We will use those later to read the output printed as a result of evaluating some F# code that prints:

open System
open System.IO
open System.Text

// Initialize output and input streams
let sbOut = new StringBuilder()
let sbErr = new StringBuilder()
let inStream = new StringReader("")
let outStream = new StringWriter(sbOut)
let errStream = new StringWriter(sbErr)

// Build command line arguments & start FSI session
let argv = [| "C:\\fsi.exe" |]

let allArgs =
    Array.append argv [| "--noninteractive" |]

let fsiConfig =

let fsiSession =
    FsiEvaluationSession.Create(fsiConfig, allArgs, inStream, outStream, errStream)

Evaluating and executing code

The F# interactive service exposes several methods that can be used for evaluation. The first is EvalExpression which evaluates an expression and returns its result. The result contains the returned value (as obj) and the statically inferred type of the value:

/// Evaluate expression & return the result
let evalExpression text =
    match fsiSession.EvalExpression(text) with
    | Some value -> printfn "%A" value.ReflectionValue
    | None -> printfn "Got no result!"

This takes a string as an argument and evaluates (i.e. executes) it as F# code.

evalExpression "42+1" // prints '43'

This can be used in a strongly typed way as follows:

/// Evaluate expression & return the result, strongly typed
let evalExpressionTyped<'T> (text) =
    match fsiSession.EvalExpression(text) with
    | Some value -> value.ReflectionValue |> unbox<'T>
    | None -> failwith "Got no result!"

evalExpressionTyped<int> "42+1" // gives '43'

The EvalInteraction method can be used to evaluate side-effectful operations such as printing, declarations, or other interactions that are not valid F# expressions, but can be entered in the F# Interactive console. Such commands include #time "on" (and other directives), open System all declarations and other top-level statements. The code does not require ;; at the end. Just enter the code that you want to execute:

fsiSession.EvalInteraction "printfn \"bye\""

The EvalScript method allows to evaluate a complete .fsx script.

File.WriteAllText("sample.fsx", "let twenty = 10 + 10")
fsiSession.EvalScript "sample.fsx"

Catching errors

EvalExpression, EvalInteraction and EvalScript are awkward if the code has type checking warnings or errors, or if evaluation fails with an exception. In these cases you can use EvalExpressionNonThrowing, EvalInteractionNonThrowing and EvalScriptNonThrowing. These return a tuple of a result and an array of FSharpDiagnostic values. These represent the errors and warnings. The result part is a Choice<_,_> between an actual result and an exception.

The result part of EvalExpression and EvalExpressionNonThrowing is an optional FSharpValue. If that value is not present then it just indicates that the expression didn't have a tangible result that could be represented as a .NET object. This situation shouldn't actually occur for any normal input expressions, and only for primitives used in libraries.

File.WriteAllText("sample.fsx", "let twenty = 'a' + 10.0")

let result, warnings =
    fsiSession.EvalScriptNonThrowing "sample.fsx"

// show the result
match result with
| Choice1Of2 () -> printfn "checked and executed ok"
| Choice2Of2 exn -> printfn "execution exception: %s" exn.Message


execution exception: Operation could not be completed due to earlier error
// show the errors and warnings
for w in warnings do
    printfn "Warning %s at %d,%d" w.Message w.StartLine w.StartColumn


Warning The type 'float' does not match the type 'char' at 1,19
Warning The type 'float' does not match the type 'char' at 1,17

For expressions:

let evalExpressionTyped2<'T> text =
    let res, warnings =

    for w in warnings do
        printfn "Warning %s at %d,%d" w.Message w.StartLine w.StartColumn

    match res with
    | Choice1Of2 (Some value) -> value.ReflectionValue |> unbox<'T>
    | Choice1Of2 None -> failwith "null or no result"
    | Choice2Of2 (exn: exn) -> failwith (sprintf "exception %s" exn.Message)

evalExpressionTyped2<int> "42+1" // gives '43'

Executing in parallel

By default the code passed to EvalExpression is executed immediately. To execute in parallel, submit a computation that starts a task:

open System.Threading.Tasks

let sampleLongRunningExpr =
async {
    // The code of what you want to run
    do System.Threading.Thread.Sleep 5000
    return 10
  |> Async.StartAsTask"""

let task1 =
    evalExpressionTyped<Task<int>> (sampleLongRunningExpr)

let task2 =
    evalExpressionTyped<Task<int>> (sampleLongRunningExpr)

Both computations have now started. You can now fetch the results:

task1.Result // gives the result after completion (up to 5 seconds)
task2.Result // gives the result after completion (up to 5 seconds)

Type checking in the evaluation context

Let's assume you have a situation where you would like to typecheck code in the context of the F# Interactive scripting session. For example, you first evaluate a declaration:

fsiSession.EvalInteraction "let xxx = 1 + 1"

Now you want to typecheck the partially complete code xxx + xx

let parseResults, checkResults, checkProjectResults =
    fsiSession.ParseAndCheckInteraction("xxx + xx")

The parseResults and checkResults have types ParseFileResults and CheckFileResults explained in Editor. You can, for example, look at the type errors in the code:

checkResults.Diagnostics.Length // 1

The code is checked with respect to the logical type context available in the F# interactive session based on the declarations executed so far.

You can also request declaration list information, tooltip text and symbol resolution:

// get a tooltip
checkResults.GetToolTip(1, 2, "xxx + xx", [ "xxx" ], FSharpTokenTag.IDENT)

checkResults.GetSymbolUseAtLocation(1, 2, "xxx + xx", [ "xxx" ]) // symbol xxx

The 'fsi' object

If you want your scripting code to be able to access the 'fsi' object, you should pass in an implementation of this object explicitly. Normally the one from FSharp.Compiler.Interactive.Settings.dll is used.

let fsiConfig2 =

Collectible code generation

Evaluating code in using FsiEvaluationSession generates a .NET dynamic assembly and uses other resources. You can make generated code collectible by passing collectible=true. However, code will only be collected if there are no outstanding object references involving types, for example FsiValue objects returned by EvalExpression, and you must have disposed the FsiEvaluationSession. See also Restrictions on Collectible Assemblies.

The example below shows the creation of 200 evaluation sessions. Note that collectible=true and use session = ... are both used.

If collectible code is working correctly, overall resource usage will not increase linearly as the evaluation progresses.

let collectionTest () =

    for i in 1 .. 200 do
        let defaultArgs =
            [| "fsi.exe"
               "--gui-" |]

        use inStream = new StringReader("")
        use outStream = new StringWriter()
        use errStream = new StringWriter()

        let fsiConfig =

        use session =
            FsiEvaluationSession.Create(fsiConfig, defaultArgs, inStream, outStream, errStream, collectible = true)

        session.EvalInteraction(sprintf "type D = { v : int }")

        let v =
            session.EvalExpression(sprintf "{ v = 42 * %d }" i)

        printfn "iteration %d, result = %A" i v.Value.ReflectionValue

// collectionTest()  <-- run the test like this
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namespace FSharp

namespace Microsoft.FSharp
namespace FSharp.Compiler
namespace FSharp.Compiler.Interactive
module Shell from FSharp.Compiler.Interactive
namespace FSharp.Compiler.Tokenization
namespace System
namespace System.IO
namespace System.Text
val sbOut : StringBuilder
Multiple items
type StringBuilder = interface ISerializable new : unit -> unit + 5 overloads member Append : value: bool -> StringBuilder + 23 overloads member AppendFormat : provider: IFormatProvider * format: string * arg0: obj -> StringBuilder + 7 overloads member AppendJoin : separator: char * [<ParamArray>] values: obj [] -> StringBuilder + 5 overloads member AppendLine : unit -> StringBuilder + 1 overload member Clear : unit -> StringBuilder member CopyTo : sourceIndex: int * destination: char [] * destinationIndex: int * count: int -> unit + 1 overload member EnsureCapacity : capacity: int -> int member Equals : span: ReadOnlySpan<char> -> bool + 1 overload ...
<summary>Represents a mutable string of characters. This class cannot be inherited.</summary>

StringBuilder() : StringBuilder
StringBuilder(capacity: int) : StringBuilder
StringBuilder(value: string) : StringBuilder
StringBuilder(capacity: int, maxCapacity: int) : StringBuilder
StringBuilder(value: string, capacity: int) : StringBuilder
StringBuilder(value: string, startIndex: int, length: int, capacity: int) : StringBuilder
val sbErr : StringBuilder
val inStream : StringReader
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type StringReader = inherit TextReader new : s: string -> unit member Close : unit -> unit member Dispose : disposing: bool -> unit member Peek : unit -> int member Read : unit -> int + 2 overloads member ReadAsync : buffer: char [] * index: int * count: int -> Task<int> + 1 overload member ReadBlock : buffer: Span<char> -> int member ReadBlockAsync : buffer: char [] * index: int * count: int -> Task<int> + 1 overload member ReadLine : unit -> string ...
<summary>Implements a <see cref="T:System.IO.TextReader" /> that reads from a string.</summary>

StringReader(s: string) : StringReader
val outStream : StringWriter
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type StringWriter = inherit TextWriter new : unit -> unit + 3 overloads member Close : unit -> unit member Dispose : disposing: bool -> unit member FlushAsync : unit -> Task member GetStringBuilder : unit -> StringBuilder member ToString : unit -> string member Write : value: char -> unit + 4 overloads member WriteAsync : value: char -> Task + 4 overloads member WriteLine : buffer: ReadOnlySpan<char> -> unit + 1 overload ...
<summary>Implements a <see cref="T:System.IO.TextWriter" /> for writing information to a string. The information is stored in an underlying <see cref="T:System.Text.StringBuilder" />.</summary>

StringWriter() : StringWriter
StringWriter(formatProvider: IFormatProvider) : StringWriter
StringWriter(sb: StringBuilder) : StringWriter
StringWriter(sb: StringBuilder, formatProvider: IFormatProvider) : StringWriter
val errStream : StringWriter
val argv : string []
val allArgs : string []
type Array = interface ICollection interface IEnumerable interface IList interface IStructuralComparable interface IStructuralEquatable interface ICloneable new : unit -> unit member Clone : unit -> obj member CopyTo : array: Array * index: int -> unit + 1 overload member GetEnumerator : unit -> IEnumerator ...
<summary>Provides methods for creating, manipulating, searching, and sorting arrays, thereby serving as the base class for all arrays in the common language runtime.</summary>
val append : array1:'T [] -> array2:'T [] -> 'T []
<summary>Builds a new array that contains the elements of the first array followed by the elements of the second array.</summary>
<param name="array1">The first input array.</param>
<param name="array2">The second input array.</param>
<returns>The resulting array.</returns>
<exception cref="T:System.ArgumentNullException">Thrown when either of the input arrays is null.</exception>
val fsiConfig : FsiEvaluationSessionHostConfig
type FsiEvaluationSession = interface IDisposable member AddBoundValue : name:string * value:obj -> unit member EvalExpression : code:string -> FsiValue option member EvalExpressionNonThrowing : code:string -> Choice<FsiValue option,exn> * FSharpDiagnostic [] member EvalInteraction : code:string * ?cancellationToken:CancellationToken -> unit member EvalInteractionNonThrowing : code:string * ?cancellationToken:CancellationToken -> Choice<FsiValue option,exn> * FSharpDiagnostic [] member EvalScript : filePath:string -> unit member EvalScriptNonThrowing : filePath:string -> Choice<unit,exn> * FSharpDiagnostic [] member FormatValue : reflectionValue:obj * reflectionType:Type -> string member GetBoundValues : unit -> FsiBoundValue list ...
<summary> Represents an F# Interactive evaluation session. </summary>
static member FsiEvaluationSession.GetDefaultConfiguration : unit -> FsiEvaluationSessionHostConfig
static member FsiEvaluationSession.GetDefaultConfiguration : fsiObj:obj -> FsiEvaluationSessionHostConfig
static member FsiEvaluationSession.GetDefaultConfiguration : fsiObj:obj * useFsiAuxLib:bool -> FsiEvaluationSessionHostConfig
val fsiSession : FsiEvaluationSession
static member FsiEvaluationSession.Create : fsiConfig:FsiEvaluationSessionHostConfig * argv:string [] * inReader:TextReader * outWriter:TextWriter * errorWriter:TextWriter * ?collectible:bool * ?legacyReferenceResolver:FSharp.Compiler.CodeAnalysis.LegacyReferenceResolver -> FsiEvaluationSession
val evalExpression : text:string -> unit
 Evaluate expression & return the result
val text : string
member FsiEvaluationSession.EvalExpression : code:string -> FsiValue option
union case Option.Some: Value: 'T -> Option<'T>
<summary>The representation of "Value of type 'T"</summary>
<param name="Value">The input value.</param>
<returns>An option representing the value.</returns>
val value : FsiValue
val printfn : format:Printf.TextWriterFormat<'T> -> 'T
<summary>Print to <c>stdout</c> using the given format, and add a newline.</summary>
<param name="format">The formatter.</param>
<returns>The formatted result.</returns>
property FsiValue.ReflectionValue: obj with get
<summary> The value, as an object </summary>
union case Option.None: Option<'T>
<summary>The representation of "No value"</summary>
val evalExpressionTyped : text:string -> 'T
 Evaluate expression & return the result, strongly typed
val unbox : value:obj -> 'T
<summary>Unbox a strongly typed value.</summary>
<param name="value">The boxed value.</param>
<returns>The unboxed result.</returns>
val failwith : message:string -> 'T
<summary>Throw a <see cref="T:System.Exception" /> exception.</summary>
<param name="message">The exception message.</param>
<returns>The result value.</returns>
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val int : value:'T -> int (requires member op_Explicit)
<summary>Converts the argument to signed 32-bit integer. This is a direct conversion for all primitive numeric types. For strings, the input is converted using <c>Int32.Parse()</c> with InvariantCulture settings. Otherwise the operation requires an appropriate static conversion method on the input type.</summary>
<param name="value">The input value.</param>
<returns>The converted int</returns>

[<Struct>] type int = int32
<summary>An abbreviation for the CLI type <see cref="T:System.Int32" />.</summary>
<category>Basic Types</category>

type int<'Measure> = int
<summary>The type of 32-bit signed integer numbers, annotated with a unit of measure. The unit of measure is erased in compiled code and when values of this type are analyzed using reflection. The type is representationally equivalent to <see cref="T:System.Int32" />.</summary>
<category>Basic Types with Units of Measure</category>
member FsiEvaluationSession.EvalInteraction : code:string * ?cancellationToken:Threading.CancellationToken -> unit
type File = static member AppendAllLines : path: string * contents: IEnumerable<string> -> unit + 1 overload static member AppendAllLinesAsync : path: string * contents: IEnumerable<string> * encoding: Encoding *?cancellationToken: CancellationToken -> Task + 1 overload static member AppendAllText : path: string * contents: string -> unit + 1 overload static member AppendAllTextAsync : path: string * contents: string * encoding: Encoding *?cancellationToken: CancellationToken -> Task + 1 overload static member AppendText : path: string -> StreamWriter static member Copy : sourceFileName: string * destFileName: string -> unit + 1 overload static member Create : path: string -> FileStream + 2 overloads static member CreateText : path: string -> StreamWriter static member Decrypt : path: string -> unit static member Delete : path: string -> unit ...
<summary>Provides static methods for the creation, copying, deletion, moving, and opening of a single file, and aids in the creation of <see cref="T:System.IO.FileStream" /> objects.</summary>
File.WriteAllText(path: string, contents: string) : unit
File.WriteAllText(path: string, contents: string, encoding: Encoding) : unit
member FsiEvaluationSession.EvalScript : filePath:string -> unit
val result : Choice<unit,exn>
val warnings : FSharp.Compiler.Diagnostics.FSharpDiagnostic []
member FsiEvaluationSession.EvalScriptNonThrowing : filePath:string -> Choice<unit,exn> * FSharp.Compiler.Diagnostics.FSharpDiagnostic []
union case Choice.Choice1Of2: 'T1 -> Choice<'T1,'T2>
<summary>Choice 1 of 2 choices</summary>
union case Choice.Choice2Of2: 'T2 -> Choice<'T1,'T2>
<summary>Choice 2 of 2 choices</summary>
Multiple items
val exn : exn

type exn = Exception
<summary>An abbreviation for the CLI type <see cref="T:System.Exception" />.</summary>
<category>Basic Types</category>
property Exception.Message: string with get
val not : value:bool -> bool
<summary>Negate a logical value. Not True equals False and not False equals True</summary>
<param name="value">The value to negate.</param>
<returns>The result of the negation.</returns>
val w : FSharp.Compiler.Diagnostics.FSharpDiagnostic
property FSharp.Compiler.Diagnostics.FSharpDiagnostic.Message: string with get
<summary> Gets the message for the diagnostic </summary>
property FSharp.Compiler.Diagnostics.FSharpDiagnostic.StartLine: int with get
<summary> Gets the start line for the diagnostic </summary>
property FSharp.Compiler.Diagnostics.FSharpDiagnostic.StartColumn: int with get
<summary> Gets the start column for the diagnostic </summary>
val evalExpressionTyped2 : text:string -> 'T
val res : Choice<FsiValue option,exn>
member FsiEvaluationSession.EvalExpressionNonThrowing : code:string -> Choice<FsiValue option,exn> * FSharp.Compiler.Diagnostics.FSharpDiagnostic []
val sprintf : format:Printf.StringFormat<'T> -> 'T
<summary>Print to a string using the given format.</summary>
<param name="format">The formatter.</param>
<returns>The formatted result.</returns>
namespace System.Threading
namespace System.Threading.Tasks
val sampleLongRunningExpr : string
val task1 : Task<int>
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type Task = interface IAsyncResult interface IDisposable new : action: Action -> unit + 7 overloads member ConfigureAwait : continueOnCapturedContext: bool -> ConfiguredTaskAwaitable member ContinueWith : continuationAction: Action<Task,obj> * state: obj -> Task + 19 overloads member Dispose : unit -> unit + 1 overload member GetAwaiter : unit -> TaskAwaiter member RunSynchronously : unit -> unit + 1 overload member Start : unit -> unit + 1 overload member Wait : unit -> unit + 4 overloads ...
<summary>Represents an asynchronous operation.</summary>

type Task<'TResult> = inherit Task new : function: Func<obj,'TResult> * state: obj -> unit + 7 overloads member ConfigureAwait : continueOnCapturedContext: bool -> ConfiguredTaskAwaitable<'TResult> member ContinueWith : continuationAction: Action<Task<'TResult>,obj> * state: obj -> Task + 19 overloads member GetAwaiter : unit -> TaskAwaiter<'TResult> member Result : 'TResult static member Factory : TaskFactory<'TResult>
<summary>Represents an asynchronous operation that can return a value.</summary>
<typeparam name="TResult">The type of the result produced by this <see cref="T:System.Threading.Tasks.Task`1" />.</typeparam>

Task(action: Action) : Task
Task(action: Action, cancellationToken: Threading.CancellationToken) : Task
Task(action: Action, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj) : Task
Task(action: Action, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj, cancellationToken: Threading.CancellationToken) : Task
Task(action: Action<obj>, state: obj, creationOptions: TaskCreationOptions) : Task
Task(action: Action<obj>, state: obj, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task

Task(function: Func<'TResult>) : Task<'TResult>
Task(function: Func<obj,'TResult>, state: obj) : Task<'TResult>
Task(function: Func<'TResult>, cancellationToken: Threading.CancellationToken) : Task<'TResult>
Task(function: Func<'TResult>, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(function: Func<obj,'TResult>, state: obj, cancellationToken: Threading.CancellationToken) : Task<'TResult>
Task(function: Func<obj,'TResult>, state: obj, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(function: Func<'TResult>, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task<'TResult>
Task(function: Func<obj,'TResult>, state: obj, cancellationToken: Threading.CancellationToken, creationOptions: TaskCreationOptions) : Task<'TResult>
val task2 : Task<int>
property Task.Result: int with get
<summary>Gets the result value of this <see cref="T:System.Threading.Tasks.Task`1" />.</summary>
<exception cref="T:System.AggregateException">The task was canceled. The <see cref="P:System.AggregateException.InnerExceptions" /> collection contains a <see cref="T:System.Threading.Tasks.TaskCanceledException" /> object. -or- An exception was thrown during the execution of the task. The <see cref="P:System.AggregateException.InnerExceptions" /> collection contains information about the exception or exceptions.</exception>
<returns>The result value of this <see cref="T:System.Threading.Tasks.Task`1" />, which is of the same type as the task's type parameter.</returns>
val parseResults : FSharp.Compiler.CodeAnalysis.FSharpParseFileResults
val checkResults : FSharp.Compiler.CodeAnalysis.FSharpCheckFileResults
val checkProjectResults : FSharp.Compiler.CodeAnalysis.FSharpCheckProjectResults
member FsiEvaluationSession.ParseAndCheckInteraction : code:string -> FSharp.Compiler.CodeAnalysis.FSharpParseFileResults * FSharp.Compiler.CodeAnalysis.FSharpCheckFileResults * FSharp.Compiler.CodeAnalysis.FSharpCheckProjectResults
property FSharp.Compiler.CodeAnalysis.FSharpCheckFileResults.Diagnostics: FSharp.Compiler.Diagnostics.FSharpDiagnostic [] with get
<summary> The errors returned by parsing a source file. </summary>
property Array.Length: int with get
<summary>Gets the total number of elements in all the dimensions of the <see cref="T:System.Array" />.</summary>
<exception cref="T:System.OverflowException">The array is multidimensional and contains more than <see cref="F:System.Int32.MaxValue" /> elements.</exception>
<returns>The total number of elements in all the dimensions of the <see cref="T:System.Array" />; zero if there are no elements in the array.</returns>
member FSharp.Compiler.CodeAnalysis.FSharpCheckFileResults.GetToolTip : line:int * colAtEndOfNames:int * lineText:string * names:string list * tokenTag:int -> FSharp.Compiler.EditorServices.ToolTipText
module FSharpTokenTag from FSharp.Compiler.Tokenization
<summary> Some of the values in the field FSharpTokenInfo.Tag </summary>
val IDENT : int
<summary> Indicates the token is an identifier (synonym for FSharpTokenTag.Identifier) </summary>
member FSharp.Compiler.CodeAnalysis.FSharpCheckFileResults.GetSymbolUseAtLocation : line:int * colAtEndOfNames:int * lineText:string * names:string list -> FSharp.Compiler.CodeAnalysis.FSharpSymbolUse option
val fsiConfig2 : FsiEvaluationSessionHostConfig
val collectionTest : unit -> unit
val i : int32
val defaultArgs : string []
val session : FsiEvaluationSession
val v : FsiValue option
property Option.Value: FsiValue with get
<summary>Get the value of a 'Some' option. A NullReferenceException is raised if the option is 'None'.</summary>