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Compiler Services: Editor services

This tutorial demonstrates how to use the editor services provided by the F# compiler. This API is used to provide auto-complete, tool-tips, parameter info help, matching of brackets and other functions in F# editors including Visual Studio, Xamarin Studio and Emacs (see fsharpbindings project for more information). Similarly to the tutorial on using untyped AST, we start by getting the InteractiveChecker object.

NOTE: The FSharp.Compiler.Service API is subject to change when later versions of the nuget package are published

Type checking sample source code

As in the previous tutorial (using untyped AST), we start by referencing FSharp.Compiler.Service.dll, opening the relevant namespace and creating an instance of InteractiveChecker:

// Reference F# compiler API
#r "FSharp.Compiler.Service.dll"

open FSharp.Compiler.CodeAnalysis
open FSharp.Compiler.EditorServices
open FSharp.Compiler.Text
open FSharp.Compiler.Tokenization

// Create an interactive checker instance
let checker = FSharpChecker.Create()

As previously, we use GetProjectOptionsFromScriptRoot to get a context where the specified input is the only file passed to the compiler (and it is treated as a script file or stand-alone F# source code).

// Sample input as a multi-line string
let input =
  open System

  let foo() =
    let msg = String.Concat("Hello"," ","world")
    if true then
      printfn "%s" msg.
// Split the input & define file name
let inputLines = input.Split('\n')
let file = "/home/user/Test.fsx"

let projOptions, _diagnostics =
    checker.GetProjectOptionsFromScript(file, SourceText.ofString input, assumeDotNetFramework=false)
    |> Async.RunSynchronously

let parsingOptions, _diagnostics2 =

To perform type checking, we first need to parse the input using ParseFile, which gives us access to the untyped AST. However, then we need to call CheckFileInProject to perform the full type checking. This function also requires the result of ParseFileInProject, so the two functions are often called together.

// Perform parsing

let parseFileResults =
    checker.ParseFile(file, SourceText.ofString input, parsingOptions)
    |> Async.RunSynchronously

Before we look at the interesting operations provided by TypeCheckResults, we need to run the type checker on a sample input. On F# code with errors, you would get some type checking result (but it may contain incorrectly "guessed" results).

// Perform type checking
let checkFileAnswer =
    checker.CheckFileInProject(parseFileResults, file, 0, SourceText.ofString input, projOptions)
    |> Async.RunSynchronously

Alternatively you can use ParseAndCheckFileInProject to check both in one step:

let parseResults2, checkFileAnswer2 =
    checker.ParseAndCheckFileInProject(file, 0, SourceText.ofString input, projOptions)
    |> Async.RunSynchronously

The function returns both the untyped parse result (which we do not use in this tutorial), but also a CheckFileAnswer value, which gives us access to all the interesting functionality...

let checkFileResults =
    match checkFileAnswer with
    | FSharpCheckFileAnswer.Succeeded (res) -> res
    | res -> failwithf "Parsing did not finish... (%A)" res

Here, we type check a simple function that (conditionally) prints "Hello world". On the last line, we leave an additional dot in msg. so that we can get the completion list on the msg value (we expect to see various methods on the string type there).

Using type checking results

Let's now look at some of the API that is exposed by the TypeCheckResults type. In general, this is the type that lets you implement most of the interesting F# source code editor services.

Getting a tool tip

To get a tool tip, you can use the GetToolTip method. The method takes a line number and character offset. Both of the numbers are zero-based. In the sample code, we want to get a tooltip for the foo function that is defined on line 3 (line 0 is blank) and the letter f starts at index 7 (the tooltip would work anywhere inside the identifier).

In addition, the method takes a tag of token which is typically IDENT, when getting a tooltip for an identifier (the other option lets you get a tooltip with full assembly location when using #r "...").

// Get tag of the IDENT token to be used as the last argument
let identToken = FSharpTokenTag.Identifier

// Get tool tip at the specified location
let tip =
    checkFileResults.GetToolTip(4, 7, inputLines.[1], [ "foo" ], identToken)

printfn "%A" tip

Aside from the location and token kind, the function also requires the current contents of the line (useful when the source code changes) and a Names value, which is a list of strings representing the current long name. For example, to get a tooltip for the Random identifier in a long name System.Random, you would use a location somewhere in the string Random and you would pass ["System"; "Random"] as the Names value.

The returned value is of type ToolTipText which contains a discriminated union ToolTipElement. The union represents different kinds of tool tips that you can get from the compiler.

Getting auto-complete lists

The next method exposed by TypeCheckResults lets us perform auto-complete on a given location. This can be called on any identifier or in any scope (in which case you get a list of names visible in the scope) or immediately after . to get a list of members of some object. Here, we get a list of members of the string value msg.

To do this, we call GetDeclarationListInfo with the location of the . symbol on the last line (ending with printfn "%s" msg.). The offsets are one-based, so the location is 7, 23. We also need to specify a function that says that the text has not changed and the current identifier where we need to perform the completion.

// Get declarations (autocomplete) for a location
let decls =
        Some parseFileResults,
        PartialLongName.Empty 23,
        (fun () -> [])

// Print the names of available items
for item in decls.Items do
    printfn " - %s" item.NameInList

NOTE: v is an alternative name for the old GetDeclarations. The old GetDeclarations was deprecated because it accepted zero-based line numbers. At some point it will be removed, and GetDeclarationListInfo will be renamed back to GetDeclarations.

When you run the code, you should get a list containing the usual string methods such as Substring, ToUpper, ToLower etc. The fourth argument of GetDeclarations, here ([], "msg"), specifies the context for the auto-completion. Here, we want a completion on a complete name msg, but you could for example use (["System"; "Collections"], "Generic") to get a completion list for a fully qualified namespace.

Getting parameter information

The next common feature of editors is to provide information about overloads of a method. In our sample code, we use String.Concat which has a number of overloads. We can get the list using GetMethods operation. As previously, this takes the zero-indexed offset of the location that we are interested in (here, right at the end of the String.Concat identifier) and we also need to provide the identifier again (so that the compiler can provide up-to-date information when the source code changes):

// Get overloads of the String.Concat method
let methods =
    checkFileResults.GetMethods(5, 27, inputLines.[4], Some [ "String"; "Concat" ])

// Print concatenated parameter lists
for mi in methods.Methods do
    [ for p in mi.Parameters do
          for tt in p.Display do
              yield tt.Text ]
    |> String.concat ", "
    |> printfn "%s(%s)" methods.MethodName

The code uses the Display property to get the annotation for each parameter. This returns information such as arg0: obj or params args: obj[] or str0: string, str1: string. We concatenate the parameters and print a type annotation with the method name.

Asynchronous and immediate operations

You may have noticed that CheckFileInProject is an asynchronous operation. This indicates that type checking of F# code can take some time. The F# compiler performs the work in the background (automatically) and when we call the CheckFileInProject method, it returns an asynchronous operation.

There is also the CheckFileInProjectIfReady method. This returns immediately if the type checking operation can't be started immediately, e.g. if other files in the project are not yet type-checked. In this case, a background worker might choose to do other work in the meantime, or give up on type checking the file until the FileTypeCheckStateIsDirty event is raised.

The fsharpbinding project has a more advanced example of handling the background work where all requests are sent through an F# agent. This may be more appropriate for implementing editor support.


The CheckFileAnswer object contains other useful methods that were not covered in this tutorial. You can use it to get location of a declaration for a given identifier, additional colorization information (the F# 3.1 colorizes computation builder identifiers & query operators) and others.

Using the FSharpChecker component in multi-project, incremental and interactive editing situations may involve knowledge of the FSharpChecker operations queue and the FSharpChecker caches.

Finally, if you are implementing an editor support for an editor that cannot directly call the .NET API, you can call many of the methods discussed here via a command line interface that is available in the FSharp.AutoComplete project.

Multiple items
namespace FSharp

namespace Microsoft.FSharp
namespace FSharp.Compiler
namespace FSharp.Compiler.CodeAnalysis
namespace FSharp.Compiler.EditorServices
namespace FSharp.Compiler.Text
namespace FSharp.Compiler.Tokenization
val checker: FSharpChecker
type FSharpChecker = member CheckFileInProject: parseResults: FSharpParseFileResults * fileName: string * fileVersion: int * sourceText: ISourceText * options: FSharpProjectOptions * ?userOpName: string -> Async<FSharpCheckFileAnswer> member ClearCache: options: FSharpProjectOptions seq * ?userOpName: string -> unit + 1 overload member ClearLanguageServiceRootCachesAndCollectAndFinalizeAllTransients: unit -> unit member Compile: argv: string array * ?userOpName: string -> Async<FSharpDiagnostic array * int> member FindBackgroundReferencesInFile: fileName: string * options: FSharpProjectOptions * symbol: FSharpSymbol * ?canInvalidateProject: bool * [<Experimental ("This FCS API is experimental and subject to change.")>] ?fastCheck: bool * ?userOpName: string -> Async<range seq> + 1 overload member GetBackgroundCheckResultsForFileInProject: fileName: string * options: FSharpProjectOptions * ?userOpName: string -> Async<FSharpParseFileResults * FSharpCheckFileResults> member GetBackgroundParseResultsForFileInProject: fileName: string * options: FSharpProjectOptions * ?userOpName: string -> Async<FSharpParseFileResults> member GetBackgroundSemanticClassificationForFile: fileName: string * options: FSharpProjectOptions * ?userOpName: string -> Async<SemanticClassificationView option> + 1 overload member GetParsingOptionsFromCommandLineArgs: sourceFiles: string list * argv: string list * ?isInteractive: bool * ?isEditing: bool -> FSharpParsingOptions * FSharpDiagnostic list + 1 overload member GetParsingOptionsFromProjectOptions: options: FSharpProjectOptions -> FSharpParsingOptions * FSharpDiagnostic list ...
<summary> Used to parse and check F# source code. </summary>
static member FSharpChecker.Create: ?projectCacheSize: int * ?keepAssemblyContents: bool * ?keepAllBackgroundResolutions: bool * ?legacyReferenceResolver: LegacyReferenceResolver * ?tryGetMetadataSnapshot: FSharp.Compiler.AbstractIL.ILBinaryReader.ILReaderTryGetMetadataSnapshot * ?suggestNamesForErrors: bool * ?keepAllBackgroundSymbolUses: bool * ?enableBackgroundItemKeyStoreAndSemanticClassification: bool * ?enablePartialTypeChecking: bool * ?parallelReferenceResolution: bool * ?captureIdentifiersWhenParsing: bool * [<Experimental ("This parameter is experimental and likely to be removed in the future.")>] ?documentSource: DocumentSource * [<Experimental ("This parameter is experimental and likely to be removed in the future.")>] ?useSyntaxTreeCache: bool * [<Experimental ("This parameter is experimental and likely to be removed in the future.")>] ?useTransparentCompiler: bool -> FSharpChecker
val input: string
val inputLines: string array
System.String.Split(separator: System.ReadOnlySpan<char>) : string array
   (+0 other overloads)
System.String.Split([<System.ParamArray>] separator: char array) : string array
   (+0 other overloads)
System.String.Split(separator: string array, options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: string, ?options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: char array, options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: char array, count: int) : string array
   (+0 other overloads)
System.String.Split(separator: char, ?options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: string array, count: int, options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: string, count: int, ?options: System.StringSplitOptions) : string array
   (+0 other overloads)
System.String.Split(separator: char array, count: int, options: System.StringSplitOptions) : string array
   (+0 other overloads)
val file: string
val projOptions: FSharpProjectOptions
val _diagnostics: FSharp.Compiler.Diagnostics.FSharpDiagnostic list
member FSharpChecker.GetProjectOptionsFromScript: fileName: string * source: ISourceText * ?previewEnabled: bool * ?loadedTimeStamp: System.DateTime * ?otherFlags: string array * ?useFsiAuxLib: bool * ?useSdkRefs: bool * ?assumeDotNetFramework: bool * ?sdkDirOverride: string * ?optionsStamp: int64 * ?userOpName: string -> Async<FSharpProjectOptions * FSharp.Compiler.Diagnostics.FSharpDiagnostic list>
module SourceText from FSharp.Compiler.Text
<summary> Functions related to ISourceText objects </summary>
val ofString: string -> ISourceText
<summary> Creates an ISourceText object from the given string </summary>
Multiple items
type Async = static member AsBeginEnd: computation: ('Arg -> Async<'T>) -> ('Arg * AsyncCallback * obj -> IAsyncResult) * (IAsyncResult -> 'T) * (IAsyncResult -> unit) static member AwaitEvent: event: IEvent<'Del,'T> * ?cancelAction: (unit -> unit) -> Async<'T> (requires delegate and 'Del :> Delegate) static member AwaitIAsyncResult: iar: IAsyncResult * ?millisecondsTimeout: int -> Async<bool> static member AwaitTask: task: Task<'T> -> Async<'T> + 1 overload static member AwaitWaitHandle: waitHandle: WaitHandle * ?millisecondsTimeout: int -> Async<bool> static member CancelDefaultToken: unit -> unit static member Catch: computation: Async<'T> -> Async<Choice<'T,exn>> static member Choice: computations: Async<'T option> seq -> Async<'T option> static member FromBeginEnd: beginAction: (AsyncCallback * obj -> IAsyncResult) * endAction: (IAsyncResult -> 'T) * ?cancelAction: (unit -> unit) -> Async<'T> + 3 overloads static member FromContinuations: callback: (('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T> ...

type Async<'T>
static member Async.RunSynchronously: computation: Async<'T> * ?timeout: int * ?cancellationToken: System.Threading.CancellationToken -> 'T
val parsingOptions: FSharpParsingOptions
val _diagnostics2: FSharp.Compiler.Diagnostics.FSharpDiagnostic list
member FSharpChecker.GetParsingOptionsFromProjectOptions: options: FSharpProjectOptions -> FSharpParsingOptions * FSharp.Compiler.Diagnostics.FSharpDiagnostic list
val parseFileResults: FSharpParseFileResults
member FSharpChecker.ParseFile: fileName: string * projectSnapshot: FSharpProjectSnapshot * ?userOpName: string -> Async<FSharpParseFileResults>
member FSharpChecker.ParseFile: fileName: string * sourceText: ISourceText * options: FSharpParsingOptions * ?cache: bool * ?userOpName: string -> Async<FSharpParseFileResults>
val checkFileAnswer: FSharpCheckFileAnswer
member FSharpChecker.CheckFileInProject: parseResults: FSharpParseFileResults * fileName: string * fileVersion: int * sourceText: ISourceText * options: FSharpProjectOptions * ?userOpName: string -> Async<FSharpCheckFileAnswer>
val parseResults2: FSharpParseFileResults
val checkFileAnswer2: FSharpCheckFileAnswer
member FSharpChecker.ParseAndCheckFileInProject: fileName: string * projectSnapshot: FSharpProjectSnapshot * ?userOpName: string -> Async<FSharpParseFileResults * FSharpCheckFileAnswer>
member FSharpChecker.ParseAndCheckFileInProject: fileName: string * fileVersion: int * sourceText: ISourceText * options: FSharpProjectOptions * ?userOpName: string -> Async<FSharpParseFileResults * FSharpCheckFileAnswer>
val checkFileResults: FSharpCheckFileResults
type FSharpCheckFileAnswer = | Aborted | Succeeded of FSharpCheckFileResults member Equals: FSharpCheckFileAnswer * IEqualityComparer -> bool
<summary> The result of calling TypeCheckResult including the possibility of abort and background compiler not caught up. </summary>
union case FSharpCheckFileAnswer.Succeeded: FSharpCheckFileResults -> FSharpCheckFileAnswer
<summary> Success </summary>
val res: FSharpCheckFileResults
val res: FSharpCheckFileAnswer
val failwithf: format: Printf.StringFormat<'T,'Result> -> 'T
val identToken: int
module FSharpTokenTag from FSharp.Compiler.Tokenization
<summary> Some of the values in the field FSharpTokenInfo.Tag </summary>
val Identifier: int
<summary> Indicates the token is an identifier </summary>
val tip: ToolTipText
member FSharpCheckFileResults.GetToolTip: line: int * colAtEndOfNames: int * lineText: string * names: string list * tokenTag: int * ?width: int -> ToolTipText
val printfn: format: Printf.TextWriterFormat<'T> -> 'T
val decls: DeclarationListInfo
member FSharpCheckFileResults.GetDeclarationListInfo: parsedFileResults: FSharpParseFileResults option * line: int * lineText: string * partialName: PartialLongName * ?getAllEntities: (unit -> AssemblySymbol list) * ?completionContextAtPos: (pos * CompletionContext option) -> DeclarationListInfo
union case Option.Some: Value: 'T -> Option<'T>
type PartialLongName = { QualifyingIdents: string list PartialIdent: string EndColumn: int LastDotPos: int option } member Equals: PartialLongName * IEqualityComparer -> bool static member Empty: endColumn: int -> PartialLongName
<summary> Qualified long name. </summary>
static member PartialLongName.Empty: endColumn: int -> PartialLongName
val item: DeclarationListItem
property DeclarationListInfo.Items: DeclarationListItem array with get
property DeclarationListItem.NameInList: string with get
<summary> Get the text to display in the declaration list for the declaration. This is a display name without backticks. </summary>
val methods: MethodGroup
member FSharpCheckFileResults.GetMethods: line: int * colAtEndOfNames: int * lineText: string * names: string list option -> MethodGroup
val mi: MethodGroupItem
property MethodGroup.Methods: MethodGroupItem array with get
<summary> The methods (or other items) in the group </summary>
val p: MethodGroupItemParameter
property MethodGroupItem.Parameters: MethodGroupItemParameter array with get
<summary> The parameters of the method in the overload set </summary>
val tt: TaggedText
property MethodGroupItemParameter.Display: TaggedText array with get
<summary> The representation for the parameter including its name, its type and visual indicators of other information such as whether it is optional. </summary>
property TaggedText.Text: string with get
<summary> Gets the text </summary>
module String from Microsoft.FSharp.Core
val concat: sep: string -> strings: string seq -> string
property MethodGroup.MethodName: string with get
<summary> The shared name of the methods (or other items) in the group </summary>

Type something to start searching.