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F# compiler service API exposed via a console application std I/O or http or Language Server Protocol
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7sharp9 and Krzysztof-Cieslak Fixed issue with array notation
let test = []
let test : int array = []
Both produce FSharEntities where the first IsArray and the second is not, so you have to unannotate the second, api is such a mess in FCS.

Second part of this commit simply does QuoteIdentifierIfNeeded only on FSharpTypes that have a TypeDefinition, anything else uses the Format.  Im not exactly sure what doesn have a TypeDefinition apart froma  generic parameter which is covered in the other branch.
Latest commit f8a6cb3 Oct 17, 2019

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The FsAutoComplete project (FSAC) provides a backend service for rich editing or 'intellisense' features for editors.

It can be hosted as command-line interface (stdio mode) or using the Language Server Protocol (lsp mode).

Currently it is used by:

It's based on:

Required software

FsAutoComplete can run on .NET/mono or .NET Core.



  • .NET Core Sdk
  • on unix/mac: Required: Mono >= 5.12, Recommended: Mono >= 5.18

Building and testing


  • .NET Core Sdk, see global.json for the exact version.
  • Mono 5.18 on unix/osx
  • Microsoft Build Tools 2013

There is a FAKE script who can be invoked with build.cmd/

  • To build fsautocomplete binaries in ~/bin directory, do run build LocalRelease
  • To build, run all tests and create packages, do run build All

The integration tests use a simple strategy of running a scripted session with fsautocomplete and then comparing the output with that saved in the repository. This requires careful checking when the test is first constructed. On later runs, absolute paths are removed using regular expressions to ensure that the tests are machine-independent.

see test/FsAutoComplete.IntegrationTests/ for more info.

Communication protocol


FsAutoComplete supports LSP as a communication protocol. To start it in LSP mode run fsautocomplete binary with --mode lsp argument.

Supported LSP endpoints

  • initialize
  • textDocument/didOpen
  • textDocument/didChange
  • textDocument/didSave
  • textDocument/hover
  • textDocument/completion & completionItem/resolve
  • textDocument/rename
  • textDocument/definition
  • textDocument/typeDefinition
  • textDocument/implementation
  • textDocument/codeAction:
    • Remove unused open
    • Resolve namespace/module
    • Replace unused symbol with _
    • Fix typo based on error message
    • Remove redundant qualifier
    • Add missing new keyword for IDisposable
    • Generate cases for all DU case in pattern matching
    • Generate empty interface implementation
    • Fixes suggested by FSharpLint
  • textDocument/codeLens & codeLens/resolve:
    • signature Code Lenses
    • reference number Code Lenses
  • textDocument/formatting - powered by fantomas
  • textDocument/references
  • textDocument/documentHighlight
  • textDocument/signatureHelp
  • textDocument/documentSymbol
  • workspace/didChangeWatchedFiles
  • workspace/didChangeConfiguration
  • workspace/symbol

Custom endpoints:

Custom endpoints are using (for messages body) PlainNotification type and string format serialized with exactly same serialization format as old JSON protocol

  • fsharp/signature - accepts TextDocumentPositionParams, returns signature of symbol at given position as formated string
  • fsharp/signatureData - accepts TextDocumentPositionParams, returns signature of symbol at given position as DTO
  • fsharp/lineLens - accepts ProjectParms (Project filed contain F# file path), returns locations where LineLenses should be displayed
  • fsharp/compilerLocation - no input, returns paths to FCS, FSI and MsBuild
  • fsharp/compile - accepts ProjectParms, tries to compile project, returns list of errors and exit status code
  • fsharp/workspacePeek - accepts WorkspacePeekRequest, returns list of possible workspaces (resolved solution files, or list of projects if there are no solution files)
  • fsharp/workspaceLoad - accepts WorkspaceLoadParms, loads given list of projects in the background, partial result notified by fsharp/notifyWorkspace notification
  • fsharp/project - accepts ProjectParms, loads given project
  • fsharp/fsdn - accepts ProjectParms (Project filed contain query string), query FSDN and returns list of functions
  • fsharp/f1Help - accepts TextDocumentPositionParams, returns URL to MSDN documentation for symbol at given position
  • fsharp/documentation - accepts TextDocumentPositionParams, returns documentation data about symbol at given position, used for InfoPanel
  • fsharp/documentationSymbol - accepts DocumentationForSymbolReuqest, returns documentation data about given symbol from given assembly, used for InfoPanel

Supported LSP notifications

  • window/showMessage
  • window/logMessage
  • textDocument/publishDiagnostics

Custom notifications:

  • fsharp/notifyWorkspace - notification for workspace/solution/project loading events
  • fsharp/notifyWorkspacePeek - notification for initial workspace peek

Additional startup options:

  • --background-service-enabled - passing this flag enables background service feature, increasing FSAC responsiveness by moving some of the operations (especially background type checking) to other process. It results in increased memory usage. Used by default in Ionide.
  • --verbose - passing this flag enables additional logging being printed out in stderr
  • DOTNET_ROOT - setting this environment variable will set the dotnet SDK root, which is used when finding references for FSX scripts.

Initialization options:

Options that should be send as initializationOptions as part of initialize request.

  • AutomaticWorkspaceInit - setting it to true will start Workspace Loading without need to run fsharp/workspacePeek and fsharp/workspaceLoad commands. It will always choose top workspace from the found list - all projects in workspace if 0 .sln files are found, .sln file if 1 .sln file was found, .sln file with most projects if multiple .sln files were found. It's designed to be used in clients that doesn't allow to create custom UI for selecting workspaces.


  • FSharp.keywordsAutocomplete - provides keywords in autocomplete list, recommended default value: true
  • FSharp.ExternalAutocomplete - provides autocomplete for symbols from not opened namespaces/modules, insert open on accept, recommended default value: false
  • FSharp.Linter - enables FSharpLint integration, provides additional warnings and code action fixes, recommended default value: true
  • FSharp.UnionCaseStubGeneration - enables code action to generate pattern matching cases, recommended default value: true
  • FSharp.UnionCaseStubGenerationBody - defines dummy body used by pattern matching generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.RecordStubGeneration - enables code action to generate record stub, recommended default value: true
  • FSharp.RecordStubGenerationBody - defines dummy body used by record stub generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.InterfaceStubGeneration - enables code action to generate interface stub, recommended default value: true
  • FSharp.InterfaceStubGenerationObjectIdentifier - defines object identifier used by interface stub generator,recommended default value: "this"
  • FSharp.InterfaceStubGenerationMethodBody - defines dummy body used by interface stub generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.UnusedOpensAnalyzer - enables unused open detections, recommended default value: true
  • FSharp.UnusedDeclarationsAnalyzer - enables unused symbol detection, recommended default value: true
  • FSharp.UseSdkScripts - enables the use of .Net Core SDKs for script file type-checking and evaluation, otherwise the .Net Framework reference lists will be used. Recommended default value: true. Current default value: false
  • FSharp.SimplifyNameAnalyzer - enables simplify name analyzer and remove redundant qualifier quick fix, recommended default value: false
  • FSharp.ResolveNamespaces - enables resolve namespace quick fix (add open if symbol is from not yet opened module/namespace), recommended default value: true
  • FSharp.EnableReferenceCodeLens - enables reference count code lenses, recommended default value: true if --background-service-enabled is used by default, false otherwise
  • FSharp.dotNetRoot - sets the root path for finding dotnet SDK references. Primarily used for FSX Scripts. Default value: operating-system dependent. On windows, C:\Program Files\dotnet; on Unix, /usr/local/share/dotnet

Old stdio protocol

It is expected that the editor will launch this program in the background and communicate over a pipe. It is possible to use interactively, although due to the lack of any readline support it isn't pleasant, and text pasted into the buffer may not be echoed. As a result, use this only for very simple debugging. For more complex scenarios it is better to write another integration test by copying an existing one.

The available commands can be listed by running fsautocomplete --commands. Commands are all on a single line, with the exception of the parse command, which should be followed by the current text of the file to parse (which may differ from the contents on disk), and terminated with a line containing only <<EOF>>.

Data is returned as JSON. An example of a simple session is:

project "Test1.fsproj"
{"Kind":"project","Data":{"Files":["<absolute path removed>/Program.fs"],"Output":"<absolute path removed>/bin/Debug/Test1.exe"}}
parse "Program.fs"
module X =
    let func x = x + 1

    let val2 = X.func 2
{"Kind":"INFO","Data":"Background parsing started"}
completion "Program.fs" 4 13

Each response is exactly one line, which simplifies the application of a JSON parser. For further insight into the communication protocol, have a look over the integration tests, which have examples of all the features. Each folder contains one or more *Runner.fsx files which specify a sequence of commands to send, and *.json files, which contain the output.


FileWatcher exceptions

You may see a stack trace finishing with System.IO.IOException: kqueue() error at init, error code = ’0’. This is due to a limitation in the number of filehandles that the Mono file watchers can keep open. Restarting FsAutoComplete or the hosting editor should help. If not, try setting export MONO_MANAGED_WATCHER=disabled in your ~/.bash_profile. Note that on OSX, this setting will only take effect if you launch emacs from the terminal.


The maintainers of this repository are:

The primary maintainer for this repository is Enrico Sada

Previous maintainers:

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