F# compiler service API exposed via a console application
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This project provides a command-line interface to the FSharp.Compiler.Service project. It is intended to be used as a backend service for rich editing or 'intellisense' features for editors. Currently it is used by:

This README is targeted at developers.

Required software

  • F# 4.0 or newer
  • MSBuild 12 or newer


This can be obtained by installing Visual Studio 2013 or downloading:


  • Required: Mono >= 4.8
  • Recommended: Mono >= 5.2

Building and testing

There is a FAKE script with chain-loaders for *nix and Windows. This can be used for both building and running the unit and integration tests. It is also the core of the CI builds running on Travis and AppVeyor, and so also has the ability to run the Emacs unit and integration tests.

The integration tests use a simple strategy of running a scripted session with fsautocomplete.exe 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.

There are unit tests for FSharp.CompilerBinding, which smoothes the integration with FSharp.Compiler.Service. The tests are simply constructed using NUnit.


Project file loading issues

Old sdk project (verbose fsproj)

Try invoking the binary for project file cracking directly:

mono FSharp.Compiler.Service.ProjectCrackerTool.exe --text <path to>/MyProject.fsproj true

Inspect the log output. At the top will be an F# data structure containing compiler options including referenced assemblies. This is then followed by a detailed MSBuild log. Take a look for any error messages to do with assembly resolution. It is common for this fake build to fail overall, because it does not actually compile the project, so latter steps such as copying of output will fail. This will not interfere with assembly resolution.

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.

Communication 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.exe --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.


The maintainers of this repository are: