FSharp source code formatter
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Latest commit 6db0f04 Dec 5, 2018



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F# source code formatter, inspired by scalariform for Scala, ocp-indent for OCaml and PythonTidy for Python.

Build Status Travis Build Status AppVeyor

How to use

Command line tool / API

Use this command to install Fantomas as a dotnet SDK global tool:

dotnet tool install -g fantomas-tool

For detailed guidelines, please read Fantomas: How to use.


If you prefer an install per project, fantomas can also be installed as a DotNetCliToolReference. See this blogpost for more info.

FAKE build system

Fantomas can be easily integrated with FAKE build system. Here is a sample build.fsx:

#r "packages/FAKE/tools/FakeLib.dll"
#r "packages/Fantomas/lib/FantomasLib.dll"

open Fake
open Fantomas.FakeHelpers
open Fantomas.FormatConfig

// Properties
let buildDir = "./build/"
let fantomasConfig =
    { FormatConfig.Default with
            PageWidth = 120
            ReorderOpenDeclaration = true }

Target "CheckCodeFormat" (fun _ ->
    !! "src/**/*.fs"
      |> checkCode fantomasConfig

Target "FormatCode" (fun _ ->
    !! "src/**/*.fs"
      |> formatCode fantomasConfig
      |> Log "Formatted files: "

RunTargetOrDefault "CheckCodeFormat"

Jetbrains Rider

The fsharp-support uses fantomas under the hood to format the source code. No need for any additional plugins.

Visual Studio Code

Install the fantomas-fmt extension.


Try the fantomas online.

Early builds

We have our own NuGet feed that contains artifacts built on the latest master branch. To install you probably need to uninstall the current version from the official NuGet feed.

dotnet tool uninstall -g fantomas-tool

Install from MyGet:

dotnet tool install -g fantomas-tool --add-source https://www.myget.org/F/fantomas/api/v3/index.json --version 2.9.1-latest

Note that the --version is important, check the latest version at MyGet. Your can check your current version with fantomas --version (since December 2018).


This project aims at formatting F# source files based on a given configuration. Fantomas will ensure correct indentation and consistent spacing between elements in the source files. We assume that the source files are parsable by F# compiler before feeding into the tool. Fantomas follows the formatting guideline being described in A comprehensive guide to F# Formatting Conventions.

Use cases

The project is developed with the following use cases in mind:

  • Reformatting an unfamiliar code base. It gives readability when you are not the one originally writing the code. To illustrate, the following example

    type Type
        = TyLam of Type * Type
        | TyVar of string
        | TyCon of string * Type list
        with override this.ToString () =
                match this with
                | TyLam (t1, t2) -> sprintf "(%s -> %s)" (t1.ToString()) (t2.ToString())
                | TyVar a -> a
                | TyCon (s, ts) -> s

    will be rewritten to

    type Type =
        | TyLam of Type * Type
        | TyVar of string
        | TyCon of string * Type list
        override this.ToString() =
            match this with
            | TyLam(t1, t2) -> sprintf "(%s -> %s)" (t1.ToString()) (t2.ToString())
            | TyVar a -> a
            | TyCon(s, ts) -> s
  • Converting from verbose syntax to light syntax. Feeding a source file in verbose mode, Fantomas will format it appropriately in light mode. This might be helpful for code generation since generating verbose source files is much easier. For example, this code fragment

    let Multiple9x9 () =
        for i in 1 .. 9 do
            printf "\n";
            for j in 1 .. 9 do
                let k = i * j in
                printf "%d x %d = %2d " i j k;
    Multiple9x9 ();;

    is reformulated to

    let Multiple9x9() =
        for i in 1..9 do
            printf "\n"
            for j in 1..9 do
                let k = i * j
                printf "%d x %d = %2d " i j k
  • Formatting F# signatures, especially those generated by F# compiler and F# Interactive.

For more complex examples, please take a look at F# outputs of 20 language shootout programs and 10 CodeReview.SE source files.


The code base is written in F# 4.X /.NET standard 2.0. The solution file can be opened in Visual Studio 2017, VS Code (with the ionide plugin) & Jetbrains Rider. Paket is used to manage external packages. The test project depends on FsUnit and NUnit. However, the library project and command line interface have no dependency on external packages.

Testing and validation

We have tried to be careful in testing the project. There are 329 unit tests and 30 validated test examples, but it seems some corner cases of the language haven't been covered. Feel free to suggests tests if they haven't been handled correctly.

Why the name "Fantomas"?

There are a few reasons to choose the name as such. First, it starts with an "F" just like many other F# projects. Second, Fantomas is my favourite character in the literature. Finally, Fantomas has the same Greek root as "phantom"; coincidentally F# ASTs and formatting rules are so mysterious to be handled correctly.

How to contribute

Would like to contribute? Discuss on issues and send pull requests. You can get started by helping us handle "You Take It" issues.

To get an understanding of the code, either:

Architectural notes

Fantomas' features are basically two commands: format a document or format a selection in the document.

They both consist in the same two stages:

  • parse the code and generate the F# AST (Abstract Syntax Tree). This is provided by the by the FSharp.Compiler.Services library (see the parse function in CodeFormatterImpl.fs).
  • rewrite the code based on the AST (previous step) and formatting settings.

The following sections describe the modules/function you will most likely be interested in looking at to get started.

The test project: Fantomas.Tests

The organization is really simple. For each F# language feature/constructs, there is a [Feature]Test.fs file. Examples:

  • StringTests.fs
  • UnionTests.fs
  • ...

Most of the tests are really simple and have this simple algorithm: assert that format [F# CODE] is equal to [FORMATTED F# CODE].

Example (from UnionTests.fs):

let ``discriminated unions declaration``() =
    formatSourceString false "type X = private | A of AParameters | B" config
    |> prepend newline
    |> should equal """
type X =
    | A of AParameters
    | B

The CodePrinter.genParsedInput function: rewrites formatted code

CodePrinter.genParsedInput (see CodePrinter.fs): what it basically does is traversing the AST corresponding to the code to format, and rewriting it according to the provided formatting options.

The FormatConfig type: format settings

Settings such as :

  • indent values in spaces
  • reorder open declarations
  • ...

See CodePrinter.fs.

How to play with fantomas on F# Interactive

The CodeFormatter.fsx script file allows you to test the code formatting behavior. See the function formatSrc: string -> unit that formats the string in input and prints it.

Contribution examples

The time it took to contribute is sometimes mentioned, as a side note.

Fixing code generation

  • Record type formatting generated invalid F# code in some cases (#197) - (2h fix)


We would like to gratefully thank the following persons for their contributions.


The library and tool are available under Apache 2.0 license. For more information see the License file.