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Solution structure

Fantomas has a modular project structure.
The parser (Fantomas.FCS), the core library (Fantomas.Core) and the command line application (fantomas) are the main components of the solution.

graph TD A[Fantomas.FCS] --> B B[Fantomas.Core] --> C[Fantomas] B --> D[Fantomas.Benchmarks] B --> E[Fantomas.Core.Tests] C --> F[Fantomas.Tests] G[Fantomas.Client] --> H[Fantomas.Client.Tests]


This is a very custom fork of the F# compiler. We only expose a single parse function to construct the untyped syntax tree. We achieve this by taking the files necessary to compile the F# parser from source (via custom code in a Fun.Build pipeline). This limits the dependency footprint that our compiler has, compared to the official F# compiler NuGet package.

Note that the AST returned by Fantomas.FCS looks identical to what the official F# compiler returns. However, the AST is not binary compatible. It is most likely that Fantomas contains a newer version of the Syntax tree than the official F# compiler.


The heart of Fantomas is the core library. It contains the core logic reconstructing source code from the AST. Fantomas can be used as a library, see CodeFormatter.fsi to learn what APIs are exposed.


The command line application is the main entry point of the solution.
It exposes the core functionality and also takes care of .editorconfig and .fantomasignore files.


A BenchmarkDotNet project used to measure the performance of the core library. We format a fixed revision of CodePrinter.fs as part of our CI process, to detect potential regressions.


A standalone library project that editors can use to interact with the fantomas commandline application. Editors do not use Fantomas.Core, instead they use the Fantomas.Client library to connect to a fantomas dotnet tool. This allows end-users to bring their "own version" of Fantomas. This selected version could then later be re-used to verify if all files were formatted in a CI scenario.


A suite of unit tests that target the core formatting functionalities of Fantomas.Core.


A suite of end-to-end tests that run the actual fantomas command line application.


A suite of end-to-end tests that will verify the Fantomas.Client code against released versions of fantomas.

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