File with an .FS extension contains source code of a program stored in the Microsoft F# - “F Sharp” language. It is used to compile files into executable EXE files or DLL libraries. F# is a version of a functional programming language – ML, modelled on OCami and Haskell, which enable also implementing objectivity and imperative code.

FS files are used since the year 2010 in the Microsoft Visual Studio environment with a proper Visual F# add-on, using MS .NET tools and libraries.