F* is written in F* and bootstrapped in OCaml.

Some files are written directly in OCaml:

 * The parser: uses the OCaml parser generator `menhir`

 * The lexer: uses the OCaml Sedlexing library

 * Some basic system utilities, like FStar.Compiler.Util only has an
   interface in F* and is implemented as FStar_Compiler_Util.ml

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History:

F* was originally implemented in F#.

Then, once F* could extract F* programs to OCaml, F* was bootstrapped
in OCaml.

For many years, the F* implementation was written in a language that
was the intersecton of F* and F#. So, although it was bootstrapped in
OCaml for typical use, it could also be directly compiled in F#.

Since March 2022, the F* implementation is no longer in the shared
subset of F* and F# and is compiled using a bootstrapped compiler in
OCaml.

We maintain an ocaml-snapshot, a last-known-good version of the F*
compiler bootstrapped in OCaml.