ML File

.ml is the standard source file extension for the OCaml programming language, a statically typed functional language with object-oriented and imperative features developed at INRIA, Paris. The extension derives from ML (Meta Language), the family of languages from which OCaml descends.

Each .ml file defines one OCaml module, automatically named after the file with its first letter capitalized - parser.ml becomes the module Parser. The companion .mli interface file declares what the module exposes publicly, acting similarly to a C header file.

Source files contain sequences of OCaml declarations: let bindings, type definitions, module and functor definitions, exception declarations, and class definitions. A let () = ... expression at the top level serves as the program entry point by convention.

Files are compiled by ocamlc to bytecode or by ocamlopt to native machine code. The Dune build system and the opam package manager are the standard project management tools. OCaml 5.x (2022 onward) introduced effect handlers and multicore parallelism, adding new syntax not accepted by older compilers.

The .ml extension is also occasionally seen for Standard ML files, though that community typically uses .sml instead. The dominant use is OCaml, which reached version 5.3 in 2025.