There is only one file type that uses the .ML extension:
ML Source Code File
Text file written in ML, a functional programming language; may be written using Standard ML (SML) or one of several varieties in the ML family, including as Caml, Objective Caml (OCaml), JoCaml, Extended ML, and MacroML; often used in language design for developing compilers or other source code interpreters; also used in academic environments for proving theories.
While there are many variants of ML, OCaml is the most popular version.
NOTE: Microsoft's F# language is also part of the ML family since it is a derivative of ML. F# is largely compatible with OCaml.
Updated: March 22, 2010