An ML file is a text file created with the general purpose functional programming language ML (oftenly known as Meta Language). It contains ML source code that is more often used in programming language research and design using completely defined semantics. There are multiple variations of ML with Caml as the widely used one. Others include Objective Caml (OCaml), JoCaml, Extended ML, and MacroML. ML language is commonly used for compiler writing, automated theorem proving, and formal verifications.