Editing ML (programming language) (section)

===Modules===
Modules are ML's system for structuring large projects and libraries. A module consists of a signature file and one or more structure files. The signature file specifies the [[API]] to be implemented (like a C header file, or [[Interface (Java)|Java interface]] file). The structure implements the signature (like a C source file or Java class file). For example, the following define an Arithmetic signature and an implementation of it using Rational numbers:

<syntaxhighlight lang="sml">
signature ARITH =
sig
        type t
        val zero : t
        val succ : t -> t
        val sum : t * t -> t
end
</syntaxhighlight>

<syntaxhighlight lang="sml">
structure Rational : ARITH =
struct
        datatype t = Rat of int * int
        val zero = Rat (0, 1)
        fun succ (Rat (a, b)) = Rat (a + b, b)
        fun sum (Rat (a, b), Rat (c, d)) = Rat (a * d + c * b , b * d)
end
</syntaxhighlight>

These are imported into the interpreter by the 'use' command. Interaction with the implementation is only allowed via the signature functions, for example it is not possible to create a 'Rat' data object directly via this code. The 'structure' block hides all the implementation detail from outside.

ML's standard libraries are implemented as modules in this way.