Editing Standard ML (section)

====Signatures====
A ''signature'' is an [[Interface (computing)|interface]], usually thought of as a type for a structure; it specifies the names of all entities provided by the structure, the [[arity]] of each type component, the type of each value component, and the signature of each substructure. The definitions of type components are optional; type components whose definitions are hidden are ''abstract types''.

For example, the signature for a [[Queue (data structure)|queue]] may be:

<syntaxhighlight lang="sml">
signature QUEUE = sig
    type 'a queue
    exception QueueError;
    val empty     : 'a queue
    val isEmpty   : 'a queue -> bool
    val singleton : 'a -> 'a queue
    val fromList  : 'a list -> 'a queue
    val insert    : 'a * 'a queue -> 'a queue
    val peek      : 'a queue -> 'a
    val remove    : 'a queue -> 'a * 'a queue
end
</syntaxhighlight>

This signature describes a module that provides a polymorphic type {{code|lang=sml|'a queue}}, {{code|lang=sml|exception QueueError}}, and values that define basic operations on queues.