The SML'97 standard of the [[Standard ML]] programming language does not provide any associative containers. However, various implementations of Standard ML do provide associative containers.
The library of the popular [[Standard ML of New Jersey]] implementation provides a signature (kind ofsomewhat like an "interface"), <code>ORD_MAP</code>, which defines a common interface for ordered functional (immutable) associative arrays. There are several general functors, <code>BinaryMapFn</code>, <code>ListMapFn</code>, <code>RedBlackMapFn</code>, and <code>SplayMapFn</code>, that allow you to create the corresponding type of ordered map (the types are a [[self-balancing binary search tree]], sorted [[association list]], [[red-black tree]], and [[splay tree]], respectively) using a user-provided structure to describe the key type and comparator. The functor returns a structure that follows the <code>ORD_MAP</code> interface. In addition, there are two pre-defined modules for associative arrays with integer keys: <code>IntBinaryMap</code> and <code>IntListMap</code>.
