Editing Standard ML (section)

===Higher-order functions===
Functions can consume functions as arguments:
<syntaxhighlight lang="sml">fun map f (x, y) = (f x, f y)</syntaxhighlight>

Functions can produce functions as return values:
<syntaxhighlight lang="sml">fun constant k = (fn _ => k)</syntaxhighlight>

Functions can also both consume and produce functions:
<syntaxhighlight lang="sml">fun compose (f, g) = (fn x => f (g x))</syntaxhighlight>

The function {{code|List.map}} from the basis [[Library (computing)|library]] is one of the most commonly used higher-order functions in Standard ML:
<syntaxhighlight lang="sml">
fun map _ [] = []
  | map f (x :: xs) = f x :: map f xs
</syntaxhighlight>

A more efficient implementation with tail-recursive {{code|List.foldl}}:
<syntaxhighlight lang="sml">
fun map f = List.rev o List.foldl (fn (x, acc) => f x :: acc) []
</syntaxhighlight>