Editing ML (programming language) (section)

===List reverse===
The following function ''reverses'' the elements in a list. More precisely, it returns a new list whose elements are in reverse order compared to the given list.

<syntaxhighlight lang="sml">
fun reverse [] = []
  | reverse (x :: xs) = (reverse xs) @ [x]
</syntaxhighlight>

This implementation of reverse, while correct and clear, is inefficient, requiring [[quadratic time]] for execution. The function can be rewritten to execute in [[linear time]]:

<syntaxhighlight lang="sml">
fun 'a reverse xs : 'a list = List.foldl (op ::) [] xs
</syntaxhighlight>
This function is an example of parametric polymorphism. That is, it can consume lists whose elements have any type, and return lists of the same type.