Editing OCaml (section)

===Summing a list of integers===
Lists are one of the fundamental datatypes in OCaml. The following code example defines a [[Recursion (computer science)|recursive]] function ''sum'' that accepts one argument, ''integers'', which is supposed to be a list of integers. Note the keyword <code>rec</code> which denotes that the function is recursive. The function recursively iterates over the given list of integers and provides a sum of the elements. The ''match'' statement has similarities to [[C (programming language)|C]]'s [[Switch statement|switch]] element, though it is far more general.

<syntaxhighlight lang="ocaml">
let rec sum integers =                   (* Keyword rec means 'recursive'. *)
  match integers with
  | [] -> 0                              (* Yield 0 if integers is the empty 
                                            list []. *)
  | first :: rest -> first + sum rest;;  (* Recursive call if integers is a non-
                                            empty list; first is the first 
                                            element of the list, and rest is a 
                                            list of the rest of the elements, 
                                            possibly []. *)
</syntaxhighlight>
<syntaxhighlight lang="OCaml">
  # sum [1;2;3;4;5];;
  - : int = 15
</syntaxhighlight>

Another way is to use standard [[fold function]] that works with lists.

<syntaxhighlight lang="ocaml">
let sum integers =
  List.fold_left (fun accumulator x -> accumulator + x) 0 integers;;
</syntaxhighlight>
<syntaxhighlight lang="OCaml">
  # sum [1;2;3;4;5];;
  - : int = 15
</syntaxhighlight>

Since the [[anonymous function]] is simply the application of the + operator, this can be shortened to:

<syntaxhighlight lang="ocaml">
let sum integers =
  List.fold_left (+) 0 integers
</syntaxhighlight>

Furthermore, one can omit the list argument by making use of a [[partial application]]:

<syntaxhighlight lang="OCaml">
let sum =
  List.fold_left (+) 0
</syntaxhighlight>