What is the difference between the `fun` and `function` keywords?

Question

Sometimes I see code like

let (alt : recognizer -> recognizer -> recognizer) =
  fun a b p -> union  (a p) (b p)

Or like:

let hd = function
    Cons(x,xf) -> x
  | Nil -> raise Empty

What is the difference between fun and function?

Solution 1

The semantics for this is the same as in F# (probably because F# is based on OCaml):

• function allows the use of pattern matching (i.e. |), but consequently it can be passed only one argument.

  function p_1 -> exp_1 | … | p_n -> exp_n
  

  is equivalent to

  fun exp -> match exp with p_1 -> exp_1 | … | p_n -> exp_n
  

• fun does not allow pattern matching, but can be passed multiple arguments, e.g.

  fun x y -> x + y
  

When either of the two forms can be used, fun is generally preferred due to its compactness.

See also OCaml documentation on Functions.

Solution 2

The way I think about it

function patterns

is shorthand for

(fun x -> match x with patterns)

where 'patterns' is e.g.

| Some(x) -> yadda | None -> blah

(And

fun args -> expr

is how you define a lambda.)

Solution 3

Russ Cam is correct in his answer.

Here is a posting on the OCaml list talking about it

http://caml.inria.fr/pub/ml-archives/ocaml-beginners/2003/11/b8036b7a0c1d082111d7a83c8f6dbfbb.en.html

function only allows for one argument but allows for pattern matching, while fun is the more general and flexible way to define a function.

I generally use fun unless there is a good reason to use function.

You can see this in the code you posted where the fun declaration takes 3 arguments and the function declaration does pattern matching on it's input

Solution 4

fun x1 ... xn -> e

is an abbreviation for

function x1 -> ... -> function xn -> e

View more solutions

Comments

[Bill the Lizard]

Removed the 'fun' tag, since it has an established meaning other than your intent here. I think the question will be better off without it, since some people filter it out using the ignore list.

[Nick Heiner]

couldn't you also do let x y z = y + z, with no fun or function at all?

[Chris Conway]

@Rosarch, yes, of course. I think the question is implicitly about anonymous function definitions.

[nlucaroni]

I didn't downvote, but, describing 'fun' as preferred because it's more compact isn't the whole story, it isn't even a description of how to use it, and in no way are you comparing the two keywords! function is the same as saying, (fun x -> match x with ...), how is that more compact if you plan to pattern match?

[newacct]

fun also allows the use of pattern matching, but with only one alternative, like let