OCaml: What is the different between `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?

Answer 1

If it's the same as F# (which I imagine it would be considering F# is based on oCaml),

function allows you to use pattern matching, but consequently it can be passed only one argument.

fun is generally preferred as it's more compact.

EDIT:

I'm not sure why this answer was downvoted, the above is absolutely correct.

the oCaml documentation on Functions

Answer 2

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

Answer 3

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.)