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

f# ocaml

Nick Heiner · Oct 22, 2009 · Viewed 18.6k times · Source

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

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.