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

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.