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.

See also OCaml documentation on Functions.

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