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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?
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The fun keyword is used to define a lambda expression, that is, an anonymous function.
void fun() is a function that does not return a value (essentially nothing). :: is a scope resolution operator. For example, cout and cin are defined in the std namespace. To qualify their names without using the using declaration we have to qualify them with std::cout and std::cin.
Similarly, OCaml functions do not have to have names; they may be anonymous. For example, here is an anonymous function that increments its input: fun x -> x + 1 . Here, fun is a keyword indicating an anonymous function, x is the argument, and -> separates the argument from the body.
OCaml doesn't have a return keyword — the last expression in a function becomes the result of the function automatically.
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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