How to read F# type signatures?

Question

I\'m struggling with the F# type signature notation. For example let\'s say you have a Fold function:

let rec Fold combine acc l =
...

that may

Answer 1

You could write a similar function in F# which has a type like you're proposing (but in F# it would be written as ('a * 'b -> 'a) * 'a * list<'b> -> 'a. However, the advantage of the existing function is that it's easy to partially apply it by only supplying a prefix of the arguments. For instance:

let sum = List.fold (+) 0

Using your definition, you'd have to write

let sum l = List.fold((fun (x,y) -> x + y), 0, l)