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

The signatures are written in that way because of what is called Currying. A slightly more accurate way of describing your function is that it takes a (function that takes a 'a and returns a function from a 'b to a 'a) and returns a function that takes a 'a and returns a function from a list<'b> to a 'a. Because of this the type signature can be rewritten as

('a -> 'b -> 'a) -> ('a -> (list<'b> -> 'a))