SML/NJ - the "fun act(f,x) = f(x) ;" signature

Question

Declare on the follow function -

fun act(f,x) = f(x);

Makes the signature -

val act = fn : ('a -> 'b) * 'a -> 'b

What does ('a -> 'b) * 'a -> 'b means ?

Answer 1

It means that act is a function that takes a pair (2-tuple)

('a -> 'b) * 'a

where the first element is a function from 'a to 'b, and the second is a thing of type 'a.
It returns a thing of type 'b.