F# difference between let add1 x = x + 1 and let add2 x = x +1

Question

What is the difference between let add1 x = x + 1 and let add2 x = x +1. The accidental removal of space changed the type of function from
val add1 : x:int->int to
val add2 : x:(int -> 'a) -> 'a

As far as I understand, the first type statement says add1 maps int onto int. But what is the meaning of the second one.

Well, 'a represents a generic type, but how is the function 'add2' returning a generic?

Thanks for your help.

Answer 1

That's a quirk of F# syntax: a plus or minus sign immediately followed by a number literal is treated as a positive or negative number respectively, and not as an operator followed by a number.

> 42
it : int = 42

> +42
it : int = 42

> -42
it : int = -42

So your second example let add2 x = x +1 is equivalent to let add2 x = x 1. The expression x 1 means that x is a function and it's being applied to the argument 1, which is exactly what your type is telling you:

add2 : x:(int -> 'a) -> 'a

This says that add2 takes a function named x, which takes an int and returns some 'a, and that add2 itself also returns the same 'a.