Question

I was following some examples on F# Wikibook on High Order Functions.

Second code snippet under title, Composition Function has following code snippet.

#light open System  let compose f g x = f (g x)  let xSquared x = x*x let negXPlusFive x = -x/2.0 + 5.0  let fog = compose xSquared negXPlusFive  // ... Console.WriteLine statements.... 

The one I am having problem understanding is

let xSquared x = x*x 

When I run it with F# interactive shell (fsi.exe) by itself, I get the following signature.

> let xSquared x = x*x;;  val xSquared : int -> int 

But when I run the whole code snippet, xSquared returns the following.

val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b val xSquared : float -> float val negXPlusFive : float -> float val fog : (float -> float) 

Why does xSquared takes float and returns float?

Answer 1

With more information, F# can determine that xSquared is called with float arguments. If you change negXPlusFive to something like ‘let negXPlusFive x = -x + 5’ you would find that it, fog and xSquared would be ‘int -> int’.