Haskell why does “Num x” require “Show x”?

Question

Recently I took a look at Haskell, using LYAH.

I was messing around with type classes and wrote this quick test function:

foo :: (Num x) => x ->         


        

Answer 1

The information in LYAH is old. The release notes for GHC 7.4.1 say that:

The Num class no longer has Eq or Show superclasses.

You will need to write,

foo :: (Num x, Show x) => x -> String

(In fact, the foo you wrote doesn't require Num x, so you can omit that to avoid an unnecessary constraint.)