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

Haskell, both 98 and 2010 both require all instances of Num to also be instances on Show and Eq. This is largely an accident of history.

GHC, the most popular Haskell compiler, diverges from the standard here without requiring any pragma. This was done to allow applicative functors to be instances of Num and enjoy the benefits of overloaded syntax.