Effective Haskell

Specifying Type Class Instances with Type Applications

A common problem when you’re using type class constraints is ambiguity about which specific type class instance should be used. Consider, for example, a program you want to have print out the additive and multiplicative identities of some Natural numbers. You might write something like this:

showIdentities =
  let mul = multiplicativeIdentity
      add = additiveIdentity
      msg = "The additive identity is: "
            <> show add
            <> " and the multiplicative identity is: "
            <> show mul
  in print msg

Unfortunately, this fails to compile! The problem is that multiplicativeIdentity and additiveIdentity both return a type that depends on the type class instance that we’re using, but the compiler ...