An example of missing some kinds of type constraints.

Lookup.hs

module Lookup where

import Prelude hiding (lookup)

lookup :: a -> [(a, b)] -> Maybe b  -- valid if the type a is a kind of Eq instances
lookup k [] = Nothing
lookup k ((k', v):ls) = if k == k' then Just v else lookup k ls

result

Lookup.hs:7:30:
    No instance for (Eq a)
      arising from a use of `=='
    In the expression: k == k'
    In the expression: if k == k' then Just v else lookup k ls
    In an equation for `lookup':
        lookup k ((k', v) : ls) = if k == k' then Just v else lookup k ls