Why is GHC complaining about non-exhaustive patterns?

Question

When I compile the following code with GHC (using the -Wall flag):

module Main where

data Tree a = EmptyTree | Node a (Tree a) (Tree a) derivin         


        

Answer 1

It's because the pattern matching is incomplete. There's no guarantee that one of x==a, x, or x>a holds. For instance, if the type is Double and x is NaN then none of them are True.