The problem here isn't quite what you think it is; (==) is not a constructor, therefore it is a *variable*. It's exactly the same problem as

    a = 5

    -- ...

    foo a = 3 -- this does NOT compare with the previous value of "a"; it's identical to the next line!

    foo x = x

Hm, I thought it is a pattern match with constant, as in f ('a':xs) ==