[jstepien]

Speaking of the first example, compiling it with -Wall provides some hints:

  $ ghc --make test.hs -Wall
  test.hs:1:1:
      Warning: Pattern match(es) are non-exhaustive
               In an equation for `fn':
                   Patterns not matched: #x with #x `notElem` [0#]

[teaspoon]

Wow, that's cool. It even works for non-Bounded argument types, as in fn [0] = 0:

  Warning: Pattern match(es) are non-exhaustive
        In an equation for `fn':
            Patterns not matched:
                []
                #x : _ with #x `notElem` [0#]
                0# : (_ : _)