Improve 'patterns not matched' warnings

[Deewiant]

Compiling the following with -fwarn-incomplete-patterns:

module Asdf where

f :: String -> Int
f "0" = 0

g :: Int -> Int
g 0 = 0

Yields:

asdf.hs:4:0:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `f':
                 Patterns not matched:
                     []
                     (GHC.Base.C# #x) : _ with #x `notElem` ['0']
                     (GHC.Base.C# '0') : (_ : _)

asdf.hs:7:0:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `g':
                 Patterns not matched: GHC.Base.I# #x with #x `notElem` [0#]

Losing the 'GHC.Base' stuff along with the various octothorpes would make the error messages a lot nicer. Ideally it'd be something like Patterns not matched: x where x notElem [0] for the second case, for instance.

[simonpj]

Yes indeed. I think it'd make sense to do this as part of #595. Which is currently awaiting a keen author.

Simon

[morabbin]

Exhibits problems described in #2204