[Haskell-cafe] OverloadedLists pattern-matching

[Konstantine Rybnikov]

Continuing playing with OverloadedLists and GHC 7.8.2. For this code:

```
{-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeFamilies #-}

import Data.Map
import Data.Text
import GHC.Exts

instance (Ord k) => IsList (Map k v) where
  type Item (Map k v) = (k,v)
  fromList = Data.Map.fromList
  toList   = Data.Map.toList

main :: IO ()
main = do
    let m = [("foo", 1), ("bar", 2)]
          :: Map Text Int
    putStrLn "My map looks like this:"
    case m of
      [] -> putStrLn "impossible!"
      [x,_] -> putStrLn $ "ok, some random elem is: " ++ show x

    print m
```

I have this output from compiler:

```
root at b575c8a9c84b:~/overloaded_lists# cabal build
--ghc-options="-fforce-recomp"
Building overloaded-lists-0.1.0.0...
Preprocessing executable 'overloaded-lists' for overloaded-lists-0.1.0.0...
[1 of 1] Compiling Main             ( src/Main.hs,
dist/build/overloaded-lists/overloaded-lists-tmp/Main.o )

src/Main.hs:19:5: Warning:
    Pattern match(es) are non-exhaustive
    In a case alternative: Patterns not matched: _

src/Main.hs:9:10: Warning:
    Orphan instance: instance Ord k => IsList (Map k v)
[1 of 1] Compiling Main             ( src/Main.hs,
dist/build/overloaded-lists/overloaded-lists-tmp/Main.o )

src/Main.hs:19:5: Warning:
    Pattern match(es) are non-exhaustive
    In a case alternative: Patterns not matched: _

src/Main.hs:9:10: Warning:
    Orphan instance: instance Ord k => IsList (Map k v)
Linking dist/build/overloaded-lists/overloaded-lists ...
```

Couple things:

1. It shows same warnings two times. Is this a bug?
2. It doesn't seem to be that warning that pattern "_" wasn't matched
should be there. Should I also create a bug report?

Thank you!
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