[Haskell-cafe] OverloadedLists pattern-matching

[Konstantine Rybnikov]

Just FYI, this still gives a warning:

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

import Data.Map (Map)
import qualified Data.Map as M
import Data.Text
import GHC.Exts

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

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



On Sun, Apr 13, 2014 at 12:05 PM, Konstantine Rybnikov <k-bx at k-bx.com>wrote:

> Oh! Right, sorry about that.
>
>
> On Sun, Apr 13, 2014 at 12:01 PM, Oliver Charles <ollie at ocharles.org.uk>wrote:
>
>> You have only pattern matched the empty list and a two element list.
>> Perhaps you meant (x : _) to match at least one element?
>>
>> - ocharles
>> On 13 Apr 2014 10:58, "Konstantine Rybnikov" <k-bx at k-bx.com> wrote:
>>
>>>  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!
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>
>>>
>
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