[Haskell-cafe] OverloadedLists pattern-matching

[Konstantine Rybnikov]

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!
>>
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>>
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