[Haskell-cafe] OverloadedLists pattern-matching

[Oliver Charles]

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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