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 wrote:

Oh! Right, sorry about that.

On Sun, Apr 13, 2014 at 12:01 PM, Oliver Charles 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" 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@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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On Sun, Apr 13, 2014 at 12:24 PM, Roman Cheplyaka wrote:

* Konstantine Rybnikov [2014-04-13 12:21:44+0200]

...

Just FYI, this still gives a warning: ... case m of [] -> putStrLn "empty" (M.toList -> (x:_)) -> putStrLn $ "ok, some random elem is: " ++ show x

Does that surprise you? The compiler doesn't have any special knowledge about the M.toList function to infer that these two cases are exhaustive.

Roman is right, and I think it's clearer if you consider this without view patterns: case m of [] -> ... m' -> case M.toList m' of (x : _) -> ... Looking at this, it's clear that the patterns are not exhaustive - you didn't match the scenario that M.toList m' produced an empty list. *You* know that M.toList (M.fromList []) == [], but GHC doesn't - and I think this is where the problem lies. With that information you might be able to have exhaustive pattern matches there. - ocharles