Re: [Haskell-cafe] OverloadedLists pattern-matching

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