Hi, I'm working on a data structure that uses Data.Sequence a lot, so views are important and I tried to simplify my code using view patterns. The problem is, that I keep getting warnings about both overlapping and non-exhaustive pattern matches. A simple test case: ===============================T.hs=============================== {-# LANGUAGE ViewPatterns #-} import Data.Sequence test :: Seq a -> Seq b -> String test (viewl -> EmptyL) (viewl -> EmptyL) = "empty, empty" test (viewl -> EmptyL) (viewl -> _ :< _) = "empty, non-empty" test (viewl -> _ :< _) (viewl -> EmptyL) = "non-empty, empty" test _ _ = "non-empty, non-empty" ==================================================================

ghci -Wall T.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( T.hs, interpreted )

T.hs:6:0: Warning: Pattern match(es) are overlapped In the definition of `test': test ((viewl -> EmptyL)) ((viewl -> EmptyL)) = ... test ((viewl -> EmptyL)) ((viewl -> _ :< _)) = ... test ((viewl -> _ :< _)) ((viewl -> EmptyL)) = ... test _ _ = ... T.hs:6:0: Warning: Pattern match(es) are non-exhaustive In the definition of `test': Patterns not matched: Ok, modules loaded: Main. *Main> test empty (singleton 'a') "empty, non-empty" *Main> test (singleton 'b') (singleton 'a') "non-empty, non-empty" *Main> test (singleton 'b') empty "non-empty, empty" *Main> test empty empty "empty, empty" There are warnings about non-exhaustive and overlapping pattern matches, but the tests show that this isn't the case. So what's the problem? I don't want to turn off or ignore warnings. //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr