#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): During the exhaustiveness checking, we already know the type of `[]`, we use `splitListTyConApp_maybe` to check whether it's type is `[a]` (list) or not and extract its element type. {{{#!hs splitListTyConApp_maybe :: Type -> Maybe Type splitListTyConApp_maybe ty = case splitTyConApp_maybe ty of Just (tc,[e]) | tc == listTyCon -> Just e _other -> Nothing }}} Indeed we don't need to know the `toList` of `[]` instance is identity function, we know the type of the pattern is `[]` then the `toList` is omitted and it is enough to know `[]` is the nil pattern. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler