#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 simonpj): Ah, hmm, I think I see. Given something like {{{ f (toList -> []) = e1 f (toList -> (a:as)) = e2 }}} we'd like to declare this function exhaustive. (Clearly it is!) The `ListPat` case of `Check.translatePat` deals with a very special sub- case. * Using `OverloadedLists`, we generate a special kind of view pattern, encoded in the `ListPat` constructor of `Pat`: {{{ | ListPat (XListPat p) [LPat p] (PostTc p Type) -- The type of the elements (Maybe (PostTc p Type, SyntaxExpr p)) -- For rebindable syntax -- For OverloadedLists a Just (ty,fn) gives -- overall type of the pattern, and the toList -- function to convert the scrutinee to a list value }}} * You are checking that the function actually has type `[t1] -> [t2]`, where `normalise t1 = normalise t2`. * In that case, you discard the `toList` view thing altogether, and pretend (for the purposes of pattern-match checking) that you wrote {{{ f [] = e1 f (a:as) = e2 }}} This would not be valid if you'd had {{{ f (toList -> []) = e1 f (weirdToList -> (a:as)) = e2 }}} with two different `toList` functions. But in this special case the view function was injected by the type-driven overloading machinery, so we can be confident (I think) that both functions are the same. So ''at least'' all this should be explained in a Note. But it also seems like a special case that is easily defeated. For example, what about this: {{{ f :: T -> blah f (toList -> []) = ... f (toList -> (x:xs) = ... }}} This too is exhaustive, but it won't be recorded as such, even if you use `OverloadedLists`. And of course if you use explicit view patterns, things are even worse. At least the Note should point out these deficiencies -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler