The overlap warning checker simply doesn't take account of GADTs. There's a long-standing project suggestion to fix this: http://hackage.haskell.org/trac/ghc/wiki/ProjectSuggestions Perhaps a good GSoc project. Simon From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Conal Elliott Sent: 31 March 2009 03:45 To: glasgow-haskell-users@haskell.org Subject: spurious "non-exhaustive" pattern warnings? -- Why do I get warnings about non-exhaustive pattern matches in the -- following code? Is the compiler just not clever enough to notice that -- the uncovered cases are all type-incorrect? (ghc 6.11.20090115) {-# LANGUAGE TypeFamilies, EmptyDataDecls, TypeOperators , GADTs, KindSignatures , FlexibleInstances, FlexibleContexts #-} {-# OPTIONS_GHC -Wall #-} import Control.Applicative (Applicative(..)) data Z data S n infixr 1 :< data Vec :: * -> * -> * where ZVec :: Vec Z a (:<) :: a -> Vec n a -> Vec (S n) a -- todo: infix op for SVec instance Functor (Vec n) where fmap _ ZVec = ZVec fmap f (a :< u) = f a :< fmap f u instance Applicative (Vec Z) where pure _ = ZVec ZVec <*> ZVec = ZVec -- Warning: Pattern match(es) are non-exhaustive -- In the definition of `<*>': -- Patterns not matched: -- (_ :< _) _ -- ZVec (_ :< _) instance Applicative (Vec n) => Applicative (Vec (S n)) where pure a = a :< pure a (f :< fs) <*> (x :< xs) = f x :< (fs <*> xs) -- Warning: Pattern match(es) are non-exhaustive -- In the definition of `<*>': -- Patterns not matched: -- ZVec _ -- (_ :< _) ZVec