#5144: Pattern synonyms -------------------------------------+------------------------------------ Reporter: simonpj | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: _|_ Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by cactus): Progress update: the following module demonstrates all the features implemented so far: {{{ {-# LANGUAGE PatternSynonyms #-} module PatSyn where -- The example from the wiki page data Type = App String [Type] deriving Show pattern Arrow t1 t2 = App "->" [t1, t2] pattern Int = App "Int" [] collectArgs :: Type -> [Type] collectArgs (Arrow t1 t2) = t1 : collectArgs t2 collectArgs _ = [] isInt Int = True isInt _ = False arrows :: [Type] -> Type -> Type arrows = flip $ foldr Arrow -- Simple pattern synonyms pattern Nil = [] pattern Cons x xs = x:xs zip' :: [a] -> [b] -> [(a, b)] zip' (Cons x xs) (Cons y ys) = Cons (x, y) (zip' xs ys) zip' Nil _ = Nil zip' _ Nil = Nil pattern One x = [x] one :: [a] -> Maybe a one (One x) = Just x one _ = Nothing singleton :: a -> [a] singleton x = One x -- Pattern only synonyms pattern Third x = _:_:x:_ third :: [a] -> Maybe a third (Third x) = Just x third _ = Nothing -- This causes a type error: invalid x = Third x -- PatSyn.hs:30:13: -- Third used in an expression, but it's a non-bidirectional pattern synonym -- In the expression: Third x -- In an equation for ‛invalid’: invalid x = Third x -- Failed, modules loaded: none. }}} The following module also works, but demonstrates the clunkiness caused by the lack of infix pattern synonyms: {{{ {-# LANGUAGE ViewPatterns, PatternSynonyms #-} import qualified Data.Sequence as Seq pattern Empty = (Seq.viewl -> Seq.EmptyL) pattern Cons x xs = (Seq.viewl -> x Seq.:< xs) pattern Snoc xs x = (Seq.viewr -> xs Seq.:> x) zipZag :: Seq.Seq a -> Seq.Seq b -> Seq.Seq (a, b) zipZag (Cons x xs) (Snoc ys y) = (x, y) Seq.<| zipZag xs ys zipZag _ _ = Seq.empty }}} Of course, implementing infix pattern synonyms should be easy. Still missing: exporting of pattern synonyms. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5144#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler