#1158: Problem with GADTs and explicit type signatures -------------------------------------+------------------------------------- Reporter: guest | Owner: simonpj Type: bug | Status: new Priority: lowest | Milestone: ⊥ Component: Compiler (Type | Version: 6.6 checker) | Keywords: Resolution: | MultiParamTypeClasses, | AllowAmbiguousTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * keywords: => MultiParamTypeClasses, AllowAmbiguousTypes * component: Compiler => Compiler (Type checker) @@ -1,3 +1,5 @@ - {{{ - - {-# OPTIONS_GHC -fglasgow-exts #-} + {{{#!hs + {-# LANGUAGE GADTs #-} + {-# LANGUAGE MultiParamTypeClasses #-} + {-# LANGUAGE FlexibleInstances #-} + {-# LANGUAGE AllowAmbiguousTypes #-} @@ -25,1 +27,0 @@ - @@ -29,11 +30,15 @@ - test.hs:45:14: - Overlapping instances for LiftToExp a a11 - arising from use of `liftToExp' at test.hs:45:14-24 - Matching instances: - instance (Floating a) => LiftToExp a b -- Defined at test.hs:19:0 - instance LiftToExp (Exp a) a -- Defined at test.hs:16:0 - (The choice depends on the instantiation of `a, a11' - Use -fallow-incoherent-instances to use the first choice above) - In the first argument of `App', namely `(liftToExp x)' - In the expression: App (liftToExp x) - In the definition of `test': test x = App (liftToExp x) + Test.hs:48:15: error: + • Overlapping instances for LiftToExp a a0 + arising from a use of ‘liftToExp’ + Matching givens (or their superclasses): + LiftToExp a a1 + bound by the type signature for: + test :: LiftToExp a a1 => a -> Exp b + at Test.hs:47:1-38 + Matching instances: + instance LiftToExp a b -- Defined at Test.hs:22:10 + instance LiftToExp (Exp a) a -- Defined at Test.hs:19:10 + (The choice depends on the instantiation of ‘a, a0’) + • In the first argument of ‘App’, namely ‘(liftToExp x)’ + In the expression: App (liftToExp x) + In an equation for ‘test’: test x = App (liftToExp x) @@ -42,4 +47,0 @@ - - Tested with GHC 6.6 (compiler and interpreter) under OS X 10.4.8 on an - iMac G5. - New description: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE AllowAmbiguousTypes #-} module Main where data Exp a where Val :: a -> Exp b App :: Exp a -> Exp b instance Show (Exp a) where show (Val _) = "Val" show (App _) = "App" class LiftToExp a b where liftToExp :: a -> Exp b instance LiftToExp (Exp a) a where liftToExp = id instance Floating a => LiftToExp a b where liftToExp v = Val v :: Exp b {- Uncommenting the type signature below causes GHCi to fail to load the file: Test.hs:48:15: error: • Overlapping instances for LiftToExp a a0 arising from a use of ‘liftToExp’ Matching givens (or their superclasses): LiftToExp a a1 bound by the type signature for: test :: LiftToExp a a1 => a -> Exp b at Test.hs:47:1-38 Matching instances: instance LiftToExp a b -- Defined at Test.hs:22:10 instance LiftToExp (Exp a) a -- Defined at Test.hs:19:10 (The choice depends on the instantiation of ‘a, a0’) • In the first argument of ‘App’, namely ‘(liftToExp x)’ In the expression: App (liftToExp x) In an equation for ‘test’: test x = App (liftToExp x) However typing :t test at the GHCi prompt gives this exact signature. -} --test :: (LiftToExp a a1) => a -> Exp b test x = App (liftToExp x) main = putStrLn $ show (test (3.0::Float)::Exp Int) }}} -- Comment: This example now requires `AllowAmbiguousTypes` (ghc-8.0.1). Is this still considered to be a bug, or are people who enable `AllowAmbiguousTypes` "asking for it"? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1158#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler