#13061: Incorrect constraints given single flexible undecidable instance -------------------------------------+------------------------------------- Reporter: JCarr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Constraint, Resolution: | UndecidableInstances, | FlexibleInstances Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by JCarr: @@ -14,1 +14,3 @@ - f will have type Eq a => a -> a -> Bool + f will have type Eq a => a -> a -> Bool. + A function of type A a => ... will always be rejected unless Eq is also + included. @@ -21,3 +23,1 @@ - Then f will have the type A a => a -> a -> Bool. - A function of type A a => ... will always be rejected unless Eq is also - included + Then f will have the correct type A a => a -> a -> Bool. New description: If a class has an instance in the form F a => G a in its file, and no other instances, then the functions in G a will have the constraint F a, rather than G a. Example file {{{#!hs {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} class A a where f :: a -> a instance {-# OVERLAPPABLE #-} Eq a => A a where f = id }}} f will have type Eq a => a -> a -> Bool. A function of type A a => ... will always be rejected unless Eq is also included. If we add: {{{#!hs instance A Int where f = id }}} Then f will have the correct type A a => a -> a -> Bool. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13061#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler