#13061: Incorrect constraints given single flexible undecidable instance. -------------------------------------+------------------------------------- Reporter: JCarr | Owner: Type: feature request | 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: @@ -17,1 +17,1 @@ - g :: A a => a -> a + g, h, i :: A a => a -> a @@ -19,1 +19,0 @@ - h :: A a => a -> a 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. Consider the functions {{{#!hs g, h, i :: A a => a -> a g = f h = let f' = f in f i = f' where f' = f }}} h and i will both fail to typecheck. If we add: {{{#!hs instance A Int where f = id }}} Then f will have the correct type A a => a -> a -> Bool. This has been closed previously, but the contrast between g and h above is worth the reconsideration, as it makes some valid functions not definable unless an instance is added. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13061#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler