#13061: Incorrect constraints given single flexible undecidable instance. Adding more clarification of inconsistency -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- Changes (by JCarr): * type: bug => feature request @@ -15,2 +15,10 @@ - A function of type A a => ... will always be rejected unless Eq is also - included. + Consider the functions + {{{#!hs + g :: A a => a -> a + g = f + h :: A a => a -> a + h = let f' = f in f + i = f' where + f' = f + }}} + h and i will both fail to typecheck. @@ -24,0 +32,4 @@ + + 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. 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 :: A a => a -> a g = f h :: A a => a -> a 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:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler