#12823: Inconsistency in acceptance of equality constraints in different forms -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: GADTs | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently, we (correctly) require a language extension to accept a declaration like foo :: a ~ b => f a -> f b foo x = x Suppose I write {{{#!hs {-# LANGUAGE GADTs, UndecidableInstances, ConstraintKinds #-} module A where class a ~ b => Equal a b instance a ~ b => Equal a b type EqualS a b = a ~ b }}} and then {{{#!hs -- No extensions module B where -- This works useEqual :: Equal a b => f a -> f b useEqual x = x -- But this does not useEqualF :: EqualF a b => f a -> f b useEqualF x = x }}} It seems that GHC expands type synonyms, but does not reduce constraints, before deciding whether enough extensions are in play to allow equality constraints. This mismatch feels weird. Is there something about `~` proper, and not `Equal`, that triggers the difficulties with let generalization? If not, I think they should either both work or both fail (probably both fail). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12823 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler