#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Keywords: Resolution: duplicate | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2893 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. This is interesting. We have {{{ $dmshowsPrec :: Show a => Int -> a -> ShowS }}} and the implied instance decl is {{{ instance (forall a. c a => Show a) => Show (Exists c) where show (Exists a) = show a showsPrec = $dmshowsPrec }}} From the RHS of `showsPred` we get `[W] Show (Exists c)`. And ''alas'' we try to solve that from the quantified constraint (which takes precedence over top-level instances), by instantiating `a` to `(Exists c)`. So we get {{{ [W} c (Exists c) }}} which we can't solve. This would happen on any recursive invocation of a method at the same type. It's not clear what we want here. The quantified constraint claims to provide an instance ''for every type a'' which is jolly suspicious. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5927#comment:33 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler