#15359: Quantified constraints do not work with equality constraints -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Simon and I discussed this today. The lack of symmetry between `Coercible` and `~` here is by design. Essentially, any equality implication constraint is guaranteed to be redundant, because GHC can already deduce all equalities from whatever assumptions are at hand. On the other hand, `Coercible` implication constraints are quite useful, because coercibility is fundamentally incomplete. So the trouble I saw isn't so bad. But I still think we should document the restriction and report a better error message here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15359#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler