#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 simonpj):
Redundant in what sense?
Well, can you show me a quantified constraint with an equality in the head that is not useless? For example what about this (by analogy with `Coercible`) {{{ f :: forall m. (forall a b. a ~ b => m a ~ m b) => blah }}} No, that redundant. If we have `[W] t1 a ~ t2 b` there is a built-in rule to decompose it to `[W] t1 ~ t2` and `[W] a ~ b`. (There is no such rule in general for `Coercible` which is why we need to allow it.) So that quantified constraint wasn't useful. Can you think of one that is? I can't. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15359#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler