#14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints, wipT2893 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): Interesting. When inferring a type, GHC finds all the unsolved constraints. Supopse they are `(Eq a, Ord a)`. Then it minimises them in a very simple-minded way, dicarding any that are implied by the superclasses of some other constraint. So, since `Eq a` is a superclass of `Ord a`, we discard it. So we infer a type like {{{ f :: Ord a => blah }}} Now suppose we have `(Bifunctor bi, Functor (bi a))`. Yes, the latter is implied by the former, but now it is much more indirect. I'm not yet sure how to do a better job here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14896#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler