#15351: QuantifiedConstraints ignore FunctionalDependencies -------------------------------------+------------------------------------- Reporter: aaronvargo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
this doesn't explain what specifically GHC "checks it against" in this case
It's worth reading the original paper -- though alas I cannot find PDF online. Given {{{ class C a b | a -> b instance C Int Bool }}} if GHC finds a wanted constraint `[W] C Int t`, it compares it against all the top-level instances to see if the fist argument of `C` matches the one in the instance. It succeeds in this case, and emits a new Derived constraint `[D] t ~ Bool`. Quantified constraints behave very like top-level instances, so they too should be examined in this search. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15351#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler