#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 goldfire): I've created #15354 about the documentation oversight -- you're absolutely right. I agree with your analysis in comment:4. By the constraint `forall a. C (f a) Int` (and the fundep), we know that any instance that begins with `C (f a)` ends with `Int`. Let's name different variables differently: {{{#!hs bar :: (forall b. C (f b) Int) => f a -> String bar = show . foo }}} So, since the usage of `foo` induces a `C (f a) alpha` constraint (for some unknown `alpha`) we can conclude that `alpha` must be `Int` by the fundep. Here is a related example, with the same `C`: {{{#!hs blah :: C Int Double => Int -> String blah = show . foo }}} This is accepted (even with no instances for `C`), because GHC can figure out that the result of `foo` must be `Double`. I don't see how quantified constraints should change that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15351#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler