#16123: QuantifiedConstraints fails to deduce trivial constraint -------------------------------------+------------------------------------- Reporter: eschnett | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While experimenting with quantified constraints, I find I need to explicitly specify a quantified constraint in an instance declaration that I think should be deduced automatically. When I leave it out, GHC complains that it cannot deduce {{{Num (f a)}}} in the code below. The full source code is attached. The relevant lines are {{{ instance ( Functor f, Dom f ~ NFun, Cod f ~ NFun , forall a. Ok NFun a => Num a -- I don't want to write this constraint ) => Functor (NIdF f) where type Dom (NIdF f) = Dom f type Cod (NIdF f) = Cod f fmap f = NFun \(NIdF xs) -> NIdF (neval (fmap f) xs) }}} where the line marked "I don't want to write ..." points to the constraint I don't want to have to write manually. The definition of the {{{NFun}}} is {{{ instance Category NFun where type Ok NFun a = Num a id = NFun P.id NFun g . NFun f = NFun (g P.. f) }}} Note that the constraint in question does not involve the type {{{NIdF}}} for which the instance is declared – it is a generic condition that is always true, given the instance declaration of {{{NFun}}} (see the definition of {{{Ok}}} there). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16123 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler