#10338: GHC Forgets Constraints -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by crockeea): I've got a new test case for what I assume is this bug. Unfortunately, I can't find a workaround in this case. {{{#!hs {-# LANGUAGE MultiParamTypeClasses, ScopedTypeVariables, ConstraintKinds, KindSignatures, TypeFamilies, FlexibleInstances #-} import GHC.Prim import Control.Monad class Unsat (a :: * -> *) c class Qux (t :: * -> *) where type QCtx t q :: Constraint qux :: (QCtx t q, Monad rnd) => v -> rnd (t q) instance Qux t where type QCtx t q = (Unsat t q) class (Qux c) => C (c :: * -> *) r mymap :: c Double -> c i mymap = undefined foo :: forall c z v rnd . (C c z, Monad rnd, Num z) => v -> rnd (c z) foo svar = liftM mymap $ qux svar I've tried a wide array of explicit type sigs on `foo`, but nothing seems to make it find the dictionary for `Qux` inherited from `C`. I really need a fix for this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10338#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler