#14799: QuantifiedConstraints: Problems with Typeable -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This compiles {{{#!hs {-# Language GADTs, ConstraintKinds, ScopedTypeVariables, QuantifiedConstraints #-} import Data.Typeable data Ex cls where MkEx :: cls xx => xx -> Ex cls a :: Ex Typeable -> TypeRep a (MkEx (_::xx)) = typeRep @_ @xx Proxy -- b :: (forall xx. cls xx => Typeable xx) => Ex cls -> TypeRep -- b (MkEx (_::xx)) = typeRep @_ @xx Proxy c :: (forall xx. cls xx => Show xx) => Ex cls -> String c (MkEx xx) = show xx }}} but uncommenting `b` is too much: {{{ $ qc hs/189-bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/189-bug.hs, interpreted ) hs/189-bug.hs:12:20: error: • Could not deduce (Typeable xx) arising from a use of ‘typeRep’ from the context: forall xx. cls xx => Typeable xx bound by the type signature for: b :: forall (cls :: * -> Constraint). (forall xx. cls xx => Typeable xx) => Ex cls -> TypeRep at hs/189-bug.hs:11:1-60 or from: cls xx bound by a pattern with constructor: MkEx :: forall xx (cls :: * -> Constraint). cls xx => xx -> Ex cls, in an equation for ‘b’ at hs/189-bug.hs:12:4-15 • In the expression: typeRep @_ @xx Proxy In an equation for ‘b’: b (MkEx (_ :: xx)) = typeRep @_ @xx Proxy | 12 | b (MkEx (_::xx)) = typeRep @_ @xx Proxy | ^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} This is maybe because of the `Typeable` solver. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14799 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler