#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Keywords: Resolution: duplicate | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2893 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Hm. That is not at all how I would have expected that instance to be solved. I agree that when typechecking `$dmshowsPrec`, we should start with: * `[W] Show (Exists c)` But don't we get that immediately? In other words, this typechecks: {{{#!hs {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE UndecidableInstances #-} module T5927 where import Text.Show data Exists c where Exists :: c a => a -> Exists c instance Show (Exists c) where show = show showsPrec = undefined showList = undefined }}} Because when typechecking the RHS of `show`, we need `[W] Show (Exists c)`, but we already have that. It's only when we strengthen the context: {{{#!hs instance (forall a. c a => Show a) => Show (Exists c) where show = show showsPrec = undefined showList = undefined }}} That GHC complains: {{{ Bug.hs:12:10: error: • Could not deduce: c (Exists c) arising from a use of ‘show’ from the context: forall a. c a => Show a bound by the instance declaration at Bug.hs:11:10-53 • In the expression: show In an equation for ‘show’: show = show In the instance declaration for ‘Show (Exists c)’ • Relevant bindings include show :: Exists c -> String (bound at Bug.hs:12:3) | 12 | show = show | ^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5927#comment:34 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler