#14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)? -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs #-} import Data.Kind data Dict c where Dict :: c => Dict c class (a => b) => Implies a b instance (a => b) => Implies a b type a :- b = Dict (Implies a b) iota :: (Implies () a) :- a iota = Dict }}} GHC claims that it can't deduce `(() :: Constraint)` :) {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/H.hs, interpreted ) /tmp/H.hs:14:8: error: • Could not deduce () :: Constraint arising from a use of ‘Dict’ from the context: Implies () :: Constraint a bound by a quantified context at /tmp/H.hs:1:1 Possible fix: add () :: Constraint to the context of the type signature for: iota :: forall (a :: Constraint). Implies () :: Constraint a :- a • In the expression: Dict In an equation for ‘iota’: iota = Dict | 14 | iota = Dict | ^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14833 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler