#2893: Implement "Quantified constraints" proposal -------------------------------------+------------------------------------- Reporter: porges | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.10.1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #5927 | Differential Rev(s): Phab:D4353 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I've added the superclass stuff {{{ commit 910dfcfeadc4f132e887bc4adf5ac2e17a29d99b Author: Simon Peyton Jones <simonpj@microsoft.com> Date: Thu Mar 1 23:32:29 2018 +0000 Add superclasses to quantified constraints This patch adds suppport for superclasses to quantified constraints. For example (contrived): f :: (forall a. Ord a => Ord (m a)) => m a -> m a -> Bool f x y = x==y Here we need (Eq (m a)); but the quantifed constraint deals only with Ord. But we can make it work by using its superclass. This behaviour finally delivers on the promise of comment:30 of Trac #9123: we can write an implication constraint that solves the problem of higher-kinded roles. Test quantified-constraints/T8123 demonstrates this in action. compiler/basicTypes/Id.hs | 2 +- compiler/typecheck/Inst.hs | 4 +- compiler/typecheck/TcCanonical.hs | 180 ++++++++++++++--------- compiler/typecheck/TcErrors.hs | 4 +- compiler/typecheck/TcEvTerm.hs | 5 +- compiler/typecheck/TcEvidence.hs | 74 ++++++---- compiler/typecheck/TcHsSyn.hs | 23 ++- compiler/typecheck/TcInstDcls.hs | 2 +- compiler/typecheck/TcInteract.hs | 61 ++++---- compiler/typecheck/TcMType.hs | 6 +- compiler/typecheck/TcPatSyn.hs | 7 +- compiler/typecheck/TcPluginM.hs | 2 +- compiler/typecheck/TcRnTypes.hs | 62 ++++++-- compiler/typecheck/TcSMonad.hs | 119 +++++++-------- compiler/typecheck/TcSimplify.hs | 2 +- compiler/typecheck/TcType.hs | 2 +- compiler/types/Class.hs | 54 ++++--- compiler/types/Kind.hs | 2 + testsuite/tests/quantified-constraints/T2893b.hs | 24 --- testsuite/tests/quantified-constraints/T2893c.hs | 15 ++ testsuite/tests/quantified-constraints/T9123.hs | 24 +++ testsuite/tests/quantified-constraints/T9123a.hs | 26 ++++ testsuite/tests/quantified-constraints/all.T | 5 + 23 files changed, 426 insertions(+), 279 deletions(-) }}} So now all the features I was planning are implemented. Give it a whirl! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:34> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler