#8634: Relax functional dependency coherence check ("liberal coverage condition") -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: feature | Status: new request | Milestone: 7.10.1 Priority: high | Version: 7.7 Component: Compiler | Keywords: Resolution: | Operating System: Unknown/Multiple Differential Revisions: Phab:D69 | Type of failure: None/Unknown Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: #1241, | #2247, #8356, #9103, #9227 | -------------------------------------+------------------------------------- Comment (by danilo2): @diatchki please do not base your opinion on the examples above - they are a little old and of course, they do not obey some basic principles. The idea with `-XDysfunctionalDependencies` is just to lift both the Paterson Conditions and the Coverage Condition - something `-XDysfunctionalDependencies` claims to do (according to documentation), but does not (as simonpj noticed above). When using this extension you can just give hints to typechecker and compile programs like the one I've posted on https://phabricator.haskell.org/D69 : {{{#!haskell {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE DysfunctionalDependencies #-} class CTest a b | a -> b where ctest :: a -> b data X = X instance Monad m => CTest X (m Int) where ctest _ = return 5 main = print (ctest X :: [Int]) -- [5] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler