#10635: -fwarn-redundant-constraints should not be part of -Wall -------------------------------------+------------------------------------- Reporter: Lemming | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.11 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #9939, #9973, | Differential Rev(s): #10100, #10183, #11370 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by konn): I need more flexible control on `-Wredundant-constraints`, too. I'm currently maintaining library to provide algebraic structures. In such a situation, we need '''empty''' class without any member function to express the additional algebraic constraint. Suppose we have following classes: {{{ class Multiplicative r where (*) :: r -> r -> r class Commutative r -- without any member functions! }}} Until here, everything goes well. Suppose we want to write algorithm relying on the commutativity of multiplication: {{{ myGreatAlgorithm :: (Commutative r) => r -> r myGreatAlgorithm = ... }}} Then GHC warns about `Commutative r` as a "redundant constraint", although it is essentially needed to work algorithm properly. Since `Commutative` class doesn't have any member functions, workaround adding "dead code", which is introduced in GHC users' guide, doesn't work at all. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10635#comment:23 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler