#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas -------------------------------------+------------------------------------- Reporter: simonpj | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.2 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): One more thing. Look at this note in `InstEnv`: {{{ Note [Incoherent instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ For some classes, the choice of a particular instance does not matter, any one is good. E.g. consider class D a b where { opD :: a -> b -> String } instance D Int b where ... instance D a Int where ... g (x::Int) = opD x x -- Wanted: D Int Int For such classes this should work (without having to add an "instance D Int Int", and using -XOverlappingInstances, which would then work). This is what -XIncoherentInstances is for: Telling GHC "I don't care which instance you use; if you can use one, use it." Should this logic only work when *all* candidates have the incoherent flag, or even when all but one have it? The right choice is the latter, which can be justified by comparing the behaviour with how -XIncoherentInstances worked when it was only about the unify-check (note [Overlapping instances]): Example: class C a b c where foo :: (a,b,c) instance C [a] b Int instance [incoherent] [Int] b c instance [incoherent] C a Int c Thanks to the incoherent flags, [Wanted] C [a] b Int works: Only instance one matches, the others just unify, but are marked incoherent. So I can write (foo :: ([a],b,Int)) :: ([Int], Int, Int). but if that works then I really want to be able to write foo :: ([Int], Int, Int) as well. Now all three instances from above match. None is more specific than another, so none is ruled out by the normal overlapping rules. One of them is not incoherent, but we still want this to compile. Hence the "all-but-one-logic". }}} That makes sense, but it doesn't implement the rule in the manual (comment:20). I think the last bullet should therefore read {{{ * If exactly one non-incoherent candidate remains, pick it. If all remaining candidates are incoherent, pick an arbitary one. Otherwise fail. }}} Do you agree? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9242#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler