#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: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: #1241, #2247, None/Unknown | #8356, #9103, #9227 Test Case: | Blocking: | Differential Revisions: Phab:D69 | -------------------------------------+------------------------------------- Comment (by simonpj): Situation is this. In comment:30 I show (using Martin's example) that lifting the liberal coverage condition (which is what `-XDysFunctionalDependencies` proposes to do) means that type inference becomes unpredicatable. On Monday it might succeed, but after a minor change to the order in which constraints are examined, on Tuesday it might fail. This is not a happy situation. I did not realise this when I wrote comment:14. On the other hand, danilo2 (I wish I knew your real name) is saying strongly that `=XDysFunctionalDependencies` would really help him. It is possible that his problem could be solved in some other way, but that would take a serious investment of time to find out, and danilo2 believes not. So what to do? We could treat it like `unsafeCoerce`: you can use it, but then you are on your own. (Maybe it should have an even more discouraging name e.g. `-XUnpredicatbleFunctionalDependencies`.) I worry, though, that people will bump into problems, and submit bug reports, ... Any other opinions? danilo2, are you still arguing for this despite the difficulties? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:50 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler