#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 simonpj): Several points * Your script in comment:37 typechecks just fine if you use `-XScopedTypeVariables`. You need the 's' in the body of `tst` to be the same as the one in the type signature. * Interestingly in this example, unlike earlier ones, the instances for `Property` really do morally satisfy the (liberal) Coverage condition: the first parameter determines the second. And GHC knows this. For example, this is accepted right now (with `UndecidableInstances` so that we get the liberal coverage condition): {{{ class Property a b | a -> b where -- Note the fundep property :: a -> b instance (y ~ Y) => Property X y where -- Suppose this was accepted property _ = Y }}} And now you don't need the `:: s` signature in the body of `tst` * It's also worth remembering that ALL fundeps do in GHC is guide the type inference engine. And you can offer the same guidance via type signatures. So you should be alle to fix your 600-module system by adding some type signatures. * Martin's excellent example, explained in detail in comment:30, is a pretty convincing reason for not rushing ahead with lifting the coverage condition altogether, especially given that there is another workaround (type signatures). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8634#comment:39 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler