#11534: Allow class associated types to reference functional dependencies -------------------------------------+------------------------------------- Reporter: ekmett | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.3 checker) | Keywords: TypeFamilies, Resolution: | FunctionalDependencies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): Replying to Simon:
This would make it possible for given constraints involving fundeps to work properly, which they don't at present (as Iavor points out).
Iavor is mostly complaining that currently you can declare instances that break FunDeps Consistency. (And because GHC knows that, it is conservative in applying type improvement via FunDeps. Also his examples either break the FunDeps Coverage Condition -- so needing `UndecidableInstances`; or use dependents (rhs of FunDeps arrow) with no free vars, so there's no algorithmic determinism.) If we desugar FunDeps to Type Families, and allow them to be Closed Type Families, then we can carry on breaking FunDeps Consistency, via overlapping equations. If OTOH we desugar FunDeps to Open Type Families, then we can't write overlapping equations; therefore we can't define (for example) a type- level Type Equality test. (This is somewhat orthogonal to whether we can write overlapping instances for the Class.) I can see the combination of FunDeps and `OverlappingInstances` is deadly for consistency. (To be precise, overlap of the determinants for FunDeps, on lhs of the arrow.) I can also see how terrifically useful it is -- for example for all sorts of record-like systems from HList onwards. I think I could make an argument that actually the combo FunDeps+Overlapping is consistent. That would need going back to the database Relational Theory from which Mark Jones borrowed FunDeps. And would require some discipline in what instances you declare. (I think most people actually observe that discipline, but GHC doesn't enforce it. I have an idea how we could help GHC enforce it -- a very old, recycled idea from Tom Schrijvers/CHRs.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11534#comment:32 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler