#12102: “Constraints in kinds” illegal family application in instance (+ documentation issues?) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Good questions. Here are my thoughts: - Satisfying kind-level equality constraints is implemented in `Inst.tcInstBinderX`, called when a type is applied to some arguments. The code there handles both unboxed equality and boxed equality. - The "no `case`" problem in Simon's comment:5 is quite true. But this is OK, because such an equality constraint can never be a Given: constraints in types can't be used within the same type, but (I believe) these constraints scope only over a type (never a term). - This last point makes these new constraints somewhat like datatype contexts, but one does not desugar into the other. - Clearly, the output from `:info` is horrible. - The "no `forall` needed" is an interaction with CUSKs. This point should be clarified in the manual. - "Constrained type families" interacts poorly with today's story for kind families: constrained type families requires the use of class constraints, but class constraints aren't currently allowed in kinds. It would seem that it's best to implement constrained type families in the context of Dependent Haskell. - Bottom line: this feature is probably a misfire. It ''is'' marginally useful, as I think the example from the manual demonstrates. (That seems useful to me, at least.) But the implementation is very ad-hoc, and the fact that these constraints never appear as Givens take much of the air out of them. It would be easy enough to remove this feature for 8.2. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12102#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler