#12088: Type/data family instances in kind checking -------------------------------------+------------------------------------- Reporter: alexvieth | Owner: Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #11348, #12239 | Differential Rev(s): Phab:D2272 Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): The nub of my idea in comment:13 and comment:15 is this: * Track dependency on ''declarations'' independently from ''definitions''. * When `T`'s declaration depends on `S`'s declaration, then `T`'s definition depends on `S`'s definition. * Dependency tracking is, as usual, transitive. So in the smaller example in comment:35, `F`'s declaration depends on `Open`'s. So `F`'s definition -- the instances -- depends on `Open`'s. This means we type check `Open`'s instances before `F`'s. There's lots more detail (comment:15) but this is the basic idea. Here is a case (due to Iavor) that would be rejected: {{{ type family Open a type instance Open Int = Bool type instance Open Char = F FLoat type instance Open Float = Type type family F a :: Open a type instance F Int = True type instance F Char = [True] type instance F Float = [Bool] }}} The instances for a given type family are checked as a clump, and these instances can be accepted only by interleaving. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12088#comment:36 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler