#14451: Type synonym of type family causes error, error jumps to (fairly) unrelated source location -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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 simonpj): I don't think this is #11203. Here's what I think is happening * `Apply`, `Comp`, and `OO` (which I will use instead of the unicode dot symbol) are mutually recursive, and so are kind-checked together. * The first two have CUSKs and so `getInitialKinds` gives them polykinds, thus {{{ kcTyClGroup: initial kinds [r1 :-> ATcTyCon Apply :: forall a b. (a ~> b) -> a -> b, r2 :-> ATcTyCon Comp :: forall k1 k2 k. (k1 ~> k) -> (k2 ~> k1) -> k2 -> k, r4 :-> ATcTyCon OO :: k_aYg[tau:1] -> k_aYh[tau:1] -> k_aYi[tau:1]] }}} Good so far. * But then pathetically, we run over all three declarations, gathering constraints on the kind variables in `OO`. In the end we make `OO` insufficiently polymorphic, giving it kind {{{ OO :: forall b. (b ~> b) -> b -> b }}} whereas it should have kind {{{ OO :: forall a b. (a ~> b) -> a -> b }}} That's stupid: `OO`'s kind should obviously be the same as `Apply`'s What we ''should'' do is exactly what we do for value declarations: * Take the SCC (the group of three declarations) * Remove edges that point to type constructors that have a CUSK * Do a new SCC on this thinned-out graph. * Now `OO` will be first, in a SCC by itself, because `Apply` and `Comp` depend on it, but not vice versa (after removing those edges) * Now kind-check each smaller SCC one by one, generalising each in turn. For bindings you can see the extra SCC analysis being done in `TcBinds.tc_group`. So all we have to do is to replicate this logic for types. Fiddly, perhaps, but not difficult. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14451#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler