This smells wrong. tcInferRho ends up calling tcInfer, which makes an OpenKind ReturnTv. But then we unify it with a liftedTypeKind ReturnTv later.
Complicated and silly. Better if tcInferRho takes the kind it is expecting. Then we have only one ReturnTv, not two; and the whole ReturnTv story is located in tcInfer, rather than having a non-return-ish hack in
#9404: tcInferRho muddies the waters -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | Blocking: | Differential Revisions: Phab:D469 | -------------------------------------+------------------------------------- Comment (by goldfire): On Phab:D469, Simon says, about line 328 of !TcExpr: the typing rule for $
Make sense?
I've decided: no, this doesn't make sense. The call to `tcInferRho` is to infer the type of the left-hand argument to `$`. This type is a function type `arg -> res`. If it turns out to be unlifted, the `matchExpectedFunTys` will fail. The silliness with the new `ReturnTy` bit isn't about the result of `tcInferRho` -- it's about the result of `matchExpectedFunTys`. So the straightforward refactoring Simon proposes doesn't work. Instead, what we should do here is to unify `arg2_ty`'s kind with `*`. But !TcUnify doesn't have a function for that -- something like `matchExpectedStar`. The closest thing -- `unifyKindX` tells us about sub- kinding relationships, instead of actually unifying the kinds. So, I propose: do nothing for now. It's working, although a little uglily, as is. When we have proper kind equalities and no sub-kinding anymore, this will be easy to fix. Please close the ticket if you agree. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9404#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler