#13601: GHC errors but hangs -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: TypeInType, | LevityPolymorphism 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): The `uo_thing` field was introduced in order to keep error messages reasonably stable during the `TypeInType` merge. Pre-`TypeInType`, kind errors reported what type had the kind that was involved in the error, and I wanted to keep this functionality post-`TypeInType`. That's the only reason the field exists. It was (and is) my hope to use `uo_thing` also for term-level error messages, as I think we can improve these through its use. One of the grim aspects of `uo_thing` is that `unifyType`'s type starts with `Outputable a => Maybe a -> ...` in order to use (possibly) something as the thing in the `uo_thing` field. But passing `Nothing` to `unifyType` causes ambiguity errors, because we can't know what `a` should be. So I have `noThing :: Maybe (HsExpr Name); noThing = Nothing` to resolve the ambiguity. I've been meaning to clean this up. Another missed opportunity after `TypeInType` is to use `TcTyCon` more fully. Type-checking can produce `TcTyCon`s instead of `TyCon`s, with the latter rewriting to the former only during the final zonk. This would mean that the type-checking knot needs to cover only zonking, instead of type- checking. That would surely fix this bug. In more direct response to comment:4, I'm dubious. If `solveEqualities` fails, then there is something ill-kinded lurking about, and I'm worried that some `failIfErrsM` and such won't fire, causing chaos. It may be possible to rein in the chaos, but I don't think it will be an easy win. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13601#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler