#9117: Coercible constraint solver misses one -------------------------------------+------------------------------------ Reporter: goldfire | Owner: nomeata Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): In the paper comments you write
Suppose module `A` uses `coerce` somewhere and contains a newtype whose constructor is not imported. We now modify `A` to import the constructor. Is it guaranteed that the use(s) of `coerce` will still work? We would want to offer such a guarantee I think.
We are unable to give this guarantee with either of the two solving strategies. Consider: {{{#!haskell module Foo where newtype Rec a = Rec a newtype Hide a = Hide (Rec a) module Bar where import Foo (Hide(Hide), Rec) ex :: Rec Age -> Hide Int ex = coerce }}} This works (`Hide` is unrolled, then we can lift through `Rec` and are done). If we also import `Rec`, then `Coercible (Rec Age) (Hide Int)` (where no lifting is possible yet, so the order does not matter) will cause it to loop. I couldn’t construct it with a less pathological newtype yet, though, as these will cause a proper data tycon to appear, then the newtypes on the RHS get unwrapped and eventually the terms on both sides match. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9117#comment:24 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler