Actually, fixing this adds a nice feature to this whole area: the ability to have free conversions within a library but not to export this capability! If we have a newtype with a nominal role annotation, its constructor might be visible only among "friendly" modules, allowing the free conversion. Then, outside of the package, the constructor is inaccessible, so no conversions are possible. This ability was a desideratum at the beginning of the design process that we thought we
#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): Replying to [comment:11 goldfire]: threw away when we went with the idea of using class instances. But, now we have it back! True, but only for newtypes, there is currently no way for „I want to coerce under `Set` in my own modules only.“, that would require the are- all-type-constructors-in-scope test again. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9117#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler