#14292: Coercing between constraints of newtypes -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: 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 RyanGlScott): Replying to [comment:3 AntC]:
`IncoherentInstances` is a module-wide setting; and nowadays deprecated in favour of the more surgical `{-# INCOHERENT #-}` pragma on the instance.
Does that also apply for the role mungling?
As far as I know, you do need to enable the `IncoherentInstances` extension proper in order to assign non-nominal roles to class parameters, as the `{-# INCOHERENT #-}` pragma only works for instance declarations, not class declarations. (This might be partly why you don't get a deprecation warning when enabling `IncoherentInstances`, but you do with `OverlappingInstances`, which doesn't have a dual purpose like `IncoherentInstances` does.)
How does the `{-# INCOHERENT #-}` pragma go for `StandaloneDeriving` or `GeneralizedNewtypeDeriving`? It seems to be accepted by GHC OK, does it have the right effect? (It's hard to put together a test case.)
I'm not sure I understand the question. Derived instances never use `{-# INCOHERENT #-}` (or even `{-# OVERLAPS #-}`/`{-# OVERLAPPING #-}`/`{-# OVERLAPPABLE #-}`, for that matter) in their emitted code, if that's what you're wondering. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14292#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler