#10715: Possible regression in Coercible a (X a) between 7.8 and 7.10 -------------------------------------+------------------------------------- Reporter: inaki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:3 simonpj]:
Maybe we should do the same for `Coercible`?
The problem is that occurs-checks for representational equality don't necessarily mean failure. For example `a ~R b a` is solvable if `b` becomes the `Identity` newtype. However, I ''do'' think that `a ~R X a` is an error if `X` is known to be a datatype. (That is, generative w.r.t. representational equality.) But this will still fail: {{{ oops :: Coercible a (b a) => a -> b a oops = coerce }}} Perhaps a way forward is to detect when an occurs-check problem has happened and add a `NB: The solver for Coercible constraints is incomplete` in the error? Not really sure what to do here, beyond making occurs-check a hard failure when there is a generative type somewhere. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10715#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler