#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 simonpj): Interesting. HEAD reports {{{ T10715.hs:8:13: error: Could not deduce: Coercible a (X a) from the context: Coercible a (X a) }}} which is even more confusing! Here's what is happening * GHC treats `Coercible a ty` as a representational equality constraint `a ~R ty`. * Given an equlity `a ~R ty`, it tries to rewrites other constraints mentioning `a`, by replacing `a` with `ty`. * But here it can't do that, because `ty` mentions `a`, so we could rewrite forever; a kind of occurs-check problem. And indeed, if we had something like {{{ f :: (a ~ [a]) => blah }}} we really wouldn't expect much to happen because the constraint can't possibly be satisfied. * However, if the given equality is `a ~R X b`, we CAN use it to rewrite the wanted constraint, to get `X b ~R X a`. And that is soluble by decomposition because `X`'s first argument is phantom. Hence your "Surprisingly to me" discovery. * If you write a type signature like {{{ f :: (a ~ [a]) => blah }}} you get an error like {{{ T10715.hs:15:8: error: Couldn't match type ‘a’ with ‘[a]’ ‘a’ is a rigid type variable bound by the type signature for: foo :: (a ~ [a]) => a -> [a] -> Bool at T10715.hs:15:8 Inaccessible code in the type signature for: foo :: (a ~ [a]) => a -> [a] -> Bool }}} Maybe we should do the same for `Coercible`? That would at least give a better error message than "can't deduce A from A"! * It wouldn't help your use-case. But do you have to use `Y` in this strange recursive way. Why not do this? {{{ newtype FY = FY (ForeignPtr Y) data Y }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10715#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler