#12040: Code that builds on 7.8.4 and 7.10.3 but fails with requiring UndecidableInstances on 8.0.1-rc4 -------------------------------------+------------------------------------- Reporter: dmcclean | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
Maybe a consequence of kind and type arguments being treated uniformly in GHC's Core language now?
Yes, probably. Writing the kind argument in we get {{{ instance (HasDimension (Proxy * d)) => Demotable (Dimensional Int d) where }}} and now indeed the bit before the arrow doesn't look smaller than the thing after. I think the solution here is to use `UndecideableInstances`. One could wonder about some kind-aware size metric might be better --- but I don't know what it would be. I'm inclined to say "wont-fix"; but if someone has a better size-metric proposal, go propose it. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12040#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler