#10592: Allow cycles in class declarations -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.1 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. I think it'd be easy to send GHC into an infinite loop using this; i.e. the current implementation is over-liberal. {{{ type instance FieldScalar x = Field x }}} Is it true that you expect `Scalar (Scalar a) ~ Scalar a)` (cf #10318). If so, you could make it better behaved by saying {{{ class (Scalar (Scalar a) ~ Scalar a, Field (Scalar a)) => Vector a class Vector x => Field x }}} This still isn't accepted but could be under comment:6 of #10318 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10592#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler