#8958: Allow role inference on datatype contexts ------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Currently, role inference on a datatype examines the use of a type parameter in all constructors of that datatype. This, of course, includes any constraint contexts on these datatypes. But, the (stupid) datatype context is not consulted, as there is no need to do so for type safety. However, by examining the datatype context, we can create a lightweight way of annotating roles, like this: {{{ class Nominal a instance Nominal a type role Nominal nominal -- this is redundant, but here for completeness class Representational a instance Representational a type role Representational representational -- this requires -XIncoherentInstances class Phantom a -- any use of this class is redundant, but here for completeness instance Phantom a type role Phantom phantom data (Nominal k, Representational v) => Map k v = ... }}} Because of the universal instances, these constraints never get in the way. This is admittedly an abuse of existing constructs, but it seems so useful that it's worth a little abuse. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8958 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler