#13140: Handle subtyping relation for roles in Backpack -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 (Type checker) | Keywords: backpack hs- | Operating System: Unknown/Multiple boot | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If I understand correctly, GHC's three currently supported roles follow the following subtyping relation: `phantom <: representational <: nominal`. So it would make sense to adjust Backpack and `hs-boot` files to handle this subtyping relation appropriately. Here's my proposal. * Today, roles in signature files default to representational. Let's change the default to nominal, as this is the most flexible implementation side. If a client of the signature needs to coerce with a type, the signature can be adjusted to have more stringent requirements. * If a parameter is declared as nominal in a signature, it can be implemented by a data type which is actually representational. * When merging abstract data declarations, we take the smallest role for every parameter. The roles are considered fix once we specify the structure of an ADT. I actually don't know if the proofs about roles actually say anything about this subtyping relation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13140 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler