#13140: Handle subtyping relation for roles in Backpack -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature request | Status: patch Priority: normal | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: backpack hs- Resolution: | boot Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3123 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Edward Z. Yang <ezyang@…>): In [changeset:"923d7ca2d90c1cb9816d14768abdd2e46adcd5dd/ghc" 923d7ca2/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="923d7ca2d90c1cb9816d14768abdd2e46adcd5dd" Subtyping for roles in signatures. Summary: This commit implements the plan in #13140: * 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. * Critically, abstract types are NOT injective, so we aren't allowed to make inferences like "if T a ~R T b, then a ~N b" based on the nominal role of a parameter in an abstract type (this would be unsound if the parameter ended up being phantom.) This restriction is similar to the restriction we have on newtypes. Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu> Test Plan: validate Reviewers: simonpj, bgamari, austin, goldfire Subscribers: goldfire, thomie Differential Revision: https://phabricator.haskell.org/D3123 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13140#comment:13> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler