[GHC] #13140: Handle subtyping relation for roles in Backpack

[GHC]

#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 at 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