[GHC] #13140: Handle subtyping relation for roles in Backpack
GHC
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Sat Feb 11 01:00:24 UTC 2017
#13140: Handle subtyping relation for roles in Backpack
-------------------------------------+-------------------------------------
Reporter: ezyang | Owner: ezyang
Type: feature request | Status: patch
Priority: normal | Milestone: 8.2.1
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: |
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Comment (by goldfire):
Sorry for not piping up sooner, but this change would be unsound. Consider
this accepted program:
{{{
data T a
type role T nominal
foo :: Coercible (T a) (T b) => a -> b
foo x = x
}}}
You can see that, if `T` were abstract, instantiating `T` with a concrete
datatype that had a phantom role would be Wrong. While there is a sub-
roling relationship, it does not induce a sub-typing relationship like the
one you seek.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13140#comment:2>
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