[GHC] #14292: Coercing between constraints of newtypes

[GHC]

#14292: Coercing between constraints of newtypes
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I wouldn't say that this is rejected due to a fundamental limitation so
 much as a deliberate design choice. The immediate reason that the former
 program is rejected is due to roles. By default, all class parameters are
 given role `nominal`, so you can't coerce between `Num a` and `Num b`
 unless `a ~ b`.

 Why is the case? Even if `a` and `b` are representationally equal, there's
 absolutely no guarantee that their corresponding `Num` dictionaries are
 also representationally equal. After all, a `Num` instance for `USD` might
 do something crazy like `fromInteger _ = USD 42`. Defaulting class
 parameters to `nominal` is thus a conservative way to avoid the
 shenanigans that would unfold if you treated a `Num USD` dictionary as a
 `Num Int` one, or vice versa.

 Now you might object: "But I'm a careful programmer! I promise to only use
 `Num Int` and `Num USD` dictionaries that are representationally
 equivalent!" In that case, there is a fallback mechanism in the form of
 `IncoherentInstances`:

 {{{#!hs
 {-# Language ConstraintKinds #-}
 {-# Language GADTs #-}
 {-# Language IncoherentInstances #-}
 {-# Language RoleAnnotations #-}

 import Data.Coerce
 import Prelude hiding (Num(..))

 newtype USD = USD Int

 data Dict c where
   Dict :: c => Dict c

 type role Num representational
 class Num a where
   -- ...

 instance Num Int
 instance Num USD

 num :: Dict (Num Int) -> Dict (Num USD)
 num = coerce
 }}}

 I hope it should be obvious from the name `IncoherentInstances` alone that
 this fallback carries significant risks. Use this trick at your own
 discretion.

 Does that answer the question satisfactorily? If so, please feel free to
 close the ticket.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14292#comment:1>
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