[GHC] #15707: More liberally kinded coercions for newtypes

[GHC]

#15707: More liberally kinded coercions for newtypes
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        Reporter:  mniip             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12369            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by mniip:

Old description:

> Consider the infinite data family (possible thanks to #12369):
>
> {{{#!hs
> data family I :: k -> k
> newtype instance I a = I0 (a)
> newtype instance I a x = I1 (a x)
> newtype instance I a x y = I2 (a x y)
> newtype instance I a x y z = I3 (a x y z)
> ...
> }}}
>
> We end up with a family of eta-contracted coercions:
> {{{#!hs
> forall (a :: *). a ~R I a
> forall (a :: k -> *). a ~R I a
> forall (a :: k -> l -> *). a ~R I a
> forall (a :: k -> l -> m -> *). a ~R I a
> ...
> }}}
>
> What if we could somehow indicate that we desire a polykinded newtype (so
> to speak) with a coercion `forall (a :: k). a ~R I a`
>
> Maybe even do so by default: `newtype I a = I0 a` would create such a
> polykinded coercion. Though the `I` data constructor and pattern would
> still only use the *-kinded restriction of it.
>
> We could then recover other constructors with:
> {{{#!hs
> i1 :: a x -> I a x
> i1 = coerce
> ...
> }}}

New description:

 Consider the infinite data family (possible thanks to #12369):

 {{{#!hs
 data family I :: k -> k
 newtype instance I a = I0 (a)
 newtype instance I a x = I1 (a x)
 newtype instance I a x y = I2 (a x y)
 newtype instance I a x y z = I3 (a x y z)
 ...
 }}}

 We end up with a family of eta-contracted coercions:
 {{{#!hs
 forall (a :: *). a ~R I a
 forall (a :: k -> *). a ~R I a
 forall (a :: k -> l -> *). a ~R I a
 forall (a :: k -> l -> m -> *). a ~R I a
 ...
 }}}

 What if we could somehow indicate that we desire a polykinded newtype (so
 to speak) with a coercion `forall (a :: k). a ~R I a`

 Maybe even do so by default: `newtype I a = I0 a` would create such a
 polykinded coercion. Though the `I0` data constructor and pattern would
 still only use the *-kinded restriction of it.

 We could then recover other constructors with:
 {{{#!hs
 i1 :: a x -> I a x
 i1 = coerce
 ...
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15707#comment:1>
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