Re: [GHC] #15782: Visible type/kind applications in declaration of data/type constructors

19 Oct 2018

      #15782: Visible type/kind applications in declaration of data/type constructors
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12045            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:
...
I'm making this ticket to keep track of this, I don't know if it's a good
idea.
Allow visible type/kind applications when declaring data/type
constructors on the LHS of `::` (from
[https://phabricator.haskell.org/D5229#144558 Phab comment]).

...
{{{#!hs
data
    Proxy :: forall k. k -> Type where
  MkProxy :: Proxy @k (a :: k)
-- can be written
data
    Proxy @k :: k -> Type where
  MkProxy @k :: Proxy @k (a :: k)
}}}
&
{{{#!hs
data
  Fin  :: N -> Type where
  FinO ::          Fin (S n)
  FinS :: Fin n -> Fin (S n)
-- can be written
data
  Fin     :: N -> Type where
  FinO @n ::          Fin (S n)
  FinS @n :: Fin n -> Fin (S n)
}}}
New description:

 I'm making this ticket to keep track of this, I don't know if it's a good
 idea.

 Allow visible type/kind applications when declaring data/type constructors
 on the LHS of `::` (from [https://phabricator.haskell.org/D5229#144558
 Phab comment]).

 {{{#!hs
 data
     Proxy @k :: k -> Type where
   MkProxy @k :: Proxy @k (a :: k)
 }}}

 &

 {{{#!hs
 data
   Fin     :: N -> Type where
   FinO @n ::          Fin (S n)
   FinS @n :: Fin n -> Fin (S n)
 }}}
 &

 {{{#!hs
 data
   Elem  @k @n  :: Vec n k -> Type where
   ElemO @a @as :: Elem @k @(S n) (a:>as)
   ElemS @a @as :: Elem @k @n     as
                -> Elem @k @(S n) (a:>as)
 }}}

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15782#comment:2
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