Re: [GHC] #9429: Alternative to type family Any

3 Nov 2017

      #9429: Alternative to type family Any
-------------------------------------+-------------------------------------
        Reporter:  mboes             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.9
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  9097, 9380        |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Apologies for reviving a long-dormant thread, but my understand is now
 that #12369 has been fixed, there is a way to achieve something like the
 `Any` of yore—just use a data family with a polymorphic return kind. That
 is:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}

 data family Any :: k
 }}}

 Since `Any` is a data family, it has a `Typeable` instance:

 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}

 import Type.Reflection

 data family Any :: k

 main :: IO ()
 main = print $ typeRep @(Any :: *)
 }}}
 {{{
 $ ~/Software/ghc/inplace/bin/runghc Foo.hs
 Any *
 }}}

 Moreover, it inhabits every kind and is a distinguishable element, so you
 can write things like this:

 {{{#!hs
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}

 import GHC.TypeLits
 import Type.Reflection

 data family Any :: k

 type family Foo (a :: Bool) :: Symbol where
   Foo False = "It's false"
   Foo Any   = "It's Any"
   Foo True  = "It's true"

 main :: IO ()
 main = do
   print $ typeRep @(Foo False)
   print $ typeRep @(Foo Any)
   print $ typeRep @(Foo True)
 }}}
 {{{
 $ ~/Software/ghc/inplace/bin/runghc Foo2.hs
 "It's false"
 "It's Any"
 "It's true"
 }}}

 Does this work for your use cases?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9429#comment:31
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