Re: [GHC] #16013: :kind! accepts unsaturated type aliases

9 Dec 2018

      #16013: :kind! accepts unsaturated type aliases
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        Reporter:  dmwit             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13962            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 My own vote is that we drop the saturation requirement only at top-level,
 calling the current implementation a bug. To my horror, GHC allows this
 interaction:

 {{{#!hs
 type family Map f x where
   Map _ '[] = '[]
   Map f (x : xs) = f x : Map f xs

 data Nat = Zero | Succ Nat

 type family Pred n where
   Pred Zero = Zero
   Pred (Succ n) = n
 }}}

 {{{
 λ> :kind! Map Pred [Succ (Succ Zero), Succ (Succ (Succ Zero)), Zero]
 Map Pred [Succ (Succ Zero), Succ (Succ (Succ Zero)), Zero] :: [Nat]
 = '[ 'Succ 'Zero, 'Succ ('Succ 'Zero), 'Zero]
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16013#comment:5
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