[GHC] #13962: GHCi allows unsaturated type family

[GHC]

#13962: GHCi allows unsaturated type family
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Taken from [http://ics.p.lodz.pl/~stolarek/_media/pl:research:promotion-
 haskell14-slides.pdf Promoting Functions to Type Families in Haskell]

 {{{#!hs
 {-# Language TypeFamilies, PolyKinds, DataKinds, TypeOperators #-}

 type family
   Map (f :: a -> b) (xs :: [a]) :: [b] where
   Map f '[]       = '[]
   Map f (x ': xs) = f x ': Map f xs

 data N = O | S N

 type family
   Pred (n :: N) :: N where
   Pred O     = O
   Pred (S n) = n

 -- The type family ‘Map’ should have 2 arguments, but has been given 1
 type MapPred = Map Pred
 }}}

 fails as expected, but it works in GHCi

 {{{
 ghci> :kind Map Pred
 Map Pred :: [N] -> [N]
 ghci> :kind! Map Pred '[S (S O), S O, O]
 Map Pred '[S (S O), S O, O] :: [N]
 = '['S 'O, 'O, 'O]
 }}}

 I can only test this in GHC 7.8, 8 now. I know GHCi has weird rules for
 unsaturated type families but I figured I'd write a ticket because the
 slides say "But it is invalid to write: `Map Pred '[O, S O]`".

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13962>
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