[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
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

Old description:

> 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]`".

New description:

 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 commenting it out and writing `Map Pred` 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#comment:1>
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