[GHC] #11594: closed empty type families fully applied get reduced lazily when in a constraint tuple and fully applied

[GHC]

#11594: closed empty type families  fully applied get reduced lazily when in a
constraint tuple and fully applied
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      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:                    |
-------------------------------------+-------------------------------------

Comment (by carter):

 the meat of it is i want the folllowing to not type check

 {{{
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}

 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeFamilies, TypeOperators #-}
 {-# LANGUAGE DataKinds, GADTs #-}
 {-# LANGUAGE TypeInType, ConstraintKinds #-}

 module MyLIbrary(sevenBad) where

 import GHC.Types (Constraint,TYPE,Levity(..))

 -- these two only only report an error once I
 ---resolve the constraint on a to something like Int etc
 sevenBad :: (ClosedStuckSilly 'True , Num a) => a
 sevenBad = 7


 type family ClosedStuckSilly (x :: a) :: b  where

 }}}

 if i instead write something like


 {{{
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}

 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeFamilies, TypeOperators #-}
 {-# LANGUAGE DataKinds, GADTs #-}
 {-# LANGUAGE TypeInType, ConstraintKinds #-}

 module MyLIbrary(sevenBadWrapped) where

 import GHC.Types (Constraint,TYPE,Levity(..))

 -- these two only only report an error once I resolve
 ---the constraint on a to something like Int etc
 sevenBad :: (ClosedStuckSilly 'True , Num a) => a
 sevenBad = 7

 sevenBadWrapped :: Num a => a
 sevenBadWrapped = sevenBad

 type family ClosedStuckSilly (x :: a) :: b  where


 }}}

 i'll get the type error i want, but that, I fear, wont scale very well in
 terms of usability for more complex codes

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11594#comment:2>
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