[GHC] #8155: Defaulting bug or unfortunate error message with closed type families

[GHC]

#8155: Defaulting bug or unfortunate error message with closed type families
------------------------------------+-------------------------------------
       Reporter:  nh2               |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 https://github.com/nh2/infinite-type-families/blob/master/Test2.hs

 Some code extracted from hmatrix:

 {{{
 {-# LANGUAGE CPP #-}

 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FlexibleInstances #-}

 module Test where


 data Vector a = Vector a
 data Matrix a = Matrix a

 class Build f where
     build' :: BoundsOf f -> f -> ContainerOf f



 #if __GLASGOW_HASKELL__ <= 706
 -- Normal type families with GHC 7.6, works well.

 type family BoundsOf x
 type instance BoundsOf (a->a)    = Int
 type instance BoundsOf (a->a->a) = (Int,Int)

 type family ContainerOf x
 type instance ContainerOf (a->a) = Vector a
 type instance ContainerOf (a->a->a) = Matrix a

 #else
 -- Closed type families for GHC 7.8

 type family BoundsOf x where
     BoundsOf (a->a->a) = (Int,Int)
     BoundsOf (a->a)    = Int

 type family ContainerOf x where
     ContainerOf (a->a)    = Vector a
     ContainerOf (a->a->a) = Matrix a

 #endif


 instance (Num a) => Build (a->a) where
     build' = buildV


 buildV :: (Integral a, Num b) => a -> (b -> c) -> Vector c
 buildV _ _ = undefined
 }}}

 This is probably related to #8154.


 With GHC 7.7, we get the following error:

 {{{
 Test2.hs:59:14:
     Could not deduce (BoundsOf (a -> a) ~ Integer)
     from the context (Num a)
       bound by the instance declaration at Test2.hs:58:10-32
     Expected type: BoundsOf (a -> a)
                    -> (a -> a) -> ContainerOf (a -> a)
       Actual type: Integer -> (a -> a) -> Vector a
     Relevant bindings include
       build' :: BoundsOf (a -> a) -> (a -> a) -> ContainerOf (a -> a)
         (bound at Test2.hs:59:5)
     In the expression: buildV
     In an equation for ‛build'’: build' = buildV
 }}}

 1) Where does this Integer come from?

 2) Change

    {{{
    BoundsOf (a->a)    = Int
    }}}

    into

    {{{
    BoundsOf (a->a)    = Integer
    }}}

    You get the same error message, although clearly {{{BoundsOf (a -> a) ~
 Integer}}}
    (you just wrote that down).[[BR]]
    Even if the order matters in closed type families, is this not the
 wrong error message?

 3) If you flip the order so that the (a->a) case comes first, it works
 (with both it Int or Integer). I don't quite understand why.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8155>
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