Re: [GHC] #11174: Traversable can't be derived for datatypes with unboxed arguments

7 Dec 2015

      #11174: Traversable can't be derived for datatypes with unboxed arguments
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by RyanGlScott:

Old description:
...
Unlike `Functor` and `Foldable`, `Traversable` cannot be derived for
datatypes that contain arguments with unboxed types. A simple example:
{{{#!hs
{-# LANGUAGE DeriveTraversable, MagicHash #-}
import GHC.Prim (Int#)
data IntHash a = IntHash Int# deriving (Functor, Foldable, Traversable)
}}}
The generated `Traversable` instance reveals the issue:
{{{#!hs
instance Traversable IntHash where
  traverse f (IntHash a1) = fmap IntHash (pure a1)
}}}
{{{
    Couldn't match kind `*' with `#'
    When matching types
      a0 :: *
      Int# :: #
    Expected type: a0 -> IntHash b
      Actual type: Int# -> IntHash b
    In the first argument of `fmap', namely `IntHash'
    In the expression: fmap IntHash (pure a1)
    When typechecking the code for  `traverse'
      in a derived instance for `Traversable IntHash':
      To see the code I am typechecking, use -ddump-deriv
}}}
We have to avoid calling `pure` on `a1`, since `pure` expects an argument
with a `*`-kinded type, not a `#`-kinded one.
One way to fix this would be restructuring the derived `traverse`
implementation such that unboxed arguments are moved to the function
initially lifted with `pure`, and doing nothing with them later. To
better articulate what I mean, envision something like this:
{{{#!hs
data IntHash2 a = IntHash2 Int# a (IntHash2 a) Int# deriving (Functor,
Foldable)
}}}
Then a derived `Traversable` instance that would typecheck would be:
{{{#!hs
instance Traversable IntHash2 where
  traverse f (IntHash2 a1 a2 a3 a4) =
    pure (\x2 x3 -> IntHash2 a1 x2 x3 a4) <*> f a2 <*> traverse f a3
}}}
Conceptually, this doesn't sound hard to implement. The tricky part is
figuring out how much of the existing `Functor`/`Foldable`/`Traversable`
deriving machinery would need to be tweaked to make this work.
New description:

 Unlike `Functor` and `Foldable`, `Traversable` cannot be derived for
 datatypes that contain arguments with unboxed types. A simple example:

 {{{#!hs
 {-# LANGUAGE DeriveTraversable, MagicHash #-}

 import GHC.Prim (Int#)

 data IntHash a = IntHash Int# deriving (Functor, Foldable, Traversable)
 }}}

 The generated `Traversable` instance reveals the issue:

 {{{#!hs
 instance Traversable IntHash where
   traverse f (IntHash a1) = fmap IntHash (pure a1)
 }}}

 {{{
     Couldn't match kind `*' with `#'
     When matching types
       a0 :: *
       Int# :: #
     Expected type: a0 -> IntHash b
       Actual type: Int# -> IntHash b
     In the first argument of `fmap', namely `IntHash'
     In the expression: fmap IntHash (pure a1)
     When typechecking the code for  `traverse'
       in a derived instance for `Traversable IntHash':
       To see the code I am typechecking, use -ddump-deriv
 }}}

 We have to avoid calling `pure` on `a1`, since `pure` expects an argument
 with a `*`-kinded type, not a `#`-kinded one.

 One way to fix this would be restructuring the derived `traverse`
 implementation such that arguments which do not mention the last type
 parameter are moved to the function initially lifted with `pure`, and
 doing nothing with them later. To better articulate what I mean, envision
 something like this:

 {{{#!hs
 data IntHash2 a = IntHash2 Int# a (IntHash2 a) Int deriving (Functor,
 Foldable)
 }}}

 Then a derived `Traversable` instance that would type-check (and kind-
 check) would be:

 {{{#!hs
 instance Traversable IntHash2 where
   traverse f (IntHash2 a1 a2 a3 a4) =
     pure (\x2 x3 -> IntHash2 a1 x2 x3 a4) <*> f a2 <*> traverse f a3
 }}}

 Conceptually, this doesn't sound hard to implement. The tricky part is
 figuring out how much of the existing `Functor`/`Foldable`/`Traversable`
 deriving machinery would need to be tweaked to make this work.

--

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11174#comment:1
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