[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
           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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 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.

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