Proposal: Add Data instance for Const

[Index Int]

You propose to add the line

  deriving instance (Data a, Data b) => Data (Const a b)

to Data.Data in base. I entered it into GHCi with -ddump-deriv and
this is what I got:

  ghci> deriving instance (Data a, Data b) => Data (Const a b)

  ==================== Derived instances ====================
  Derived instances:
    instance (Data.Data.Data a_a53b, Data.Data.Data b_a53c) =>
             Data.Data.Data (Data.Functor.Const.Const a_a53b b_a53c) where
      Data.Data.gfoldl k_a53f z_a53g (Data.Functor.Const.Const a1_a53h)
        = (z_a53g Data.Functor.Const.Const `k_a53f` a1_a53h)
      Data.Data.gunfold k_a53i z_a53j _
        = k_a53i (z_a53j Data.Functor.Const.Const)
      Data.Data.toConstr (Data.Functor.Const.Const _)
        = Ghci4.$cBHDVlUg6FI9L4iOviG5mer
      Data.Data.dataTypeOf _ = Ghci4.$tBHDVlUg6FI9L4iOviG5mer

    Ghci4.$tBHDVlUg6FI9L4iOviG5mer :: Data.Data.DataType
    Ghci4.$cBHDVlUg6FI9L4iOviG5mer :: Data.Data.Constr
    Ghci4.$tBHDVlUg6FI9L4iOviG5mer
      = Data.Data.mkDataType "Const" [Ghci4.$cBHDVlUg6FI9L4iOviG5mer]
    Ghci4.$cBHDVlUg6FI9L4iOviG5mer
      = Data.Data.mkConstr
          Ghci4.$tBHDVlUg6FI9L4iOviG5mer
          "Const"
          ["getConst"]
          Data.Data.Prefix

So GHC does not generate a definition for dataCast2 (and it default to
`const Nothing`, I suppose). I tried the same with the constraint
relaxed to Typeable and it worked. Am I missing something?

On Wed, Nov 2, 2016 at 10:27 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>> The proposed instance is overconstrained. You only need Typeable for the
>> second argument of Const (because it is phantom).
>
> It's a phantom type, but the Data constraint is necessary because the way
> deriving Data works. If  You can read this [1] for an explanation behind
> this design decision. Essentially, if GHC sees that a datatype has two type
> parameters of kind *, then it generates a definition for the dataCast2
> method, which allows for a higher-order version of the cast function. But
> implementing dataCast2 requires that both type parameters be Data instances.
>
> A separate question would be whether implementing dataCast2 could be done
> without these Data constraints (and thus allowing the second type parameter
> to only be an instance of Typeable, rather than Data). But that is outside
> my area of expertise; I'd need someone more knowledgeable in the arts of
> Data.Data than I.
>
> For now, I am proposing what GHC currently considers to be a canonical Data
> instance for Const. We can revisit the exact instance context details later
> if need be.
>
> Ryan S.
> -----
> [1] https://ghc.haskell.org/trac/ghc/ticket/4028
>
> On Wed, Nov 2, 2016 at 3:16 PM, Index Int <vlad.z.4096 at gmail.com> wrote:
>>
>> The proposed instance is overconstrained. You only need Typeable for
>> the second argument of Const (because it is phantom).
>>
>> On Wed, Nov 2, 2016 at 7:05 PM, Edward Kmett <ekmett at gmail.com> wrote:
>> > Definitely an oversight.
>> >
>> > On Wed, Nov 2, 2016 at 11:40 AM, Ryan Scott <ryan.gl.scott at gmail.com>
>> > wrote:
>> >>
>> >> GHC Trac #12438 [1] exists because there's no Data instance for Const
>> >> in base. I found this quite surprising, since we have Data instances
>> >> for just about every other type combinator out there (Identity, Sum,
>> >> Product, Compose, etc.), but not for Const. The fix for #12438 would
>> >> be quite simple: I propose we add
>> >>
>> >>     deriving instance (Data a, Data b) => Data (Const a b)
>> >>
>> >> to Data.Data in base. Any objections?
>> >>
>> >> Ryan S.
>> >> -----
>> >> [1] https://ghc.haskell.org/trac/ghc/ticket/12438
>> >> _______________________________________________
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>> >
>> >
>> >
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>
>