Re: Proposal: Add Data instance for Const

2 Nov 2016

      Ah, right. I forgot about this additional wrinkle - GHC only checks the
kinds of each type variable at the definition site, not at the site of the
Data instance. And Const is poly-kinded, so its second type variable is of
kind k, not *, so that's why GHC forgoes defining dataCast2 here. My bad.

So in this case, I suppose it doesn't hurt to relax the second constraint
to Typeable. There's the separate issue of whether this is a "correct" Data
instance, but GHC won't allow us to derive dataCast2 for the Data (Const a
b) instance in the first place, so it's a bit of a moot point.

Ryan S.

On Wed, Nov 2, 2016 at 4:17 PM, Index Int  wrote:
...
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?
...
...
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
On Wed, Nov 2, 2016 at 10:27 PM, Ryan Scott 
wrote:
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  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  wrote:
...
Definitely an oversight.
On Wed, Nov 2, 2016 at 11:40 AM, Ryan Scott 
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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