Re: Proposal: Add Data instance for Const

2 Nov 2016

      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  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  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
_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries