On Wed, Aug 29, 2012 at 12:01 PM, Philip Holzenspies <pkfh@st-andrews.ac.uk> wrote:

Dear GHCers,

I'm performing traversals over GHC-API results (HsSyn et al). For this purpose, I'm using SYB generics.

I found that I couldn't use "ext1Q" for a function with type "Data x => Bag x -> String", i.e. that this function was never applied. The source of Bag's instance of the Data class seems to explain why:

instance Data a => Data (Bag a) where
gfoldl k z b = z listToBag `k` bagToList b -- traverse abstract type abstractly
toConstr _ = abstractConstr $ "Bag("++show (typeOf (undefined::a))++")"
gunfold _ _ = error "gunfold"
dataTypeOf _ = mkNoRepType "Bag"

Is there a rationale to not allow gunfolds and to keep toConstr abstract?

As far as I understand, this is to keep `Bag` itself abstract, preventing users from inspecting its internals.

More to the point for my needs, is there a reason to not allow dataCast1 casting of Bags?

That is a separate issue; I believe this instance is just missing a `dataCast1 = gcast1` line.
All datatypes of kind `* -> *` should have such a definition.

(Having a look at Data.Data, I guess the same applies to `Ptr a` and `ForeignPtr a`.
And `Array a b` seems to be missing the `dataCast2` method. I propose fixing all of these.)

Cheers,
Pedro

Regards,
Philip
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