The constructors exposed via Data do not have to be the actual constructors of the data type. With this you can quotient out the portions of the structure you don't want the user to be able to inspect.
See the libraries@ proposal that I put in 3-4 weeks ago (which will have just passed) to fix all the broken Data instances for containers by using virtual constructors such as 'fromList', (which incidentally led to Milan finding huge space and time improvements in fromList).
Effectively allowing the user to use the 'listToBag' as a "constructor" loses no information violates no invariants, and prevents code written for uniplate, SYB, etc. from having to crash, panic or give up upon the sight of a mkNoRepType.
My reaction for years to the sight of a mkNoRepType and undefined gunfold has been to hang my head. Now I just fix them.
-Edward
On Wed, Aug 29, 2012 at 7:11 AM, José Pedro Magalhães
<jpm@cs.uu.nl> wrote:
Hi Philip,
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
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