[Hs-Generics] xml deserialization using generics?

[oleg at pobox.com]

Hugh Perkins wrote:
> Cant decide whether this list is appropriate for questions related to
> generics usage, or only to generics design?

I think your question is quite appropriate -- actually, it is a good
question. It asks essentially for a monadic gmap, which seems quite
important.  This operation is also the `inverse' of one of our
comparison tests: listify. The latter collects values from the leaves
of a data type in a list. That list is a `state' of a value, so to
speak. The state can be transported over the network, and, on the
other end, can be infused into a prototype object (assuming it has the
same structure).

The monadic gmap subsumes two traversals: generic map and
reduction. So, your question looks even better.

I wonder if we should add monadic gmap to our test suite?

I have just committed the monadic gmap implementation and the test for
one generic library, the one I'm more familiar with. The test is in
the file

 http://darcs.haskell.org/generics/comparison/SmashA/Deserialize.hs

We start with the data type of a company 

C [D "Research" (E (P "Laemmel" "Amsterdam") (S 8000.0)) 
    [PU (E (P "Joost" "Amsterdam") (S 1000.0)),
     PU (E (P "Marlow" "Cambridge") (S 2000.0))],
   D "Strategy" (E (P "Blair" "London") (S 100000.0)) []]

(see http://darcs.haskell.org/generics/comparison/CompanyDatatypes.hs)

We can serialize it:

 test1 = serialize genCom

obtaining a list

 ["Research","Laemmel","Amsterdam","8000.0","Joost","Amsterdam",
  "1000.0","Marlow","Cambridge","2000.0","Strategy","Blair",
 "London","100000.0"]

OTH, we can take the list

retro = ["Metaphysics",
	 "Kant","Koeningsberg","800.0",
	 "Hume","Edinburgh","100.0",
	 "Marlowe","Cambridge","200.0",
	 "Ruling","Thatcher","London","50000.0"]

and use it to `upgrade' the company genCom:

test2 = deserialize genCom retro

giving us

 *Deserialize> test2
 C [D "Metaphysics" (E (P "Kant" "Koeningsberg") (S 800.0)) 
    [PU (E (P "Hume" "Edinburgh") (S 100.0)),
     PU (E (P "Marlowe" "Cambridge") (S 200.0))],
    D "Ruling" (E (P "Thatcher" "London") (S 50000.0)) []]