Hello Henry, The paper "A Lightweight Approach To Datatype-Generic Rewriting" [1] describes a way to generically add a constructor to any regular datatype using type-indexed datatypes [2]. A similar technique could be used to add a new field to each constructor. Then you get something like:
data Foo > type Extended f = ... > |Extended Foo| represents your |Foo| datatype with an added |z| field of type |Int|. Since the underlying generic programming library used (regular [3]) has Template Haskell generation, you don't even have to write the generic representations for your many datatypes. (As far as I know, SYB does not mix with type-indexed datatypes.) Cheers, Pedro [1] Thomas van Noort, Alexey Rodriguez, Stefan Holdermans, Johan Jeuring, Bastiaan Heeren. A Lightweight Approach to Datatype-Generic Rewriting. Submitted to the Workshop on Generic Programming 2008. http://www.cs.uu.nl/wiki/bin/view/Alexey/ALightweightApproachToDatatype-GenericRewriting [2] http://www.iai.uni-bonn.de/~ralf/publications/SCP2004.pdf [3] http://www.cs.uu.nl/wiki/GenericProgramming/Regular On Tue, Jul 28, 2009 at 16:29, Henry Laxen <nadine.and.he...@pobox.com>wrote: > Dear Group, > > It seems to me this should be easy, but I can't quite figure out > how to do it without a lot of typing. Here is the question: > > Suppose you have a data type like: > Data Foo = Foo { a :: Int, b :: Int, > ... many other fields ... > y :: Int } deriving (Eq, Read, Show, Typeable, Data) > > Now I would like to add a field z :: Int to the end of Foo. If > I have a ton of data out on disk, which I wrote with, say > writeFile "a.data" (show foo) -- where foo is a [Foo] say 1000 > long, I would like to get a new "a.data" file which has a new > z::Int field. > > So far the only way I can think of is to make a new Data Foo1, > which includes the z::Int, read in a.data as a list of Foo, > write a function like: > > fooTofoo1 :: Foo -> Foo1 > fooTofoo1 xx = Foo1 {a = a xx, ... y = y xx, z = 1} > > then write the file back out, and perhaps use emacs to > query-replace all the Foo1's back to Foo's, add the z::Int field > back into Foo, and read it back. > > Please tell me there is a better way. Thanks in advance. > Best wishes, > Henry Laxen > > PS: > I have read syb1, and syb2 a couple of times now, but so far > haven't been able to connect it with this kind of problem. > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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