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-Gene...
[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
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.
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