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.
This seems to depend on what you want to accomplish. Is your goal just to rewrite this whole file? If it is, the idea of just adding a field to Foo would be enough. You could then add that 'z' field in your file using 'sed' (or, as you said, emacs) and then read it back. In general, however, if you want to deal with this kind of translation of text to data, what you really want is to take some time to learn something like Parsec. http://www.haskell.org/ghc/docs/latest/html/libraries/parsec/Text-ParserComb...
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}
Note that this would not work exactly like that. 'a' is a "field" of Foo, and that means it's a function like a :: Foo -> Int So, you can't use it as a field of Foo1, as that would imply a :: Foo1 -> Int Best, Maurício