[Haskell-cafe] JSON serialization/deserialization

[ntu...@googlemail.com]

Dear list members,

I try to use Text.JSON
(http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json) to
serialize and deserialize record types. As I understand it, the data
types need to be instances of class JSON. However I have difficulties
to come up with a nice implementation of showJSON and readJSON. For
example:

module Test where

import Text.JSON

data Record =
One {
field1 :: String,
field2 :: String }
| Two {
field :: String } deriving (Eq, Ord, Show, Read)

showJSON (One x y) = toJSObject [("field1", x), ("field2", y)]
showJSON (Two x) = toJSObject [("field", x)]

readJSON x = -- ???

This lacks the instance declaration itself but my problem is more
fundamental. How can I write the readJSON method? In general how would
one approach the problem of serialization/deserialization? I am
puzzled that the type classes Show and Read can be derived
automatically. I can not think of any way to do this.

Thanks for your help!
_______________________________________________
Haskell-Cafe mailing list
Haskel...@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

[Marc Weber]

The trouble:
module A
data ABC = A { a :: String }
| B { a :: String }
| C { a :: String }

module B
data ABC = A { a :: String }
| B { a :: String }
| C { a :: String }

is valid haskell.
so how should you know having the serialized String { a : "abc" }
wether to use the constructor A,B,C ? or wether to use ABC from module A
or from B?
There are many ways to solve this:
One is to serialize it this way: { "datatype": "A.ABC", constructor : "C"
, a :"text" }
or such..
Maybe also read about Data.Typable, have a look at derive or DrIft (or
the lib itself, I'm not sure wethere there is one which has choosen one
way to do this)..

Basically JavaScript doesn't have real typing (such as classes). But
there exist some libraries to emulate it such as mootools or
scriptaculous..

I hope you'll find a solution which works for you.

Sincerly
Marc Weber