Re: [Haskell-cafe] serialize an unknown type

21 Oct 2012

      On Sun, Oct 21, 2012 at 08:11:50PM +0200, Corentin Dupont wrote:
...
Hi Iustin,
yes I want to deserialize an unknown type based on the content of the file
(in this example).
Let's say I can reduce the spectum of types to: Strings, all the types in
Enum, Ints. Is it possible?
Maybe :) I don't know how to do "all the types in enum". For Strings,
Ints and Float, you could have something like: serialise a pair instead
of just the type, and decode:

data Event = EventString String | EventInt Int | EventFloat Float
…
  (kind, val) <- read in_data
  return $ case kind of
    "string" -> EventString val
	"int -> EventInt (read val)
	"float" -> EventFloat (read val)

But this is only for a very few specific types.
...
My real problem is that on my web interface I want to use web routes to
allow a user to pass an Event back to the engine.
The problem is that it seems that web-routes only accepts Strings (or some
known types) to be passed on the web route, whereas I need to pass random
types.
Are you sure you need to pass random types? Can't you define a (large)
set of known types, for example?

regards,
iustin
...
On Sun, Oct 21, 2012 at 8:00 PM, Iustin Pop  wrote:
...
On Sun, Oct 21, 2012 at 07:20:10PM +0200, Corentin Dupont wrote:
...
Hi,
Sorry if it was not enough explicit.
I want to write functions like this:
serialize :: (Show a) => Event a -> IO ()
deserialize :: (Read a) => IO () -> Event a
The functions would write and read the data in a file, storing/retrieving
also the type "a" I suppose...
Can't you simply, when defining the type event, add a "deriving (Show,
Read)"? Then the standard (but slow) read/show serialisation would work
for you.
If you're asking to de-serialise an unknown type (i.e. you don't know
what type it should restore a-priori, but you want to do that based on
the contents of the file), things become a little more complex. Unless
you can further restrict the type 'a', really complex.
Maybe stating your actual problem, rather than the implementation
question, would be better?
regards,
iustin