On Wed, Apr 27, 2011 at 11:16 AM, John Obbele
<john.obbele@gmail.com> wrote:
Hi Haskellers,
I'm currently serializing / unserializing a bunch of bytestrings
which are somehow related to each others and I'm wondering if
there was a way in Haskell to ease my pain.
The first thing I'm looking for, is to be able to automatically
derive "Serializable" objects, for example:
Happstack has "Serialize" type class, and uses TemplateHaskell to automate deriving instances. I don't know if they are binary compatible with cereal (i.e., that you could serialize with one and deserialize with the other, or vice-versa)
---------------------------------------------------------------
import Data.Serialize -- using cereal as an example
data MyFlag = One | Two | Three
instance Serialize [MyFlag] where
put = putWord16le . marshalFlags
get = unmarshal `fmap` getWord16le
data ObjectA = ObjectA { attribute0 :: Word8
, attribute1 :: Word16le
, attribute2 :: [MyFlag]
} deriving (Serialize) -- magic goes here!
---------------------------------------------------------------
Unfortunately ghci complains that 'Serialize' is not a derivable
class. Yet, deriving the Serialize instance for ObjectA should be
simple, since all the three attributes are already serializable
themselves...
Second issue, I would like to find a way to dispatch parsers. I'm
not very good at expressing my problem in english, so I will use
another code example:
This sounds very hard in the general case. Others have shown you how to dispatch on two types. But there is no general data type which combines all (or even arbitrarily many) types. Somehow, "Read" is able to do this, but I don't know what kind of magic it uses.
---------------------------------------------------------------
-- let's say we have two objects with almost the same structure:
data ObjectA = ObjectA { objLength :: Int
, objType :: TypeId
, attribute2a :: [MyFlag]
}
data ObjectB = ObjectB { objLength :: Int
, objType :: TypeId
, attribute2b :: Word32le
}
---------------------------------------------------------------
When we begin to deserialize theses objects, we don't know their
final type, we just know how to read their length and their
typeId.
Only then can we determine if what we are parsing is an ObjectA
or an ObjectB.
Once we now the object type, we can resume the parsing and return
either an ObjectA or ObjectB.
Oki, so I may have read too much of Peter Seibel's chapter on
binary-data parsing in Common Lisp or spent too much time working
on object-oriented code, but currently, I have no idea on how to
write this 'simply' in Haskell :(
any help would be welcome
/john
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