Generics

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generics@haskell.org

July 2007

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2 discussions

xml deserialization using generics?
by Hugh Perkins 03 Jul '07

03 Jul '07

Cant decide whether this list is appropriate for questions related to generics usage, or only to generics design? So let me know if I should use haskell(a)haskell.org instead, but on the other hand I figure you guys are the experts :-) Trying to write an xmlserialization function for arbitrary (Data) objects, and not requiring a DTD or a code generation step. It's possible in C# so it really ought to be possible in Haskell ;-) xml serialization is pretty easy, see section 9.1.1 of http://www.haskell.org/haskellwiki/HXT On the other hand deserialization is still a work in progress. At least I'm not smart enough to figure it out, and nothing comes up in Google. The xml parsing bit is done, see section 9.1.2 of http://www.haskell.org/haskellwiki/HXT So, we can get a list of pairs/triples etc containing the data of our choice, such as field names, field values (in string format), data types, constructors, etc. What's remaining is to take makeConstrM, our final data type, and the list of field values, and to create the final object. Here's my non-working attempt at this bit so far. Most of the working bits come from a demonstration by kpreid in irc yesterday night of using makeConstrM with a pair of strings. The rest of the code is my feeble attempt to get this working for the Config custom data type. runM' :: (MonadState [String] m, Monad m, Data a) => m a runM' = do value <- gets head modify tail -- then one of: (pick the non-working function of your choice ;-) : -- return read (fromJust value) -- return (fromJust $ cast value ) -- return (fst $ head $ gread( "(" ++ value ++ ")" ) ) -- return (fromConstrM runM' constr) -- return (fromConstr contr) testConstrM' :: (Read a, Data a, Read c, Read b, Data b, Data c) => [String] -> a -> (b,c) testConstrM' fieldvalues object = evalState( fromConstrM runM' (toConstr object) ) fieldvalues data Config = Config{ name :: String, age :: Int } deriving( Data, Show, Typeable, Ord, Eq, Read ) createConfig = Config "blah" 3 test = testConstrM' ["qsdfqsdf", "7"] createConfig Note that whilst I'm a total Haskell newbie (this is week 2), I've used Reflection extensively in other languages, eg wrote a fast async rpc layer over guaranteed udp for C# http://metaverse.svn.sourceforge.net/viewvc/metaverse/Trunk/Source/Metavers…, so I know more or less what I'm aiming for, just not exactly how to do it ;-)

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Making a call to a function specified as a string???
by Hugh Perkins 01 Jul '07

01 Jul '07

Next question: any way to call a function specified as a string? gshow( somefunction ) produces gshow on the output of the function, rather than on the function itself, so not got very far so far. For background, the idea is to be able to serialize a function call to something we can pass over the network ,deserialize on the other end, and process appropriately. One could presumably do this using a generator of some sort. Is this the only option? I can live with using a generator for this; it's the usual way most implementations of this kind of thing work (ICE, WCF, ...); but if we can avoid using a generator, *** and stay strongly typed ***, it's better. For example, in traditional imperative languages, we'd specify an interface - so ensuring we are strongly typed - and then we are free to implement a serialization/deserialization scheme of our choice.

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