Re: [Haskell-cafe] handling multiple versions of a data structure

On Thu, Dec 16, 2010 at 12:56 PM, Daniel Peebles wrote:

I haven't tested this idea, but it occurred to me that this might be a good place for data families: data Z = Z newtype S n = S n -- Some nastiness to avoid having to see into the future? type family Pred n :: * type instance Pred (S n) = n type instance Pred Z = Z class MyDataClass ver where   data MyData ver :: *   upgrade :: MyData (Pred ver) -> MyData ver   -- other methods that won't change from version to version class Less a b instance Less Z a instance Less a b => Less a (S b) convert :: Less a b => MyData a -> MyData b -- you may need a method in Less to witness the less-ness, but then you'd iterate your upgrade until you reach the version you want. -- Then you might want to abstract over this class with an existential, so it doesn't infect other things: data MyDataGeneral = forall ver. MyDataClass ver => MyDataGeneral ver -- now make instances for versions you have, with data instances for your current version of the structure.

This might not even compile as I just wrote it into my email client, but it seems like it could work. Any comments? Dan

The Happstack project includes versioning in its serialization libraries - Here is the API reference: http://happstack.com/docs/0.5.0/happstack-data/Happstack-Data-Serialize.html Although I don't know if there are any good tutorials on using it separated from the Happstack persistence layer. Antoine

On Thu, Dec 16, 2010 at 1:26 PM, Dmitry V'yal wrote:

...

Greetings,

while developing my neural net simulator I stumbled upon a problem.

I have a data type NeuralNet and use Show and Read instances for saving and loading configurations. As time passed, I changed the data type, so the program can no longer load files saved in previous versions.

I want fix it. My current idea looks as follows. I'm going to create a bunch of types NN1, NN2, NN3..NNn for different versions and write converters c12 :: N1 -> N2, c23 :: N2 -> N3 and so on.

But how to organize the whole process of parsing String into NNn so it's easy to change formats? Something based on using a list of parsers [read, c43 . read, c43 . c23 . read, c43, c23 . c12 . read, c43 . c32 . c21 . read]

looks rather verbose and grows quadratically with N.

I'm sure there must be a more elegant way. Any ideas?

Dmitry

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe