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

[Jeremy Shaw]

Hello,

You should use happstack-data for this (you do not need the other  
happstack components to use happstack-data)*. It was created to solve  
this exact problem.

happstack-data builds on type of the 'binary' library and adds  
versioned  data types and automatic version migration.

You can get an idea as to how it works by reading this old blog post,

http://nhlab.blogspot.com/2008/12/data-migration-with-happs-data.html

The modules names have changed from HAppS.* to Happstack.*, but  
otherwise it is still pretty accurate. The upcoming happstack 7  
release cycle will be focusing on this area of happstack. Especially  
improved documentation. But, it is quite usable right now.

If you have questions about happstack-data, feel free to ask on the  
happstack mailing list or irc channel. (http://happstack.com/community)

I am happy to answer any questions or concerns you may have.

- jeremy

* the version on hackage depends on happstack-util, but the darcs  
version does not.

On Dec 16, 2010, at 12:26 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
>
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