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

[Daniel Peebles]

Have you considered moving these packages that are unrelated to web
development into a separate namespace? I know that I never considered
looking under the happstack namespace simply because I never do webapps.

On Thu, Dec 16, 2010 at 5:09 PM, Jeremy Shaw <jeremy at n-heptane.com> wrote:

> 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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