[Haskell-cafe] How to avoid keeping old data formats in your code?

Roman Cheplyaka roma at ro-che.info
Sun Dec 21 09:09:21 UTC 2014

Yeah, I agree with Evan, it does sound too complicated.

I also have a SafeCopy module where I keep my SafeCopy instances and old
versions of data types.

On 21/12/14 07:29, Evan Laforge wrote:
> I haven't heard of anything like that, it sounds too complicated for me.
> You still need to convert the old format to the new one, so it seems
> like you still need a bit of code for each version, which needs access
> to both old and new.
> On Dec 21, 2014 9:10 AM, "Corentin Dupont" <corentin.dupont at gmail.com
> <mailto:corentin.dupont at gmail.com>> wrote:
>     On Sun, Dec 21, 2014 at 1:50 AM, Evan Laforge <qdunkan at gmail.com
>     <mailto:qdunkan at gmail.com>> wrote:
>         On Sat, Dec 20, 2014 at 3:29 AM, Corentin Dupont
>         <corentin.dupont at gmail.com <mailto:corentin.dupont at gmail.com>>
>         wrote:
>         > Hi,
>         > I didn't had so much luck with this question. The question was:
>         > -> "How to avoid keeping old data formats in your code?"
>         > Indeed those old data formats are often needed for retro-compatibility with
>         > previous versions of your software.
>         >
>         > Say you have a data structure that is serialized in a file, and then you add
>         > a field in a later version. You are obliged to keep both versions of the
>         > data structure if you want to be able to read both versions of the file.
>         > Potentially, if you update very often your structure between releases, you
>         > are obliged to keep N versions of the data structure in your code.
>         > See the exemple at: http://acid-state.seize.it/safecopy
>         I just keep both versions.  It doesn't seem like too much hassle.  I
>         move the old version to the Serialize module so modules that aren't
>         concerned with serialization aren't cluttered.
>         In the fully general case, I think there's no way around it.  But if
>         you are just adding a field, or or changing a field to a super-type
>         (e.g. 'a' becomes 'Maybe a') then you can trivially upgrade in place
>         in the deserialization code.  Or you can use something like
>         protobufs,
>         which supports that kind of thing automatically.
>     What I'm thinking about is to put in the Setup.hs  some code that
>     will extract from GIT the older versions of the data structure. This
>     way you are able to keep only the last version of it in the HEAD,
>     and still be able to build migration code.
>     Do you know of any such custom build?
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