[Haskell-beginners] Haskell way of defining and implementing OO interfaces

[Kim-Ee Yeoh]

On Sun, Jan 4, 2015 at 8:00 PM, Julian Birch <julian.birch at gmail.com> wrote:

Consider that
>
> interface PasswordStore {
>   void store(Path path, String secret, Map metadata);
> }
>
> is identical to
>
> void store (PasswordStore store, Path path, String secret, Map metadata)
>
> or
>
> store :: PasswordStore -> Path -> secret -> MetaData -> IO ()
>
>
In fact, Thomas's original PasswordStore definition uses record syntax to
simultaneously define 'store' as exactly that.

That is, PasswordStore is a data triple and the store function picks out
the first one.


> So, you can treat PasswordStore as a pure data structure (that has things
> like connection details) and just define functions that use it.  I wouldn't
> worry about grouping the functions together.(*) I'm going to assume you
> don't really need an actual interface, but if you did, you could
> investigate typeclasses.
>

If I understand correctly he wants the functions together because there's
this mysterious piece of Java (?):

dbusClient.export(PasswordStore store)

I agree that type classes are probably not a good idea here. Thomas, good
job on using a record of functions !

But this is the sort of discussion better suited on haskell-cafe, a strict
superset of this list, where there are domain experts on dbus to contribute.

Haskell-beginners is not better, merely more responsive on language
rudiments and libraries circa haskell 98 and LYAH.



> Julian.
>
> (*) In general terms, the only reason to group functions together is to
> enforce laws that relate the behaviours together e.g. that you can retrieve
> something you stored.
>
> On 4 January 2015 at 11:14, Thomas Koch <thomas at koch.ro> wrote:
>
>> Hi,
>>
>> I'm writing a password manager that implements a dbus-api using the
>> dbus[1]
>> package. I'd like to separate the code that implements from the dbus api
>> from
>> the code that stores and retrieves the secrets (passwords). In Java I'd
>> use an
>> interface, e.g.:
>>
>> interface PasswordStore {
>>   void store(Path path, String secret, Map metadata);
>>   (String secret, Map metadata) retrieve(Path path);
>>   (String secret, Map metadata) search(Map criteria);
>> }
>>
>> And the dbus-api would export this interface:
>>
>> dbusClient.export(PasswordStore store)
>>
>> What would be a Haskell way to do the same? My only idea is to define a
>> record:
>>
>> data PasswordStore {
>>     store :: Path -> Secret -> MetaData -> IO ()
>>   , retrieve :: Path -> IO (Secret, MetaData)
>>   , search :: Criteria -> IO (Secret, MetaData)
>> }
>>
>> Thank you for any suggestions! Thomas Koch
>>
>> [1] http://hackage.haskell.org/package/dbus-0.10.9
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>
>
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