On Sun, Jan 4, 2015 at 8:00 PM, Julian Birch <julian.birch@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@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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