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Date: -- (:)
From: Richard Jones <rich@a...>
Subject: Re: [Caml-list] Functors and classes
On Tue, Aug 03, 2004 at 12:23:07PM -0400, John Prevost wrote:
> Yes, that's the same problem I'm seeing.  I think it *might* be
> possible to get around it by using a connection type with type
> parameters.  Maybe.  I'll investigate more in a few hours.
> 
> This is an upsetting problem, but I believe that it's known that there
> are some drawbacks to mixing classes and functors.  (And, well, if
> it's not, it should be now.)  Anyway, we'll see what can be done.
> 
> If one of the approaches I'm thinking of would work, it might either
> suggest a rather painful (and non-obvious) change to the Dbi API, or
> it might be possible to define a functor or simple wrapper to turn a
> Dbi module into a Dbi' module that works well for this.
> 
> And maybe someone who's done a lot of work with classes and modules
> will mention a really nifty solution.  :)

One "solution" I've come up with is to add a functional interface to
Dbi_postgres.  ie: adding this to the end:

  let connect ?host ?port ?user ?password database =
    new connection ?host ?port ?user ?password database
  let close (dbh : connection) = dbh#close ()
  let closed (dbh : connection) = dbh#closed
  let commit (dbh : connection) = dbh#commit ()
  let ping (dbh : connection) = dbh#ping ()
  let rollback (dbh : connection) = dbh#rollback ()

then I've changed the functor input signature to:

  module type DbiDriverT = sig
    type connection
    val connect : ?host:string -> ?port:string ->
      ?user:string -> ?password:string -> string ->
      connection
    val close : connection -> unit
    val closed : connection -> bool
    val commit : connection -> unit
    val ping : connection -> bool
    val rollback : connection -> unit
  end

with appropriate changes to the functor itself to use the functions
instead of methods.

It works, but it's not particularly elegant, and a better solution
would be welcome.

Rich.

-- 
Richard Jones. http://www.annexia.org/ http://www.j-london.com/
Merjis Ltd. http://www.merjis.com/ - improving website return on investment
'There is a joke about American engineers and French engineers. The
American team brings a prototype to the French team. The French team's
response is: "Well, it works fine in practice; but how will it hold up
in theory?"'

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