Re: Thinking of using Ada for a job at home. Couple of questions first.

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 03 Mar 2010 16:19:31 +0000, John McCabe wrote:

> So the first question is do any of you know of any bindings to MS
> Windows MIDI (presumably the multimedia system) functions that already
> exist and are open source or something like that? I searched on Google
> for "ada midi" but it looks like there's a product called that so it
> came up with loads of stuff. I'm aware there are some binding to Win32
> from Ada but, as I've not been doing Ada in anger for a long time, I'm
> not sure if there's something for Ada 2005.

I vaguely remember that there is no proper API for that (either in Ada or
in C). It is communicated over the Windows messages or worse at the process
level. I don't remember. Anyway if there exists proper C API, there is no
problem to convert it to Ada. It is a matter of a pair minutes of work.

> So, in principle, would that be the way to do this, or would you be
> able to suggest a better mechanism?

In practice that usually does not work well. The approach I am using is
reading the octet stream or else what is natural for the device or the
protocol at hand, and then constructing the necessary Ada objects on the
fly. I don't mess with representation clauses. If 100 nanoseconds overhead
impose a serious performance problem to your program it won't work anyway.
And under Windows we are talking about 1ms jitter at least. So no reason to
worry about that.

> Finally, is there a straightforward way to do that sort of thing using
> tagged records?

Yes, as above. This is the way I usually do it.

>  E.g. could I define a record which is the header
> format, then extend that in a number of different tagged extensions to
> create each individual message as a separate tagged record and use the
> OO and dispatching facilities to work with them?

No, why do you need message objects? What for? Once a message is received
and interpreted there is no need to keep it in any form, so to have a
Message type [or an hierarchy of]. Instead of that you create a tagged
Connection type, with primitive higher-level operations invoked by the
driver as it reads incoming messages.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de