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From: Lawrence =?iso-8859-13?q?D=FFOliveiro?= <ldo@nz.invalid>
Newsgroups: comp.lang.ada
Subject: Re: In precision typing we trust
Date: Thu, 28 Aug 2025 01:45:58 -0000 (UTC)
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On Wed, 27 Aug 2025 19:17:43 -0600, Alex // nytpu wrote:

> On 8/27/25 6:33 PM, Lawrence D’Oliveiro wrote:
>>
>> But can you do type checking dynamically, at run-time?
>>
>> For example, in one project (a Python wrapper for receiving
>> fanotify events from the Linux kernel), I had to deal with a
>> sequence of packets of variable length and type. I could dispatch
>> on the packet-type field and return correspondingly typesafe
>> results in my wrapper.
>
> Yes, that's sorta the entire premise of Ada's OOP lol. Use the
> `'Class` type to give you a root class and then you can dispatch
> with a method call or you sorta can match directly (nowhere even
> near the convenience of more modern functional-style matching
> admittedly though). Or just have a variant record that you can match
> on with a `case` if one prefers---I usually do this unless I need
> dynamic dispatch or inheritance.

But the function has to return the base class type, it cannot have
different return types dynamically. E.g. in the above case, I have an
expression like

    info_type = infoptr.contents.info_type
    infoptr = ct.cast \
      (
        infoptr,
        ct.POINTER
          (
            {
                FANOTIFY.EVENT_INFO_TYPE_FID : FANOTIFY.event_info_fid,
                FANOTIFY.EVENT_INFO_TYPE_DFID_NAME : FANOTIFY.event_info_fid,
                FANOTIFY.EVENT_INFO_TYPE_DFID : FANOTIFY.event_info_fid,
                FANOTIFY.EVENT_INFO_TYPE_PIDFD : FANOTIFY.event_info_pidfd,
                FANOTIFY.EVENT_INFO_TYPE_ERROR : FANOTIFY.event_info_error,
            }[info_type]
          )
      )
    additional.append(infoptr)
    if info_type == FANOTIFY.EVENT_INFO_TYPE_DFID_NAME :
        nameptr = ct.cast \
          (
                ct.cast(infoptr, ct.c_void_p).value
            +
                ct.sizeof(FANOTIFY.event_info_fid)
            +
                infoptr.contents.handle.handle_bytes,
            ct.c_char_p
          )
        additional.append(nameptr.value)
    #end if

Note how you have a switch-expression that uses the value of the
event-type field to return the corresponding type object, and I cast
the info pointer to point to that type. And then the “additional”
variable-length array can contain pointers to additional data,
depending on the event type.

> But in the specific case of interacting directly with hardware[1],
> Ada is far and away the best language in existence for it.

Look at MicroPython, though. It makes it so easy to write just a few
lines of code to twiddle some bits or something. Ada is for the big
stuff, not the small stuff.