[ros-users] rosmsg and rossrv prototype

[Ken Conley]

On Wed, Nov 2, 2011 at 1:41 PM, Thibault Kruse <kruset at in.tum.de> wrote:
> On 11/02/2011 08:29 PM, Ken Conley wrote:
>>
>> There is a question I have, which is whether or not arrays of
>> messages should be populated with a default instance to make them
>> easier to use (those are the hardest case, IMHO, to type).
>
> It could be useful, but indeed my code is inconsistent in that
> it does this only for arrays at the root level. It is more of a bug
> really, I did not think too much about it then.
>>
>>   This may
>> or may not be related to your comment re: Quaternions and Headers.
>> For Quaternion and Headers, I'll note that we learned the hard way
>> that making the core libraries have specific knowledge of messages
>> creates annoying future problems, no matter how well intentioned, so
>> perhaps a plugin system is in order, as well as possible changes to
>> the generated messages themselves.
>
> I think there is nothing wrong with the general idea of defining
> defaults for ros message elements, following convention over
> configuration. That would be the right fix for Quaternions IMO,
> but I guess a lot of work.
>
> I believe the issue of giving initial array elements in the completion
> is a separate issue that is only relevant for completion. An option
> for the prototype command would do for that, traversing the
> message and creating instances in empty arrays.
>
> A plugin system traversing the message and replacing values in
> specific message types is thinkable, but I'd say not a priority.
>
> What's more important I think is to manage the order of elements
> to match the .msg file definition. I think my code creates them in
> arbitrary order, which I find a bit confusing.

The Python message generator generates the __slot__ definitions in
field message order, so the approach I described earlier with
instantiating the actual message would be beneficial here.  It would
mainly mean changing the iterator from "for k in dir(obj)" to "for k
in obj.__slots__"

 - Ken

>>
>> Some union of roslib.messages, rosmsgproto, and rosh bagys seems like
>> a good strawman.
>
> Seems like a lot of work, good luck with it.
>
> If you want me to clean up the script given your input and create
>  unit tests, I can do so.
>
> cheers,
>  Thibault
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