RE: [EXT] PMT thoughts

Hi John,

I think, in general, there is a consensus that PMTs are an imperfect solution and exactly for the reasons you pointed out (naming consistency, OOP, type safety, etc)

My gut says that they haven’t been replaced yet because, first, it’s unclear what a better solution would look like and, second, someone would have to implement it :p

Having no dog in this fight (other than constantly cursing my PMT formatting code and/or carrying numpy as dependency on deployments just for PMT) I’m very open to a ‘try it and see’ approach – I would love to see something of a more generalized message transport.

There is a short discussion on the GREP repo about using flatbuffers you may want to chime in on [1]. If you think there’s a reliable way to use stdlib instead of an external library I would be very open to using it.

Thanks,

Jason

[1] https://github.com/gnuradio/greps/pull/31

From: Discuss-gnuradio <discuss-gnuradio-bounces+jason.uher=jhuapl.edu@gnu.org> On Behalf Of John Sallay
Sent: Thursday, March 25, 2021 18:43
To: discuss-gnuradio@gnu.org
Subject: [EXT] PMT thoughts

APL external email warning: Verify sender discuss-gnuradio-bounces+jason.uher=jhuapl.edu@gnu.org before clicking links or attachments

I appreciate the effort that you guys have put into cleaning up gnuradio and making it easier to work with over the past few years (python2 - python3, swig to pybind11). I see that you are going to improve the logging as well which is great.

I've recently been doing some message based processing, and been using PMTs. I think that PMTs could use an overhaul too.

Here are some of my concerns:

The interface is inconsistent. Here are a few examples:

is_integer, but from/to_long

is_dict, is_vector, not no is_list

make_dict, make_vector, but list1, list2, etc

Type safety doesn't really work in several cases:

# apt install of gr 3.9 on ubuntu 20.04

a = pmt.make_dict()

a = pmt.dict_add(a, pmt.intern("a"), pmt.intern("a"))

pmt.is_dict(a) # True

a = pmt.list_add(a, pmt.intern("b")) # I believe that this changes the dictionary to a list

pmt.is_dict(b) # False

I could come up with more examples, but they illustrate roughly the same point. I should note that something changed in GR3.9. It was even worse in 3.8. For example, on a list, pmt.is_dict() returns True. I can call some of the dict functions which return unexpected results, while others raise an error.

The interface doesn't feel like modern C++. The pmt object doesn't really have any useful functions associated with it. You have to use the functions in pmt namespace to operate on any pmt. I believe that PMTs are using boost::any under the hood. I can get why some of the choices were made, but I think that there may be some alternatives available that could lead to an easier interface.

I've been playing around with std::variant (in c++17, boost::variant prior to that). I have some ideas for how we could use it to wrap the scalar types as well as the vector and dynamic types. I would be happy to explore the idea if there is interest. We could also use variadic templates to avoid things like list1, ..., list6.

Using std::variant, I would envision an interface like:

pmt my_dict();

my_dict["abc"] = 4.0 // Automatically convert the string and the number to the proper type.

std::get<float>(my_dict["abc"]))

pmt_list my_list();

my_list.append(pmt(my_dict));

for (auto& element : my_list) {

// Do something

}

std::cout << my_list.type() << " " << my_list << std::endl;

We should be able to deprecate, but maintain the current functions for dealing with PMTs. I believe that these changes could be made without breaking existing code.

To be clear, I haven't implemented any of this. My goal right now is just to see if there is interest in doing something like this. If there is, I am happy to look into it and try to create a proof of concept. I'm also open to suggestions and feedback.

Thanks,

John

From:	Uher, Jason J.
Subject:	RE: [EXT] PMT thoughts
Date:	Mon, 29 Mar 2021 13:47:51 +0000