better type system based on py3 type hints

[asmodehn]

We should take inspiration from python3 type hints and https://www.python.org/dev/peps/pep-0483/

It would make things more solid, and would also provide simple python3 compatibility.

The type checker would operate at runtime, with the types providing a "contract" regarding the message structure and content.

Later we might want to extend it to provide contracts for complex node behaviors...

[asmodehn]

typing is supported for python 2.7 and mypy can be used for checking types (could be integrated with catkin_pip)

[asmodehn]

Note there is two distinct topics here :

• improve our typechecker by matching types / structures / API from types hints PEP, hypothesis, etc.
• add type hints to our implementation, so that, if people use mypy, they will detect type problems when using messages... but this is just a "nice to have" and not the core functionality. We still need to implement a dynamic type checking, so that we can extend it later to check for concurrency problems...

[asmodehn]

Another (complementary ?) option : https://andreacensi.github.io/contracts/

[asmodehn]

pycontracts seems to be lacking quite a few features, especially when comparing with hypothesis...
We have currently implemented a very basic functional type checker, following hypothesis strategies, not sure if the effort in improving pycontracts is worth it.

Not sure what should be done here to improve/solidify this... maybe generating a type checker for python from a more rigorously typed language like idris ?

Anyway it seems to be quite distinct from defining messages for pyros : We want to give python a nice type system that can :

• currently support py3 type hints
• be extended later to support more dynamic contracts (like TLA+ : specifying a state machine for the type - we want to use it like contracts work in erlang, upon message reception to guarantee perceived state from another node )

This will help overall stability for our distributed system.

[asmodehn]

Python3 is now in progress in #14
However we are not doing anything with python3 types yet.