Implement generics

[certik]

We need to allow writing generic functions (templates) and template constraints.

One idea that we can explore is to also support something like: #152 (comment):

>>> def str(val: i32 | f64):
...   # "isinstance" type guard
...   if isinstance(val, i32):
...     # type of `val` is narrowed to `i32`
...     return str_int(val)
...   elif isinstance(val, f64):
...     # else, type of `val` is narrowed to `f64`
...     return str_float(val)

[Smit-create]

This should now be possible with overload decorator.

[certik]

This particular case can be. We still need generics/templates.

The overload in a sense is a subset of generics: you provide specific implementations ahead of time. The templates do not provide specific implementations, the compiler instantiates them.

[Aser-Abdelfatah]

Was this picked up by any person? Or is it still open for contribution?

[certik]

Yes, it's still open. We have quite solid generics in libasr thanks to LFortran, and we should hook them into LPython.

[Aser-Abdelfatah]

Hi,
I'm doing more research into this. I found some generics implemented in GSoC 2022. I'd just appreciate if there were more details on which generics have higher priority or where to start

[certik]

They are implemented in LFortran, you can start here: https://docs.lfortran.org/en/asr/generics/.