Actually, I believe this is expressible in Python's type system:

    # Python 3.8
    from typing import Protocol, TypeVar

    T = TypeVar('T')

    class Summable(Protocol[T]):
        def __add__(self, other: T) -> T: ...

In Python 3.4-3.7 you'd need to install typing-extensions library:

    from typing_extensions import Protocol

Interesting. It seems that "every second language" gets "type-classes" these days…

C++ added such feature, this above looks like a Python version, Rust has it, Scala had them as one of the first "OOP" languages.

It's nice to see that Haskell pioneered a very useful and now spreading language feature!

Ada had them before Haskell, and I'm not even sure it was the first language with them.

> There's no language in which you would be able to write a function for arbitrary data structures using sum(), and get a reasonable outcome.

Computing the number of bytes used by a data type seems perfectly reasonable use of sum in that context. This is also achievable in many languages. All 'sum' needs is a map from a value to a number, which can have many sensible meanings even for arbitrary data types.

And that works in python to, even if the source type is strings:

  sum(map(mappingFunction, iterable))

works fine as long as mappingFunction maps the elements of an iterable to numbers.