Or Rust! Everything that implements the Iterator trait gets access to all of Iterator’s goodies, like map, filter, reduce, etc. Implementing iterator just requires adding a single next(&mut self) -> Option<Item> method on your type.

Lifetimes and async are a massive pain in rust. But the trait system is a work of art.

ruby is closer to what i meant because you can't add methods to rust's iterator, can you? but people add stuff to enumerable all the time

You can!

    trait MyIterHelpers: Iterator {
        fn dance(&self) {
            println!("wheee");
        }
    }
    
    // And tell rust that all Iterators are also MyIterHelpers.
    impl<I: Iterator> MyIterHelpers for I {}

The one caveat is that using it in a different context will need a use crate::MyIterHelpers; line, so the namespace isn't polluted.

neat, i didn't know that was possible

I like Rust's struct + traits approach, because they avoid inheritance and encourage composition. I am sure people have built bad workarounds though to do inheritance anyway.

Or its inspiration, Smalltalk.