Index atom

[DaddyWesker]

Index-atom function added. Usage:

!(index-atom (5 2) 1)
[2]

So it allows to get atom from expression (created by cons-atom) using index.

[vsbogd]

1. I would use Number as an index argument instead of converting atom via string
2. I would use internal logic of the Vec::get method to check index bounds
3. It is more proper to return Error instead of printing it on the screen. In the former case caller can check for the error automatically:

Suggested change

	let expr = TryInto::<&ExpressionAtom>::try_into(args.get(0).ok_or_else(arg_error)?)?;
	let atom = args.get(1).ok_or_else(arg_error)?;
	let children = expr.children();
	let index = atom_to_string(atom).parse::<i32>().unwrap();
	if index < 0 {
	println!("Negative indexes currently not supported");
	return unit_result()
	}
	else if (index as usize) >= children.len() {
	println!("Index is out of bounds");
	return unit_result()
	}
	Ok(vec![children.get(index as usize).clone().expect("Unknown error").clone()])
	let children = TryInto::<&ExpressionAtom>::try_into(args.get(0).ok_or_else(arg_error)?)?.children();
	let index = AsPrimitive::from_atom(args.get(1).ok_or_else(arg_error)?).as_number().ok_or_else(arg_error)?;
	match children.get(Into::<i64>::into(index) as usize) {
	Some(atom) => Ok(vec![atom.clone()]),
	None => Err(ExecError::from("Index is our of bounds")),
	}

In order to make this code compile one should also make AsPrimitive public inside arithmetics.rs

[vsbogd]

I would use Number as an index type like in all arithmetic operations.

Suggested change

	Atom::expr([ARROW_SYMBOL, ATOM_TYPE_EXPRESSION, ATOM_TYPE_ATOM, ATOM_TYPE_ATOM])
	Atom::expr([ARROW_SYMBOL, ATOM_TYPE_EXPRESSION, ATOM_TYPE_NUMBER, ATOM_TYPE_ATOM])

[vsbogd]

Please add:

1. Rust unit tests to check it works
2. Documentation into both stdlib.metta and stdlib_minimal.metta

[vsbogd]

Why do you have commits from other branches in this one?

[DaddyWesker]

Why do you have commits from other branches in this one?

I'm working in several branches simultaneously and somehow it got really messy =(

[DaddyWesker]

Thanks for your comments. I'll look into them and make suggested changes.