Comment by thaumasiotes

> Map and filter usually have only one arg and if they have 2, the 2nd is almost always a 0-based index. They look identical in most languages, even when Microsoft chooses to call them Select and Where.

I don't understand. Map takes input of type a and size n and returns output of type b and size n.

Filter takes input of type a and size n and returns output of type a and size ≤ n.

They look nothing alike?