take_while

In std/seq.

fn take_while<S : Seq<T>>(self: S, by: T -> Bool) : S
fn take_while(self: Iterable<T>, by: T -> Bool) : List<T>

The leading values of self that by accepts, up to the first it rejects: [1, 2, 5, 1] | take_while(x => x < 3) is [1, 2]. A sequence comes back at its own type, and any other iterable as a List.