With most people, it is impossible to gauge how good they are just by observing what they do in half a day on the job. Thus, any interview process must necessarily be somewhat artificial in trying to gain insight into a candidate's abilities in the span of a half a day.

A real life task might be implementing a much more complex library over several days or weeks. Asking the person to implement a simple function like shuffle is the closest you can get in the span of an hour.