This series is about how to write a programming language. An evaluator takes in the syntax tree from the parser (see previous video) and magically turns it into behaviour.

Andy Balaam walks through the evaluator of Cell, a little programming language he wrote. More info: http://www.artificialworlds.net/blog/2016/10/16/how-to-write-a-programming-language-part-3-the-evaluator/

Read the code here: https://github.com/andybalaam/cell/blob/master/pycell/eval_.py