They missed one: def agent(data, predicate, fns): ps = map(predicate, data) fs = map(lambda x:fns[x], ps) return map(fs, data) Basically

They missed one: def agent(data, predicate, fns): ps = map(predicate, data) fs = map(lambda x:fns[x], ps) return map(fs, data) Basically you want to apply one of N functions to each of the items in your data iterable, so you have a predicate to figure out which function to apply first. The degenerate case is when you have just 2 functions and your predicate returns a boolean (0 or 1). This is "agent" from J: https://code.jsoftware.com/wiki/Vocabulary/atdot#agent https://code.jsoftware.com/wiki/Vocabulary/atdot#agent