Higher Order Functions

Higher-order functions

A common higher-order function is reduce, which is more commonly known as a fold. This function reduces a list to a single value.

compose	f1.compose(f2) -> first run f2, then pass the result to f1
andThen	f1.andThen(f2) -> first run f1, then pass the result to f2. So,
f2.andThen(f1) is same as f1.compose(f2).
identity	Function.isdentity() creates a function whose apply method just
returns the argument unchanged

Methods that Return Functions

Two common examples are filter and map.