Why Functional Programming Matters is a classic programming paper by John Hughes. It's a highly readable and fun introduction to some of the most useful techniques in functional programming. When the paper was published in 1990, the concepts covered were very advanced. Hughes had to use a Haskell-like, high-level functional language Miranda to make his points.
In recent years, features such as higher-order functions and lazy evaluation have become available in mainstream languages such as Python. However, it still takes some brain-rewiring to take advantage of these abstract constructs. I decided to translate Hughes' Miranda code to Python, as an exercise to become more fluent in functional thinking. Since 90% of Hughes' paper is about lazy evaluation, this is basically an intermediate-level tutorial on Python's generators.
It should be noted that Python's generator really is a poor-person's version of lazy evaluation. A lot of the elegance of the original code was unfortunately lost in translation (at least in my best effort). However, Hughes' main argument - that higher-order functions combined with lazy evaluation allow programmers to decompose densely-linked logic into modular components - remains wise.
I embedded the Python code in a series of "articles" that should be read in the following order:
- Work with trees using higer-order functions
- Numerical approximation using lazy evaluation
- Numerican differentiation using iterators of iterators
- Numerical integration using lazy recursion
- Lazy tree operations using higher-order functions for iterators
- Play games using lazy trees
- Play Tic-tac-toe
The code was written in orgmode - a markup language with a lightweight literate programming system. It's popular with Emacs users. orgmode documents (*.org) can be viewed directly on Github. The code blocks in the documents can be extracted and assembled (or "tangled" in the jargon of literate programming) into regular Python source code under src/ with the M-x org-babel-tangle command in Emacs. make tangle defined in the Makefile also does the trick. The source code assembled by orgmode doesn't quite follow Python's PEP 8 style guideline, so I use yapf to reformat them.