Functional programming for imperative programmers

This series is for programmers who know an imperative language, such as C, Go, Python or Rust, and have never used a pure functional one. By the end you will be able to read and write ordinary Geng, and you will know why it is written the way it is.

It is a long read, in five parts. Each part builds on the one before, so read them in order.

How to read it#

Most examples are in Python, since nearly everyone can read it. Many come in three versions:

Python as most people write it.
The same Python, written in a functional style. Python can do almost everything in this series; it just doesn’t insist.
The Geng version. Geng does insist, and that is what lets its compiler check so much.

The middle step is the important one. It shows that each idea is something you can already do, not a new kind of magic.

Notes for Go, C and Rust programmers appear in short asides. Rust programmers already know a good part of this, and the asides say what to skim.

The running example#

Across the series we build one small program: an expense report. It reads a file like this:

date,category,amount,note
2026-11-02,food,12.50,lunch with the team
2026-11-03,travel,48.00,train to Boston
2026-11-03,food,abc,coffee
2026-11-04,office,23.99,printer paper

and prints a total for each category, along with the lines it could not read:

food      $12.50
office    $23.99
travel    $48.00
total     $84.49

Skipped line 4: "abc" is not an amount

Each part adds a piece: a record for one expense, totals, a type for categories, parsing that reports bad lines instead of crashing, and finally reading the file and printing the report.

The parts#

1. Thinking in values
Values that never change, functions with no hidden effects, and code made of expressions.
2. Loops without loops
map, filter and fold in place of for loops, state carried through a loop, and functions as values.
3. Modeling data
Records instead of classes, custom types and when, Maybe and Result instead of None and exceptions, and modules instead of private fields.
4. Real programs
How a pure program does I/O with Task, the finished expense report, and a mental model for performance.
5. What Geng adds
Checks moved from run time into the compiler, debugging and testing, and what you give up and get.