This document discusses the problems of complexity in software development and proposes functional programming as a solution. It outlines three main sources of complexity: time, state, and architecture. Functional programming aims to master these through an immutable data and stateless function model where functions take inputs and produce outputs without side effects. This reduces complexity by avoiding unintended interactions between different parts of a program over time and through state changes.

1. What is
Functional
Programming?
Eric Normand

2. Outline
The problem with software: complexity
Mastering time
Mastering (state)space
Mastering architecture
A model of functional programming

3. The problem with software: complexity

4. Essential complexity Accidental complexity

6. Sources of complexity
Possible histories
Possible codepaths
Possible changes
Mastering time
Mastering (state)space
Mastering architecture

7. For a video and the transcript
of this presentation,
click here:
https://lispcast.com/what-is-functional-programming-2/

8. Mastering time

11. JavaScript has this problem, too
var ovenTemperature = 100;
ajaxGet("http://api.com/number", function(number) {
ovenTemperature *= number;
});
ajaxGet("http://api.com/number", function(number) {
ovenTemperature += number;
});

12. Where do timelines come from?
Multiple threads
Multiple processes
Multiple machines
Async operations

13. What's the problem?
Many histories are more than we can keep in our heads
Different histories give different results
Sometimes we can't reproduce the bad history (heisenbug)

21. Mastering (state)space
Each conditional creates at least 2 branches
Branches multiply the number of possible codepaths
More codepaths means it's harder to hold in your head
Do all codepaths do the right thing?

27. Mastering architecture
Guarding against unforeseen change

28. Stratified design
Layers built on layers
Each layer adds domain meaning to the layer below it

29. Chemistry
Fundamental cooking techniques
Cuisine building blocks
Dishes
protein, acid, heat, etc.
chopping, slicing, applying heat, etc.
sauces, trinity, roux, browning, etc.
gumbo, jambalaya, étouffée, etc

30. A model of functional programming

32. Action + Action => Action
Action + Calculation => Action
Action + Data => Action
Calculation + Calculation => Calculation
Calculation + Data => Calculation
Data + Data => Data

33. Eric Normand
Follow Eric on:
Eric Normand @EricNormand
eric@lispcast.comlispcast.com