Functional programming is interesting. On the one hand it’s very (extremely!) disciplined. But on the other hand, people can’t agree on certain definitions. As an example, here are the “three principles of functional programming,” from this tweet:
1. Orthogonal composability
2. Maximum polymorphism
3. Maximum deferment
Conversely, here are the “three pillars of functional programming,” from Functional and Reactive Domain Modeling:
1. Referential transparency
2. Substitution model
3. Equational reasoning
When I learned OOP I saw that it was based on several principles that everyone agrees upon. When I started learning FP (and later took two years to write Functional Programming, Simplified) I was surprised there wasn’t a single accepted definition of functional programming. I ran across the principles/pillars in the last two days and was reminded of that again.