# 15-150: Principles of Functional Programming

# Lecture 18: Representation Invariants (Red/Black Trees)

We demonstrated a complicated representation invariant using
Red/Black Trees.

The main lesson is to understand the subtle
interactions of invariants, data structures, and reliable code
production.

In order to write code satisfying a strong invariant, it
is useful to proceed in stages. Each stage satisfies a simple
invariant, and is provably correct. Together the stages satisfy the
strong invariant.

### Key Concepts

- Representation invariants
- Weakening an invariant temporarily

- Chapter 32: Data Abstraction

This chapter covers the code and ideas of today's lecture closely.