• Event
Date
Description
Course Material
• Lecture
09/08/2021
Wednesday
Introduction

We’ll talk about what Lean is and see what it can do, and also go over some organizational points about the course.

Takeaways: verified programming is fun and powerful, and this course is very experimental!

• Lecture
09/13/2021
Monday
The basics of Lean syntax

In this lecture we’ll learn the basics of the Lean programming and specification language: types and terms, type inhabitation, and writing and evaluating very simple functional programs. No proving yet!

We covered HHG 1.1 and 1.2.

• Assignment
09/13/2021
Monday
Assignment #1 released!
• Lecture
09/15/2021
Wednesday
The basics of Lean syntax, continued

We’ll finish Chapter 1 of the HHG, and get a head start on Chapter 2, where we’ll actually start proving some theorems. Today’s topics: inductive types (continued), function definition and evaluation, specifications, and basic tactics. We talked about `intro` and `apply`, and how tactic mode is like a proving minigame.

More on Chapter 2 next time!

• Lecture
09/20/2021
Monday
Backward (tactic) proofs

We’ll dive into the meat of the HHG Ch. 2: what are some of the moves available to us in the tactic proving minigame, beyond `intro` and `apply`? How do we deal with logical connectives: `and`, `or`, `not`, and so on?

• Assignment
09/20/2021
Monday
Assignment #2 released!
• Lecture
09/22/2021
Wednesday
Backward (tactic) proofs, continued

We’ll continue talking about tactic proofs. How do we deal with equality? What about the natural numbers?

We’ll also talk about classical vs constructive logic.

• Due
09/22/2021 15:00
Wednesday
Assignment #1 due
• Lecture
09/27/2021
Monday
Forward proofs

We’ll see another way to write proofs in Lean, incorporating forward reasoning.

Structured (“proof-term”) proofs are a little closer to the underlying logic. Surprise: proofs in Lean are, literally, just terms in the type theory.

• Assignment
09/27/2021
Monday
Assignment #3 released!
• Lecture
09/29/2021
Wednesday
Dependent types

The type theory that Lean is based on, the Calculus of Inductive Constructions, is an instance of dependent type theory. In DTT, we follow the PAT principle: propositions as types, proofs as terms. (Buzzword: the Curry-Howard correspondence!) We’ll look deeper today into these foundations.

• Due
09/29/2021 15:00
Wednesday
Assignment #2 due
• Lecture
10/04/2021
Monday
Functional programming: data structures

Chapter 4 of the Hitchhiker’s Guide introduces some paradigms – inductive types, structures, recursive definitions, type classes – that might be familiar from other functional programming languages. The interesting thing for us is how these paradigms interact with writing proofs. For instance, how do we mix properties into data structures?

• Assignment
10/04/2021
Monday
Assignment #4 released!
• Lecture
10/06/2021
Wednesday
Functional programming: type classes, lists, trees

Type classes are a language feature inspired by Haskell with equivalents in Scala, ML, and other languages. They allow us a kind of ad hoc polymorphism: we can define functions on types that implement certain interfaces, and can declare that certain types implement these interfaces, without bundling the interfaces into the data type itself.

We’ll see how this interacts with some of the data structures we like to use, as we implement and specify functions on these types.

• Due
10/06/2021 15:00
Wednesday
Assignment #3 due
• No Lecture
10/11/2021
Monday
Indigenous Peoples' Day
• Lecture
10/13/2021
Wednesday
Inductive predicates

We’ll cover ch. 5 of the Hitchhiker’s Guide today, on inductive predicates. This will complete what we need to know about foundations for now: inductive predicates give us a way to introduce new propositions and prove things about them.

Inductive predicates are also the source of most of the propositional symbols we’ve used so far – `and`, `or`, `exists`, `eq`, ….

• Due
10/13/2021 15:00
Wednesday
Assignment #4 due
• Lecture
10/18/2021
Monday
Big-step operational semantics

We’re jumping ahead to Chapter 8 today! Time to start putting what we’ve learned into practice. We’ll define the syntax of a toy programming language inside of Lean, discussing the difference between shallow and deep embeddings. Using inductive predicates, we’ll define a transition system and use this to prove things about the execution of programs in this toy language.

• Assignment
10/18/2021
Monday
Assignment #5 (chs. 5 and 8) released!
• Lecture
10/20/2021
Wednesday
Small-step operational semantics

The big-step semantics we saw on Monday aren’t fine-grained. We can’t reason about intermediate states. An alternative is using a small-step semantics, where our program execution path is broken down much further. This comes with upsides and downsides.

• Lecture
10/25/2021
Monday
Denotational semantics

Operational semantics define the meaning of a program by the process it follows to evaluate. In contrast, denotational semantics define the meaning of a program as a mathematical object, a relation between possible inputs and outputs. Today we’ll cover all of Ch 10 of the HHG.

• Assignment
10/25/2021
Monday
Assignment #6 (chs. 8 and 10) released!
• Lecture
10/27/2021
Wednesday
Algebraic structures

We’ll jump ahead again to chapter 12, where we’ll start talking about algebraic structures. But we’ll also improvise a bit here. After we see some basic structures, we’ll define some mathematical types of our own.

• Due
10/27/2021 15:00
Wednesday
Assignment #5 due
• Lecture
11/01/2021
Monday
Numbers and sets

We’ll continue the Ch 12 material we started last week, including a little more with the complex number playground. We’ll also talk about embeddings between different numerical structures, and some different kinds of “set-like” objects.

• Assignment
11/01/2021
Monday
Assignment #7 (ch 12) released!
• Lecture
11/03/2021
Wednesday
Logical foundations

As this course has progressed, we’ve gotten some insight into the foundations of Lean and its type theory. But some features have remained mysterious. In the next few lectures we’ll poke some more at this foundational theory. Today we’ll be focusing in particular on the type universe `Prop`, what we’re allowed and disallowed in this universe compared to the others.

• Due
11/03/2021 15:00
Wednesday
Assignment #6 due
• Assignment
11/05/2021
Friday
Final project released!
• Lecture
11/08/2021
Monday
Logical foundations, continued

We’ll continue with chapter 11 today, talking about more foundational constructs. As we discussed last class, there’s a grab bag of features that we can take or leave: proof irrelevance, impredicative Prop, the axiom of choice, and others. Why should we be convinced that the collection we choose is consistent? We’ll introduce the notion of a model of the type theory to answer questions like this.

• Assignment
11/08/2021
Monday
Assignment #8 (ch 11) released!
• Lecture
11/10/2021
Wednesday
Quotients, rationals, and reals

The last bit of Ch. 11, on quotient types, is very relevant to what we want to do next! We’ll wrap up that discussion (including talking a bit about the computability properties of quotients) and then immediately use quotient types to define some familiar things. Rational and real numbers are interesting mathematically, and for programming purposes, they can be a very convenient tool for writing specifications. Even if we don’t compute with real numbers they’re useful to have around.

• Due
11/10/2021 15:00
Wednesday
Assignment #7 due
• Lecture
11/15/2021
Monday
Real numbers

We finished last week with the rational numbers. Now we need to complete them to get the reals. This will take yet another quotient. The reals bring to light some computability issues that we’ve touched on briefly before: what does it mean to compute with real numbers? How do we do it in normal languages? If time permits, we’ll look at mathlib’s implementation of the reals and see some generalizations.

• Assignment
11/16/2021
Tuesday
Assignment #9 (ch 13) released!
• Lecture
11/17/2021
Wednesday

Lean has a very powerful framework for writing custom tactics. These tactics are written in Lean itself, with a number of catches to make this possible. Today we’ll see the fundamentals of this approach. We’ll learn the (very) basics about monads, a technique used in some functional languages to simulate programming with side effects. (But this isn’t an FP class and we’re not going to dwell on monads, beyond what we need to know.)

Chapter 6 of the HHG is a more detailed introduction to monads. We’ll cover a bit of this, but mainly take an alternate approach to Chapter 7.

• Due
11/17/2021 15:00
Wednesday
Assignment #8 due
• Lecture
11/22/2021
Monday

More from chapters 6 and 7 of the HHG: we’ll look at the `expr` type, which reflects Lean expressions as a Lean datatype. There’s a big API around creating, modifying, and using expressions – unsurprisingly, since this is what’s meta about metaprogramming!

• Assignment
11/22/2021
Monday
Assignment #10 (ch 7) released!
• No Lecture
11/24/2021
Wednesday
Thanksgiving break
• Lecture
11/29/2021
Monday

There are lots of subtleties to writing metaprograms that we’ve skimmed over so far. In particular, there’s a disconnect between the syntax we use when writing tactics and the syntax we use within `begin...end` blocks. We’ll touch on these subtleties today. Time permitting, we’ll talk about design strategies for metaprograms, including certification and proof by reflection.

• Lecture
12/01/2021
Wednesday
Linear arithmetic

The tactic `linarith` solves linear programs over ordered rings. It’s a great example of a “large” metaprogram that shows off a number of interesting design principles. We’ll talk both about the algorithm it implements and the strategy used in implementing the tactic itself.

• Due
12/01/2021 15:00
Wednesday
Assignment #9 due
• Lecture
12/06/2021
Monday
Lecture

Our final class will have a guest lecturer: Jeremy Avigad will tell us about a project he and others have been working on to verify smart contract executions. The work he’ll talk about is also described in this paper (code). No prior experience with blockchains will be assumed: I know nothing about them myself!

Jeremy will join us via Zoom, which I’ll project to the class. Join in person or remotely, whichever you prefer. After Jeremy’s guest talk, we’ll have some time to chat about your final projects.

• Due
12/08/2021 15:00
Wednesday
Assignment #10 due
• Due
12/15/2021 23:59
Wednesday
Final project due