[fg-arc] FME Teaching Tutorial on June 6, 2025, 3pm CET: Prof Philip Wadler, University of Edinburgh, UK: Lambda, the Ultimate Teaching Assistant (Agda version)

["João F. Ferreira"]

*[Apologies for cross-posting]*

Dear all,

We continue our Formal Methods Teaching tutorials series with a lecture on Friday, June 6, at 3 pm CET.

Prof. Philip Wadler from The University of Edinburgh will talk about his experience with his textbook Programming Language Foundations in Agda (PLFA).

Abstract:

Proof assistants, such as Agda, Coq, HOL, Isabelle, Lean, or Twelf, permit formal statements of propositions and proofs in a way that can be checked by a computer. Some assistants, such as Coq, also provide a way to write tactics to describe how to search for a proof. The use of mathematics to describe programs and programming languages, known as formal methods, can benefit from a proof assistant. In particular, use of a proof assistant in teaching provides immediate feedback to students.

One of the leading textbooks for formal methods is Software Foundations (SF), written by Benjamin Pierce in collaboration with others, and based on the proof assistant Coq. After five years using SF in the classroom, I have come to the conclusion that Coq is not the best vehicle for this purpose, as too much of the course needs to focus on learning tactics, to the cost of learning programming language theory. Accordingly, I have written a textbook, Programming Language Foundations in Agda (PLFA), based on the proof assistant Agda. PLFA covers much of the same ground as SF, although it is not a slavish imitation.

What did I learn from writing PLFA? First, that it is possible. One might expect that without tactics proofs become too long, but in fact proofs in PLFA are about the same length as those in SF. Proofs in Coq require an interactive environment to be understood, while proofs in Agda can be read on the page. Second, I compared two standard methods of formalising programming, one with extrinsically-typed terms and one with inherently-typed terms. SF uses the former presentation, while PLFA presents both; the former uses about 1.6 as many lines of Agda code as the latter, roughly the golden ratio. Third, that constructive proofs of preservation and progress give immediate rise to a prototype evaluator. This fact is obvious in retrospect but it is not exploited in SF (which instead provides a separate normalise tactic) nor can I find it in the literature. Fourth, that if the final exam is administered online with a proof assistant, students can achieve perfection.

The textbook is written as a literate Agda script, and can be found here:

http://plfa.inf.ed.ac.uk

The event will last about an hour. The Zoom link is: https://aboakademi.zoom.us/j/64254430116 

More details can be found in the tutorial series webpage:
https://fme-teaching.github.io/2021/08/24/tutorial-series-of-the-fme-teaching-committee/

Everyone is warmly welcome!

Best wishes,
João
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