# Spring 2017 PL Junior Retrospective

:: PL Junior

The PL Junior Seminar is for beginning PhD and interested undergrad and masters students to understand the foundations of programming languages research. It serves to fill in background knowledge and get up to speed with different areas of PL research.

For the spring 2017 instance of PL Junior we chose program synthesis, the sequent calculus, and logic programming as topics we wanted to learn more about. We also did two group paper readings for Luca Cardelli’s Typeful Programming and Alan Kay’s Early History of Smalltalk. At the same time, we changed up the format from the previous semester.

## Format

As discussed in last fall’s retrospective, we wanted to move from group reading and discussion towards weekly presentations. Reading a paper to prepare a presentation is quite a different experience compared to the effort that goes in when it is just for a group discussion (in our experience). With any luck, the presentation will convey some of this deeper knowledge to the rest of the group, with the result being a deep understanding on the part of the presenter and an informed, if somewhat shallower, understanding in the rest of the group. Ideally, the end result should compare favorably to simply reading the paper individually.

One idea from last semester that we decided to keep is to spend a length of time (possibly an entire semester) on a topic rather than having a new topic each week. Staying on the same theme helps with retention as well as allowing for deeper investigation.

In that spirit, we chose three themes for the semester: program synthesis, the sequent calculus, and logic programming. Mostly by chance, these topics have interesting connections to each other, and we even had several PL Grown-Up Seminars this semester on program synthesis!

## Synthesis

The first paper on program synthesis that we looked at was A Deductive Approach to Program Synthesis by Manna and Waldinger. We chose this paper because it’s old and has a lot of citations so it’s probably Important. It was interesting and provided an OK introduction to proof search but the method presented seems far removed from modern synthesis techniques.

The next paper was Programmatic and Direct Manipulation, Together by Chugh, Hempel, Spradlin, and Alders, which presents the Sketch-n-Sketch system. Sketch-n-Sketch is a cool system. It demonstrates that a narrow application of synthesis - trying to fill in the constant values in a program (sketching) - can be used for great effect. We were left wondering, however, if it was too narrow an application of synthesis to give much of an indication of what the entire field is like.

We concluded our program synthesis segment with Type-and-Example-Directed Program Synthesis by Osera and Zdancewic, another relatively recent paper. This seems like a relevant paper because we are under the impression that using examples to do synthesis is a big thing right now. Using types to constrain the search is another interesting perspective on techniques for synthesis.

While each of theses papers had merits, none was so comprehensive as to be a necessary inclusion in any future look at program synthesis for pl junior

## Sequent Calculus

We followed up the program synthesis unit with a week on the sequent calculus. The seminar presentation was based on a paper by Herbelin. Gabriel’s thesis (chapter 4) includes maybe a more suitable modern introduction to the sequent calculus.

It might have been better to do sequent calculus first because there is a modern branch of proof search based on the sequent calculus. Presenting this first would have allowed us to look into proof search for program synthesis.

An additional problem is that it was insufficiently motivated. Either skipping the topic or spending more time on it would be preferable, since one week was just enough to describe the sequent calculus but not enough to apply it. For this topic to be worthwhile, it would best be used as the basis for subsequent readings that directly reference it.

## Logic Programming

The topic was presented over two weeks. The first session presented/demoed Prolog as a language, and we got a sense of what logic programming could do. But it was a whirlwind tour, and we were left wondering about specific details (how proof search runs, what cut does).

The second session presented the paper The Semantics of Predicate Logic as a Programming Language. It was interesting and insightful but left wondering how it relates to the implementation of real logic programming languages.

In hindsight this was about as far as we could have gotten in just two weeks. However, complications such as the cut rule seem prevalent enough in practice that more time would be required to build up a useful understanding of logic programming

## Bonus Rounds

We also used a few weeks to read and discuss specific papers as a group.

The first paper we read was Cardelli’s Typeful Programming. We picked typeful programming because Matthias has mentioned on occasion how important he thinks it is.

It was an interesting read; more of an essay than a paper. It really stood out as different from the other academic publications that we have looked at. It’s a walk through of a language design motivating each design decision in practical terms, as in things that actually help the programmer.

Cardelli places great importance on polymorphism (subtyping in addition to parametric), as well as features for programming in the large such as modules and interfaces. Several features are interesting in their omission, like type inference and macros.

After reading it it’s not clear why Matthias thinks it’s so important. From the perspective of modern researchers, many of the features in Cardelli’s language seem rather mundane. However, it’s likely that at the time he published it, these ideas were significantly newer and much less widespread.

The other paper we read as a group was Alan Kay’s The Early History of Smalltalk. It seems like the Smalltalk project investigated a plethora of interesting ideas about designing programming languages and environments. This article seems to confirm that but does not delve into many particulars.

## Final Thoughts

Overall this semester of pl junior went well enough that we think it makes a reasonable template for future semesters. The topics were interesting and relevant, and we mostly picked appropriate material for presentations. One downside is that we didn’t quite ‘fill out’ the semester with presentations due to scheduling and not wanting to make some people present twice. Here’s a lesson: recruit more people to the phd program (or get more undergrads) so you don’t have this problem!

Having papers in a theme helped a lot over previous paper-presentation iterations of pl junior. It helped each week being able to build on what we learned last week, as opposed to having a whirlwind of unrelated topics.

Writing this retrospective has also proven to be a beneficial exercise. Especially with our sequences of connected topics, looking back has allowed us to put the earlier papers into perspective and better assess both their relevance and presentation.