On compositionality

Jules Hedges has written a thought-provoking blog post, On compositionality where he connects the familiar idea of compositionality to the idea of emergent effects in nature, where systems can be understood as either having compositional properties or emergent properties.

The key point about emergent systems is that they are hard to understand, and this is as true for engineering as it is for science. He goes on to say "As a final thought, I claim that compositionality is extremely delicate, and that it is so powerful that it is worth going to extreme lengths to achieve it", so that avoiding emergent effects is a characteristic of good programming language design.

Some thoughts:

  1. His examples of emergent systems are biology and game theory from an economic perspective. I would add to this list physics: of his co-authored paper showing that the spectral gap is undecidable, David Pérez-García said "For example, our results show that adding even a single particle to a lump of matter, however large, could in principle dramatically change its properties."
  2. Spolsky's famous characterisation of interfaces built on shaky foundations as Leaky abstractions to me makes the distinction between compositional and emergent systems a little less than sharp.
  3. We could talk endlessly about the list of what he regards as compositionality-breaking features of PLs. The evils of global state are well-documented, but I find dmbarbour's argument that Local state is poison a very interesting alternative way to look at what properties do we want from code; more generally, what kind of compositionalty PLs offer is very much paradigm dependent. Gotos are considered harmful, but the Linux kernel has little trouble with longjmp because of its mandated coding style: compositionality in engineering is a not just a matter of semantics but also of use. He targets OO and Haskell's type classes - I think he is quite correct - note that within these paradigms one can regain compositionality by restricting to LSP or algebraic classes, and supporting his thesis we see that these ideas have spawned ideas for the design of new, cleaner PLs.

Notes on notation and thought

(via HN)

A nice collection of quotes on notation as a tool of thought. Mostly not programming related, which actually makes them more interesting, offering a richer diversity of examples. We used to have quite a few discussions of notation in the early days (at least in part because I never accepted the prevailing dogma that syntax is not that interesting or important), which is a good reminder for folks to check the archives.

Sequent Calculus as a Compiler Intermediate Language

Sequent Calculus as a Compiler Intermediate Language
2016 by Paul Downen, Luke Maurer, Zena M. Ariola, Simon Peyton Jones

The typed λ-calculus arises canonically as the term language for a logic called natural deduction, using the Curry-Howard isomorphism: the pervasive connection between logic and programming languages asserting that propositions are types and proofs are programs. Indeed, for many people, the λ-calculus is the living embodiment of Curry-Howard.

But natural deduction is not the only logic! Conspicuously, natural deduction has a twin, born in the very same paper, called the sequent calculus. Thanks to the Curry-Howard isomorphism, terms of the sequent calculus can also be seen as a programming language with an emphasis on control flow.

Why Is SQLite Coded In C

We are nearing the day someone quips that C is an improvement on most of its successors (smirk). So reading this page from the SQLite website is instructive, as is reading the page on the tooling and coding practices that make this approach work.

I think none of this is news, and these approaches have been on the books for quite a bit. But still, as I said: an improvement on most of its successors. Hat tip: HN discussion.

Proceedings of the ACM on Programming Languages

Proceedings of the ACM on Programming Languages (PACMPL) is a Gold Open Access journal publishing research on all aspects of programming languages, from design to implementation and from mathematical formalisms to empirical studies. Each issue of the journal is devoted to a particular subject area within programming languages and will be announced through publicized Calls for Papers.

See the ToC of the September 2017, ICFP issue, here. Some very cool stuff.


Graydon Hoare: What next for compiled languages?

Since everybody is talking about this post,we might as well.

Key topics discussed: modules(you know, real ones); errors ("there are serious abstraction leakages and design trade-offs in nearly every known approach"); Coroutines, async/await, "user-visible" asynchronicity; effect systems, more generally (you could see that coming, couldn't you?); Extended static checking (ESC), refinement types, general dependent-typed languages; and formalization ("we have to get to the point where we ship languages -- and implementations -- with strong, proven foundations").

He goes on to discuss a whole grab bag of "potential extras" for mainstream languages, including the all time favorite: units of measure.

Feel free to link to the relevant discussions from the LtU archive...


p5.js is a JavaScript library inspired by Processing. Seems it could be a fun way to introduce non-CS types to programming. The demo is particularly well done; check it out first. The actual home of the project is here.

Idris 1.0 Released

What do we mean by “1.0”?

Idris version 1.0 corresponds to the language as described in Type-Driven Development with Idris, published last week by Manning.

Stroustrup's Rule and Layering Over Time

Dave Herman is the voice of the oppressed: syntax is important, contrary to what you have been told!

To illustrate he discusses what he calls Stroustrup's Rule:

  • For new features, people insist on LOUD explicit syntax.
  • For established features, people want terse notation.

Joe Armstrong Interviews Alan Kay

Youtube video (via HN)

By far not the best presentation of Kay's ideas but surely a must watch for fans. Otherwise, skip until the last third of the interview which might add to what most people here already know.

It is interesting that in this talk Kay rather explicitly talks about programming languages as abstraction layers. He also mentions some specifics that may not be as well known as others, yet played a role in his trajectory, such as META.

I fully sympathize with his irritation with the lack of attention to and appreciation of fundamental concepts and theoretical frameworks in CS. On the other hand, I find his allusions to biology unconvincing.
An oh, he is certainly right about Minsky's book (my first introduction to theoretical CS) and in his deep appreciation of John McCarthy.

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