Just out, an essay on the Cambridge University Press author’s blog about “programming paradigms”, and why I did not structure Practical Foundations for Programming Languages around them.

Thoughts from an existential type.

Just out, an essay on the Cambridge University Press author’s blog about “programming paradigms”, and why I did not structure Practical Foundations for Programming Languages around them.

It is well-known that constructivists renounce “proof by contradiction”, and that classicists scoff at the critique. “Those constructivists,” the criticism goes, “want to rule out proofs by contradiction. How absurd! Look, Pythagoras showed that the square root of two is irrational by deriving a contradiction from the assumption that it is rational. There is nothing wrong with this. Ignore them!”

On examination that sort of critique fails, because *a proof by contradiction is not a proof that derives a contradiction*. Pythagoras’s proof is valid, one of the eternal gems of mathematics. No one questions the validity of that argument, even if they question proof by contradiction.

Pythagoras’s Theorem expresses a negation: *it is not the case that* the square root of two can be expressed as the ratio of two integers. Assume that it can be so represented. A quick deduction shows that this is impossible. So the assumption is false. Done. This is a* direct proof* of a negative assertion; it is *not* a “proof by contradiction”.

What, then, *is* a proof by contradiction? It is the *affirmation* of a positive statement by refutation of its denial. It is a *direct proof* of the negation of a negative assertion that is then pressed into service as a *direct proof* of the assertion, which it is not.* *Anyone is free to ignore the distinction for the sake of convenience, as a philosophical issue, or as a sly use of “goto” in a proof, but the distinction nevertheless exists and is important. Indeed, part of the beauty of constructive mathematics is that one can draw such distinctions, for, once drawn, one can selectively disregard them. Once blurred, forever blurred, a pure loss of expressiveness.

For the sake of explanation, let me rehearse a standard example of a proof by contradiction. The claim is that there exists irrationals a and b such that a to the b power is rational. Here is an indirect proof, a true proof by contradiction. Move number one, let us prove instead that it is impossible that any two irrationals a and b are such that a to the b is irrational. This is a negative statement, so of course one proves it be deriving a contradiction from assuming it. But it is not the original statement! This will be clear from examining the information content of the proof.

Suppose, for a contradiction, that every two irrationals a and b are such that a to the b power is irrational. We know from Pythagoras that root two is irrational, so plug it in for both a and b, and conclude that root two to the root two power is irrational. Now use the assumption again, taking a to be root two to the root two, and b to be root two. Calculate a to the power of b, it is two, which is eminently rational. Contradiction.

We have now proved that it is not the case that every pair of irrationals, when exponentiated, give an irrational. There is nothing questionable about this proof as far as I am aware. But it does not prove that there are two irrationals whose exponent is rational! If you think it does, then I ask you, please name them for me. That information is not in this proof (there are other proofs that do name them, but that is not relevant for my purposes). You may, if you wish, disregard the distinction I am drawing, that is your prerogative, and neither I nor anyone has any problem with that. But you cannot claim that it is a *direct proof*, it is rather an *indirect proof*, that proceeds by refuting the negative of the intended assertion.

So why am I writing this? Because I have learned, to my dismay, that in U.S. computer science departments–of all places!–students are being taught, *erroneously,* that any proof that derives a contradiction is a “proof by contradiction”. It is not. Any proof of a negative must proceed by contradiction. A proof by contradiction in the long-established sense of the term is, contrarily, an indirect proof of a positive by refutation of the negative. This distinction is important, even if you want to “mod out” by it in your work, for it is only by drawing the distinction that one can even define the equivalence with which to quotient.

That’s my main point. But for those who may not be familiar with the distinction between direct and indirect proof, let me take the opportunity to comment on why one might care to draw such a distinction. It is entirely a matter of intellectual honesty: the information content of the foregoing indirect proof does not fulfill the expectation stated in the theorem. It is a kind of boast, an overstatement, to claim otherwise. Compare the original statement with the reformulation used in the proof. The claim that it is not the case that every pair of irrationals exponentiate to an irrational is uncontroversial. The proof proves it directly, and there is nothing particularly surprising about it. One would even wonder why anyone would bother to state it. Yet the supposedly equivalent claim stated at the outset appears much more fascinating, because most people cannot easily think up an example of two irrationals that exponentiate to rationals. Nor does the proof provide one. Once, when shown the indirect proof, a student of mine blurted out “oh that’s so cheap.” Precisely.

Why should you care? Maybe you don’t, but there are nice benefits to keeping the distinction, because it demarcates the boundary between constructive proofs, which have direct interpretation as functional programs, and classical proofs, which have only an indirect such interpretation (using continuations, to be precise, and giving up canonicity). Speaking as a computer scientist, this distinction matters, and it’s not costly to maintain. May I ask that you adhere to it?

*Edit: **rewrote final paragraph, sketchy and irrelevant, and improved prose throughout. *

I gave a presentation at the Workshop on Categorical Logic and Univalent Foundations held in Leeds, UK July 27-29th 2016. My talk, entitled *Computational Higher Type Theory*, concerns a formulation of higher-dimensional type theory in which terms are interpreted directly as programs and types as programs that express specifications of program behavior. This approach to type theory, first suggested by Per Martin-Löf in his famous paper *Constructive Mathematics and Computer Programming *and developed more fully by The NuPRL Project, emphasizes constructive mathematics in the Brouwerian sense: proofs are programs, propositions are types.

The now more popular accounts of type theory emphasize the *axiomatic freedom* given by making fewer foundational commitments, such as not asserting the decidability of every type, but give only an indirect account of their computational content, and then only in some cases. In particular, the computational content of Voevodsky’s Univalence Axiom in Homotopy Type Theory remains unclear, though the Bezem-Coquand-Huber model in cubical sets carried out in constructive set theory gives justification for its constructivity.

To elicit the computational meaning of higher type theory more clearly, emphasis has shifted to *cubical type theory* (in at least two distinct forms) in which the higher-dimensional structure of types is judgmentally explicit as the higher cells of a type, which are interpreted as identifications. In the above-linked talk I explain how to construe a cubical higher type theory directly as a programming language. Other efforts, notably by Cohen-Coquand-Huber-Mörtberg, have similar goals, but using somewhat different methods.

For more information, please see my home page on which are linked two arXiv papers providing the mathematical details, and a 12-page paper summarizing the approach and the major results obtained so far. These papers represent joint work with Carlo Angiuli and Todd Wilson.

I am building a web page devoted to the 2nd edition of *Practical Foundations for Programming Languages*, recently published by Cambridge University Press. Besides an errata, the web site features a commentary on the text explaining major design decisions and suggesting alternatives. I also plan to include additional exercises and to make sample solutions available to faculty teaching from the book.

The purpose of the commentary is to provide the “back story” for the development, which is often only hinted at, or is written between the lines, in *PFPL* itself. To emphasize enduring principles over passing fads, I have refrained from discussing particular languages in the book. But this makes it difficult for many readers to see the relevance. One purpose of the commentary is to clarify these connections by explaining *why* I said what I said.

As a starting point, I explain why I ignore the familiar concept of a “paradigm” in my account of languages. The idea seems to have been inspired by Kuhn’s (in)famous book *The Structure of Scientific Revolutions*, and was perhaps a useful device at one time. But by now the idea of a paradigm is just too vague to be useful, and there are many better ways to explain and systematize language structure. And so I have avoided it.

I plan for the commentary to be a living document that I will revise and expand as the need arises. I hope for it to provide some useful background for readers in general, and teachers in particular. I wish for the standard undergraduate PL course to evolve from a superficial taxonomy of the weird animals in the language zoo to a systematic study of the general theory of computation. Perhaps *PFPL* can contribute to effecting that change.

Today I received my copies of *Practical Foundations for Programming Languages, Second Edition* on Cambridge University Press. The new edition represents a substantial revision and expansion of the first edition, including these:

- A new chapter on type refinements has been added, complementing previous chapters on dynamic typing and on sub-typing.
- Two old chapters were removed (general pattern matching, polarization), and several chapters were very substantially rewritten (higher kinds, inductive and co-inductive types, concurrent and distributed Algol).
- The parallel abstract machine was revised to correct an implied extension that would have been impossible to carry out.
- Numerous corrections and improvements were made throughout, including memorable and pronounceable names for languages.
- Exercises were added to the end of each chapter (but the last). Solutions are available separately.
- The index was revised and expanded, and some conventions systematized.
- An inexcusably missing easter egg was inserted.

I am grateful to many people for their careful reading of the text and their suggestions for correction and improvement.

In writing this book I have attempted to organize a large body of material on programming language concepts, all presented in the unifying framework of type systems and structural operational semantics. My goal is to give precise definitions that provide a clear basis for discussion and a foundation for both analysis and implementation. The field needs such a foundation, and I hope to have helped provide one.

A recent discussion of introductory computer science education led to the topic of teaching recursion. I was surprised to learn that students are being taught that recursion requires understanding something called a “stack” that is nowhere in evidence in their code. Few, if any, students master the concept, which is usually “covered” only briefly. Worst, they are encouraged to believe that recursion is a mysterious bit of esoterica that is best ignored.

And thus is lost one of the most important and beautiful concepts in computing.

The discussion then moved on to the implementation of recursion in certain inexplicably popular languages for teaching programming. As it turns out, the compilers mis-implement recursion, causing unwarranted space usage in common cases. Recursion is dismissed as problematic and unimportant, and the compiler error is elevated to a “design principle” — to be snake-like is to do it wrong.

And thus is lost one of the most important and beautiful concepts in computing.

And yet, for all the stack-based resistance to the concept, *recursion** has nothing to do with a stack*. Teaching recursion does not need any mumbo-jumbo about “stacks”. Implementing recursion does not require a “stack”. The idea that the two concepts are related is simply mistaken.

What, then, is recursion? It is nothing more than *self-reference*, the ability to name a computation for use within the computation itself. *Recursion is what it is*, and nothing more. No stacks, no tail calls, no proper or improper forms, no optimizations, just self-reference pure and simple. Recursion is not tied to “procedures” or “functions” or “methods”; one can have self-referential values of all types.

Somehow these very simple facts, which date back to the early 1930’s, have been replaced by damaging myths that impede teaching and using recursion in programs. It is both a conceptual and a practical loss. For example, the most effective methods for expressing parallelism in programs rely heavily on recursive self-reference; much would be lost without it. And the allegation that “real programmers don’t use recursion” is beyond absurd: the very concept of a digital computer is grounded in recursive self-reference (the cross-connection of gates to form a latch). (Which, needless to say, does not involve a stack.) Not only do real programmers use recursion, there could not even be programmers were it not for recursion.

I have no explanation for why this terrible misconception persists. But I do know that when it comes to programming languages, attitude trumps reality every time. Facts? We don’t need no stinking facts around here, amigo. You must be some kind of mathematician.

If all the textbooks are wrong, what is right? How *should* one explain recursion? It’s simple. If you want to refer to yourself, you need to give yourself a name. “I” will do, but so will any other name, by the miracle of α-conversion. A computation is given a name using a *fixed point* (not *fixpoint*, dammit) operator: *fix x is e* stands for the expression *e* named *x* for use within *e*. Using it, the textbook example of the factorial function is written thus:

fix f is fun n : nat in case n {zero => 1 | succ(n') => n * f n'}.

Let us call this whole expression *fact,* for convenience. If we wish to evaluate it, perhaps because we wish to apply it to an argument, its value is

fun n : nat in case n {zero => 1 | succ(n') => n *factn'}.

The recursion has been *unrolled* one step ahead of execution. If we reach *fact* again, as we will for a positive argument, *fact* is evaluated again, in the same way, and the computation continues. *There are no stacks involved in this explanation*.

Nor is there a stack involved in the implementation of fixed points. It is only necessary to make sure that the named computation does indeed name itself. This can be achieved by a number of means, including circular data structures (non-well-founded abstract syntax), but the most elegant method is by *self-application*. Simply arrange that a self-referential computation has an implicit argument with which it refers to itself. Any use of the computation unrolls the self-reference, ensuring that the invariant is maintained. No storage allocation is required.

Consequently, a self-referential functions such as

fix f is fun (n : nat, m:nat) in case n {zero => m | succ(n') => f (n',n*m)}

execute without needing any asymptotically significant space. It is quite literally a loop, and *no special arrangement* is required to make sure that this is the case. All that is required is to implement recursion properly (as self-reference), and you’re done. *There is no such thing as tail-call optimization. *It’s not a matter of optimization, but of proper implementation. Calling it an optimization suggests it is optional, or unnecessary, or provided only as a favor, when it is more accurately described as a matter of getting it right.

So what, then, is the source of the confusion? The problem seems to be a too-close association between compound expressions and recursive functions or procedures. Consider the classic definition of factorial given earlier. The body of the definition involves the expression

n *factn'

where there is a pending multiplication to be accounted for. Once the recursive call (to itself) completes, the multiplication can be carried out, and it is necessary to keep track of this pending obligation. *But this phenomenon has nothing whatsoever to do with recursion.* If you write

n *squaren'

then it is equally necessary to record where the external call is to return its value. In typical accounts of recursion, the two issues get confused, a regrettable tragedy of error.

Really, the need for a stack arises the moment one introduces compound expressions. This can be explained in several ways, none of which need pictures or diagrams or any discussion about frames or pointers or any extra-linguistic concepts whatsoever. The best way, in my opinion, is to use Plotkin’s structural operational semantics, as described in my *Practical Foundations for Programming Languages (Second Edition)* on Cambridge University Press.

There is no reason, nor any possibility, to avoid recursion in programming. But folk wisdom would have it otherwise. That’s just the trouble with folk wisdom, everyone knows it’s true, even when it’s not.

*Update*: Dan Piponi and Andreas Rossberg called attention to a pertinent point regarding stacks and recursion. The conventional notion of a run-time stack records two distinct things, the *control state* of the program (such as subroutine return addresses, or, more abstractly, pending computations, or continuations), and the *data state* of the program (a term I just made up because I don’t know a better one, for managing multiple simultaneous activations of a given procedure or function). Fortran (back in the day) didn’t permit multiple activations, meaning that at most one instance of a procedure can be in play at a given time. One consequence is that α-equivalence can be neglected: the arguments of a procedure can be placed in a statically determined spot for the call. As a member of the Algol-60 design committee Dijkstra argued, successfully, for admitting multiple procedure activations (and hence, with a little extra arrangement, recursive/self-referential procedures). Doing so requires that α-equivalence be implemented properly; two activations of the same procedure cannot share the same argument locations. The data stack implements α-equivalence using de Bruijn indices (stack slots); arguments are passed on the data stack using activation records in the now-classic manner invented by Dijkstra for the purpose. It is not self-reference that gives rise to the need for a stack, but rather re-entrancy of procedures, which can arise in several ways, not just recursion. Moreover, recursion does not always require re-entrancy—the so-called tail call optimization is just the observation that certain recursive procedures are not, in fact, re-entrant. (Every looping construct illustrates this principle, albeit on an *ad hoc* basis, rather than as a general principle.)