







February 2008



Arc's been out for a few days now. Everyone seems to have an opinion about it. The low end are much the same as the low end of opinions about anything online. There's no high end yet, because no one has had enough time yet to be able to speak from experience about what it's like to program in Arc. Those are the responses I'm really interested in. But in the meantime, we have the medium-level responses: the opinions of people who have some understanding of the issues involved, but who are writing based on first impressions.



I've noticed a strange pattern in these. The main fault they find with Arc is that I don't seem to have had to work hard enough writing it. Ron Garret writes: And that is my main gripe about Arc: it has been so long in the making and set such lofty goals and then it seems to pretty much punt on all the hard problems of language design. I don't usually refute criticisms directly. Refutations tend to be more gratifying to write than to read. But in this case I'm going to, because in this case Ron & Co are mistaken in an illuminating way. Explaining why will clarify some important issues about language design.



Ron's right. I didn't decide what problems to work on based on how hard they were. Instead I used what might seem a rather mundane test: I worked on things that would make programs shorter.



Why would I do that? Because making programs short is what high level languages are for. It may not be 100% accurate to say the power of a programming language is in inverse proportion to the length of programs written in it, but it's damned close. Imagine how preposterous it would sound if someone said "The program is 10 lines of code in your language and 50 in my language, but my language is more powerful." You'd be thinking: what does he mean by power, then?



I'm not claiming that power is the only criterion by which to judge programming languages. It would also be a legitimate goal, for example, to design a language to be easy for kids to learn, or to compile efficiently (though this is less common than it used to be), or to limit the damage that can be done by individual bad programmers within a group. But power is the test of languages not designed for such special purposes.



So working on what makes programs short rather than what's hard to implement translates to: I chose what to work on based on the value to the user, rather than the cost to me. Surely this is the right order of priorities to have in designing not just programming languages, but anything meant to be used by other people.



This is not to say none of the stuff I did was hard. Some of it seemed hard to me. But in language design, solving problems, whether hard or easy, is not the goal. Making a good language is. The real test of Arc—and any other general-purpose high level language—is not whether it contains feature x or solves problem y, but how long programs are in it.



The programs that get shorter should be the ones users actually need to write. So my m.o. while working on Arc was to write applications in it, then comb through them line by line trying to imagine language features that would make them shorter. Then I'd implement those features, rewrite the program to use them, and start over. Here's a comment from the source of Hacker News where I occasionally kept track: ; results of (codetree "news.arc"): ; 8787, 8760, 8738, 8726, 8823, 8755, 8703, 8628, 8587, 8565 ; 8633, 8573, 8552, 8520, 8510, 8498, 8549, 8515, 8684, 9025 ; 9573, 12196, 12469, 12648, 12373 (The numbers go up sometimes because I'd added features to News.)



This is one reason the source code of Arc itself is so short, incidentally. I did the same thing to it. But my first priority was making applications shorter, not the language. There are features, most notably Prolog-style pattern-matching, that seem to promise great savings in length, but turn out only to be useful for writing a few basic sequence operations like append, remove, and so on. Prolog is a great language for writing append; after that it's all downhill.



Another goal I had while writing Arc was to continue as long as possible in the mode in which McCarthy began. In his original 1960 paper he built Lisp up from "axioms" like car, cdr, and cons, through "theorems" like assoc and mapcar. [ 1 ] There must be some optimal path all the way up to a complete language. What is it? McCarthy didn't get very far along it in his paper. And after that the language passed into the hands of his grad students, who at the time were more worried about the exigencies of making an interpreter run on the IBM 704 than continuing McCarthy's axiomatic approach. We've been living with their hacks ever since. Steele and Sussman tried to start over when they first began working on Scheme, but they seem to have been practically the only ones. And they made, at least from the point of view of brevity/power, some serious mistakes early on.



This seemed a territory worth exploring. And I hoped that with the axioms pushing from below and the demands of brevity in real applications pushing from above, I'd be able to grow an optimal core of operators. I'm not claiming I have yet, just that that's the goal: to find an optimal path from a small number of axioms up to a complete language for everyday use. I've made compromises for efficiency. I'm not using Church numerals. [ 2 ] But I've tried to preserve as much of the spirit of the original 1960 paper as I could.



Building up the language from axioms is not an end in itself either. I'm only doing it because I suspect that's the way to get maximum expressive power.



How well does Arc deliver so far? Does it make programs shorter than they'd be in other languages? Let's try measuring.



I'm going to propose a simple problem as a challenge. We'll collect solutions in each of the popular languages, and compare their lengths. Here it is: Write a program that causes the url said (e.g. http://localhost:port/said) to produce a page with an input field and a submit button. When the submit button is pressed, that should produce a second page with a single link saying "click here." When that is clicked it should lead to a third page that says "you said: ..." where ... is whatever the user typed in the original input field. The third page must only show what the user actually typed. I.e. the value entered in the input field must not be passed in the url, or it would be possible to change the behavior of the final page by editing the url. Though simple, as such tests have to be, this is not a contrived example. Web apps have to do this sort of thing all the time. Nor does it depend on some sort of esoteric libraries that Arc has and other languages don't; this is all stuff that any language used to write Web apps would have to have already.



Here's the answer in Arc: (defop said req (aform [w/link (pr "you said: " (arg _ "foo")) (pr "click here")] (input "foo") (submit))) If you're not used to Arc you'll have to take my word for it, but this is not code that has been compressed using coding tricks. This would be the standard way to write it in Arc.



The most meaningful test of the length of a program is not lines or characters but the size of the codetree-- the tree you'd need to represent the source. The Arc example has a codetree of 23 nodes: 15 leaves/tokens + 8 interior nodes. How long is it in your favorite language?



(Code to import standard libraries doesn't count, of course; you can assume those are already loaded.)



I've posted this problem on the Arc Forum at http://arclanguage.org/item?id=722. If you have a solution in a language that's not yet represented, or a shorter or more correct solution for a language that is, please add it. It will be interesting to compare languages not just to Arc but to one another.







Update: May 2009



The Arc version would now be slightly shorter: (defop said req (aform [onlink "click here" (pr "you said: " (arg _ "foo"))] (input "foo") (submit))) This is 21 nodes: 14 leaves + 7 interior.











Notes



[ 1 ] "Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I," CACM, April 1960.



http://www-formal.stanford.edu/jmc/recursive/recursive.html



[ 2 ] I did once try representing the integer n as a list of length n, with horrifying results.







































