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Growing a Language
Guy L. Steele Jr. Sun Microsystems Laboratories 1 Network Drive Burlington, Massachusetts 01803 [email protected] October 1998
[This is the text of a talk I once gave, but with a few bugs fixed here and there, and a phrase or two changed to make my thoughts more clear. The talk as I first gave it can be had on tape [12].] I think you know what a man is. A woman is more or less like a man, but not of the same sex. (This may seem like a strange thing for me to start with, but soon you will see why.) Next, I shall say that a person is a woman or a man (young or old). To keep things short, when I say “he” I mean “he or she,” and when I say “his” I mean “his or her.” A machine is a thing that can do a task with no help, or not much help, from a person. (As a rule, we can speak of two or more of a thing if we add an “s” or “z” sound to the end of a word that names it.)
〈noun〉 ::= 〈noun that names one thing〉 “s” | 〈noun that names one thing〉 “es”
These are names of persons: Alan Turing, Alonzo Church, Charles Kay Ogden, Christo- pher Alexander, Eric Raymond, Fred Brooks, John Horton Conway, James Gosling, Bill Joy, and Dick Gabriel. The word other means “not the same.” The phrase other than means “not the same as.” A number may be nought, or may be one more than a number. In this way we have a set of numbers with no bound.
〈number〉 ::= 0 | 1 + 〈number〉
There are other numbers as well, but I shall not speak more of them yet. These numbers—nought or one more than a number—can be used to count things. We can add two numbers if we count up from the first number while we count down from the number that is not the first till it comes to nought; then the first count is the sum.
4 + 2 = 5 + 1 = 6 + 0 = 6
Four plus two is the same as five plus one, which is the same as six plus nought, which is six. We shall take the word many to mean “more than two in number.”
Think of a machine that can keep track of two numbers, and count each one up or down, and test if a number be nought and by such a test choose to do this or that. The list of things that it can do and of choices that it can make must be of a known size that is some number.
A
B
Q 0 up A Q 1 Q 1 down B Q 0 Q 2 Q 2 up A Q 3 Q 3 up B Q 4 Q 4 down A Q 4 Q 4
STATE
Here you can see the two numbers and a list. The machine starts its work with the first row of the list. Each row in the list has a state name; the word “up” or “down”; and which number to count up or count down. For “up,” we have the name of the next state to go to (and the machine counts the number up by one); for “down,” the machine first tests the number, and so we have the name of the new state to go to if the number be nought and the name of the new state to go to if the number be other than nought (in which case the machine counts the number down by one). Note that no two rows of the list have the same state name. A computer is a machine that can do at least what the two number machine can do— and we have good cause to think that if a computer task can be done at all, then the two number machine can do it, too, if you put numbers in and read them out in the right way. In some sense, all computers are the same; we know this thanks to the work of such persons as Alan Turing and Alonzo Church. A vocabulary is a set of words. A language is a vocabulary and rules for what a string of words might mean to a person or a machine that hears them. To define a word is to tell what it means. When you define a word, you add it to your vocabulary and to the vocabulary of each person who hears you. Then you can use the word.
terms of the word “person”, and it seemed least hard to define “person” as “a man or a woman.” (I know, I know: this does not quite give the true core meaning of the word “person”; but, please, cut me some slack here.) By chance, the word “man” has one syllable and so is a primitive, but the word “woman” has two syllables and so I had to define it. In a language other than English, the words that mean “man” and “woman” might each have one syllable—or might each have two syllables, in which case one would have to take some other tack. We have to do this a lot when we write real computer programs: a thought that seems like a primitive in our minds turns out not to be a primitive in a programming language, and in each new program we must define it once more. A good example of this is max, which yields the more large of two numbers. It is a primitive thought to me, but few programming languages have it as a primitive; I have to define it. This is the sort of thing that makes a computer look like a person who is but four years old. Next to English, all computer programming languages are small; as we write code, we must stop now and then to define some new term that we will need to use in more than one place. Some persons find that their programs have a few large chunks of code that do the “real work” plus a large pile of small bits of code that define new words, so to speak, to be used as if they were primitives. I hope that this talk brings home to you—in a way you can feel in your heart, not just think in your head—what it is like to have to program in that way. This should show you, true to life, what it is like to use a small language. Each time I have tried this sort of thing, I have found that I can not say much at all till I take the time to define at least a few new terms. In other words, if you want to get far at all with a small language, you must first add to the small language to make a language that is more large. (In some cases we will find it more smooth to add the syllable “er” to the end of a word than to use the word “more” in front of it; in this way we might say “smoother” in place of “more smooth” or “larger” in place of “more large.” Let me add that better means “more good.”)
〈word that can change what a noun means〉 ::= 〈word that can change what a noun means〉 “er” 〈word that can change what a noun means〉 “r”
In truth, the words of one syllable form quite a rich vocabulary, with which you can say many things. You may note that this vocabulary is much larger than that of the language called Basic English, defined by Charles Kay Ogden in the year one nine three nought [10]. I chose not to use Basic English for this talk, for the cause that Basic English has many words of two or more syllables, some of them quite long, handed down to us from the dim past of Rome. While Basic English has fewer words, it does not give the feel, to one who speaks full English, of being a small language. By the way, from now on I shall use the word because to mean “for the cause that.” If we look at English and then at programming languages, we see that all our program- ming languages seem small. And yet we can say, too, in some cases, that one programming language is smaller than some other programming language. A design is a plan for how to build a thing. To design is to build a thing in one’s mind but not yet in the real world—or, better yet, to plan how the real thing can be built. The main thing that I want to ask in this talk is: If I want to help other persons to write all sorts of programs, should I design a small programming language or a large one?
I stand on this claim: I should not design a small language, and I should not design a large one. I need to design a language that can grow. I need to plan ways in which it might grow—but I need, too, to leave some choices so that other persons can make those choices at a later time. This is not a new thought. We have known for some time that huge programs can not be coded from scratch all at once. There has to be a plan for growth. What we must stop to think on now is the fact that languages have now reached that large size where they can not be designed all at once, much less built all at once. Let me pause to define some names for numbers. Twenty is twice ten. Thirty is thrice ten. Forty is twice twenty. A hundred is ten times ten. A million is a hundred times a hundred times a hundred. Eleven is ten plus one. Thirteen is ten plus three. Fourteen is ten plus four. Sixteen is twice eight. Seven is one plus six. Fifty is one more than seven squared. One more thing: ago means “in the past, as one counts back from now.” In the past, it made sense to design a whole language, once for all. Fortran was a small language forty years ago, designed for tasks with numbers, and it served well. PL/I was thought a big language thirty years ago, but now we would think of it as small. Pascal was designed as a small, whole language with no plan to add to it at a later time. That was five and twenty years ago. What came to pass? Fortran has grown and grown. Many new words and new rules have been added. The new design is not bad; the parts fit well, one with the other. But to many of those who have used Fortran for a long time, the Fortran of here and now is not at all the same as the language they first came to know and love. It looks strange. PL/I has not grown much. It is, for the most part, just as it was when it first came out. It may be that this is just from lack of use. The flip side is that the lack of use may have two causes. Number one: PL/I was not designed to grow—it was designed to be all things to all who program right from the start. Number two: for its time, it started out large. No one knew all of PL/I; some said that no one could know all of PL/I. Pascal grew just a tad and was used to build many large programs. One main fault of the first design was that strings were hard to use because they were all of fixed size. Pascal would have been of no use for the building of large programs for use in the real world if this had not been changed. But Wirth had not planned for the language to change in such ways, and in fact few changes were made. At times we think of C as a small language designed from whole cloth. But it grew out of a smaller language called B, and has since grown to be a larger language called C plus plus. A language as large as C plus plus could not have spread so wide if it had been foisted on the world all at once. It would have been too hard to port. (One more rule for making words: if we add the syllable “er” to a verb stem, we make a noun that names a person or thing that does what the verb says to do. For example, a buyer is one who buys. A user is one who does use.)
〈noun〉 ::= 〈verb stem〉 “er”
As you may by now have guessed, I am of like mind with my good friend Dick Gabriel, who wrote the well known screed “Worse Is Better” [5, 6]. (The real name was “Lisp: Good News, Bad News, How to Win Big,” which is all words of just one syllable—which might seem like good luck for me, but the truth is that Dick Gabriel knew how to choose words with punch. Yet what first comes to mind for most persons is the part headed “Worse Is Better” and so that is how they cite it.) The gist of it is that the best way to get a
choosing with care from the work of many users. And Lisp grew much faster than APL did, because many users could try things out and put their best code out there for other users to use and to add to the language. Lisp is not used quite as much as it used to be, but parts of it live on in other languages, the best of which is called garbage collection (and I will not try to tell you now what that means in words of one syllable—I leave it to you as a task to try in your spare time). This leads me to claim that, from now on, a main goal in designing a language should be to plan for growth. The language must start small, and the language must grow as the set of users grows. I now think that I, as a language designer who helps out with the design of the Java programming language, need to ask not “Should the Java programming language grow?” but “How should the Java programming language grow?” There is more than one kind of growth and more than one way to do it. But, as we shall see, if the goal is to be quick and yet to do good work, one mode may be better by far than all other modes. There are two kinds of growth in a language. One can change the vocabulary, or one can change the rules that say what a string of words means. A library is a vocabulary designed to be added to a programming language to make the vocabulary of the programming language larger. Libraries means more than one library. A true library does not change the rules of meaning for the language; it just adds new words. (You can see from this that, in my view, the code that lets you do a “long jump” in C is not a true library.) Of course, there must be a way for a user to make libraries. But the key point is that the new words defined by a library should look just like primitives of the language. Some languages are like this and some are not. Those that are not are harder to grow with the help of users. It may be good as well to have a way to add to the rules of meaning for a language. Some ways to do this work better than other ways. But the language should let work done by a user look just like what was designed at the start. I would like to grow the Java programming language in such a way that users can do more of this. In the same way, there are two ways to do the growing. One way is for one person (or a small group) to be in charge and to take in, test, judge, and add the work done by other persons. The other way is to just put all the source code out there, once things start to work, and let each person do as he wills. To have a person in charge can slow things down, but to have no one in charge makes it harder to add up the work of many persons. The way that is faster and better than all other ways does both. Put the source code out there and let all persons play with it. Have a person in charge who is a quick judge of good work and who will take it in and shove it back out fast. You don’t have to use what he ships, and you don’t have to give back your work, but he gives all persons a fast way to spread new code to those who want it. The best example of this way to do things is Linux, which is an operating system, which is a program that keeps track of other programs in a computer and gives each its due in space and time. You ought to read what Eric Raymond had to say of how Linux came to be. I shall tell you a bit of it once I define two more words for you. A cathedral is a huge church. It may be made of stone; it may fill you with awe; but the key thought, in the mind of Eric Raymond, is that there is but one design, one grand plan, which may take a long time—many years—to make real. As the years pass, few
changes are made to the plan. Many, many persons are needed to build it, but there is just one designer. A bazaar is a place with many small shops or stalls, where you can buy things from many persons who are there to sell their wares. The key thought here is that each one sells what he wants to sell and each one buys what he wants to buy. There is no one plan. Each seller or buyer may change his mind at whim. Eric Raymond wrote a short work called “The Cathedral and the Bazaar” in which he looks at how programs are built or have been built in the past. (You can find it on his web site [11].) He talks of how he built a mail fetching program with the help of more than two hundred users. He quotes Fred Brooks as saying, “More users find more bugs,” and backs him up with tales from this example of building a program in the bazaar style. As for the role of the programmer in charge, Eric Raymond says that it is fine to come up with good thoughts, but much better to know them when you see them in the work of other persons. You can get a lot more done that way. Linux rose to its heights of fame and wide use in much the same way, though on a much larger scale. (To take this thought to the far end of the line: it may be that one could write an operating system by putting a million apes to work at a million typing machines, then just spotting the bits of good work that come out by chance and pasting them up to make a whole. That might take a long time, I guess. Too bad!) But the key point of the bazaar is not that you can get many persons to work with you at a task, for cathedral builders had a great deal of help, too. Nor is the key point that you get help with the designing as well as with the building, though that in fact is a big win. No, the key point is that in the bazaar style of building a program or designing a language or what you will, the plan can change in real time to meet the needs of those who work on it and use it. This tends to make users stay with it as time goes by; they will take joy in working hard and helping out if they know that their wants and needs have some weight and their hard work can change the plan for the better. Which brings me to the high point of my talk. It seems, in the last few years, at least, that if one is asked to speak on design, one ought to quote Christopher Alexander. I know a bit of his work, though not a lot, and I must say thanks to Dick Gabriel for pointing out to me a quote that has a lot to do with the main point of this talk. I am sad to say that I do not know what this quote means, because Christopher Alexander tends to use many words of more than one syllable and he does not define them first. But I have learned to say these words by rote and it may be that you out there can glean some thoughts of use to you. Christopher Alexander says [1]:
Master plans have two additional unhealthy characteristics. To begin with, the existence of a master plan alienates the users... After all, the very exis- tence of a master plan means, by definition, that the members of the community can have little impact on the future shape of their community, because most of the important decisions have already been made. In a sense, under a master plan people are living with a frozen future, able to affect only relatively trivial details. When people lose the sense of responsibility for the environment they live in, and realize that they are merely cogs in someone else’s machine, how can they feel any sense of identification with the community, or any sense of purpose there?
I think this means, in part, that it is good to give your users a chance to buy in and
and many meanings. The definition of a word may be polymorphic, but the word as such is not. An operator can be overloaded in C plus plus, but right now operators in the Java pro- gramming language can not be overloaded by the programmer, though names of methods may be overloaded. I would like to change that. I have said in the past, and will say now, that I think it would be a good thing for the Java programming language to add generic types and to let the user define overloaded operators. Just as a user can code methods that can be used in just the same way as methods that are built in, the user ought to have a way to define operators for user defined classes that can be used in just the same way as operators that are built in. What is more, I would add a kind of class that is of light weight, one whose objects can be cloned at will with no harm and so could be kept on a stack for speed and not just in the heap. Classes of this kind would be well suited for use as user defined number types but would have other uses, too. You can find a plan for all this on a web page by James Gosling [7]. (There are a few other things we could add as well, such as tail calls and ways to read and write those machine flags for numbers whose points float. But these are small language tweaks next to generic types and overloaded operators.) If we grow the language in these few ways, then we will not need to grow it in a hundred other ways; the users can take on the rest of the task. To see why, think on these examples. A complex number is a pair of numbers. There are rules for how to find the sum of two complex numbers, or a complex number times a complex number:
(a, b) + (c, d) = (a + c, b + d)
which says that a paired with b, plus c paired with d, is the same as a plus c paired with b plus d, and
(a, b) · (c, d) = (a · c − b · d, a · d + b · c)
which says that a paired with b, times c paired with d, is the same as a times c less b times d paired with a times d plus b times c. Some programmers like to use complex numbers a lot; other programmers do not use them at all. So should we make “complex number” a type in the Java programming language? Some say yes, of course; other persons say no. A rational number is a pair of numbers. There are rules (not the same as the rules for complex numbers, of course) for how to find the sum of two rational numbers, or a rational number times a rational number:
(a, b) + (c, d) = (a · d + b · c, b · d)
which says that a paired with b, plus c paired with d, is the same as a times d plus b times c paired with b times d, and
(a, b) · (c, d) = (a · c, b · d)
which says that a paired with b, times c paired with d, is the same as a times c paired with b times d. A few programmers like to use rational numbers a lot; most do not use them at all. So should we make “rational number” a type in the Java programming language? An interval is a pair of numbers. There are rules (not the same as the rules for complex numbers or rational numbers, of course) for how to find the sum of two intervals, or an interval times an interval:
(a, b) + (c, d) = (a + c, b + d)
(a, b) · (c, d) = (min(a · c, a · d, b · c, b · d), max(a · c, a · d, b · c, b · d))
A few programmers like to use them a lot and wish all the other programmers who use numbers would use them, too; but most do not use them at all. So should we make “interval” a type in the Java programming language? John Horton Conway once defined a game to be a pair of sets of games (see his book On Numbers and Games [4]), then pointed out that some games may be thought of as numbers that say how many moves it will take to win the game. There are rules for how to find the sum of two games, and so on:
(A, B) + (C , D) =
a + (C , D) | a ∈ A
(A, B) + c | c ∈ C
b + (C , D) | b ∈ B
(A, B) + d | d ∈ D
−(A, B) =
−b | b ∈ B
−a | a ∈ A
A − B = A + (−B)
(A, B) · (C , D) =
a · (C , D) + (A, B) · c − a · c | a ∈ A, c ∈ C
b · (C , D) + (A, B) · d − b · d | b ∈ B, d ∈ D
a · (C , D) + (A, B) · d − a · d | a ∈ A, d ∈ D
b · (C , D) + (A, B) · c − b · c | b ∈ B, c ∈ C
(I will not try to state here in words what these rules mean!) From this he worked out for hundreds of kinds of real games how to know which player will win. I think, oh, three persons in the world want to use this kind of number. Should we make it a type in the Java programming language? A vector is a row of numbers all of the same type, with each place in the row named by the kind of number we first spoke of in this talk. There are rules... In fact, for vectors of length three there are two ways to do “a vector times a vector,” so you can have twice the fun!
(a, b, c) + (d, e, f ) = (a + d, b + e, c + f )
(a, b, c) · (d, e, f ) = a · d + b · e + c · f
(a, b, c) × (d, e, f ) = (b · f − c · e, c · d − a · f, a · e − b · d)
Vectors of length three or four are a great aid in making bits on the screen look like scenes in the real world. So should we make “vector” a type in the Java programming language? A matrix is a set of numbers laid out in a square. And there are rules (not shown here!). So should we make “matrix” a type in the Java programming language? And so on, and so on, and so on.
sure that I would have to define hundreds of new words. It should give no one pause to note that the writing of a program a million lines of code in length might need many, many hundreds of new words—that is to say, a new language built up on the base language. I will be so bold as to say that it can be done in no other way. Well—there may be one other way, which is to use a large, rich programming language that has grown in the course of tens or hundreds of years, that has all we need to say what we want to say, that we are taught as we grow up and take for granted. It may be that a hundred years from now there will be a programming language that by then has stood the test of time, needs no more changes for most uses, and is used by all persons who write programs because each child learns it in school. But that is not where we are now. So. Language design is not at all the same kind of work it was thirty years ago, or twenty years ago. Back then, you could set out to design a whole language and then build it by your own self, or with a small team, because it was small and because what you would then do with it was small. Now programs are big messes with many needs. A small language won’t do the job. If you design a big language all at once and then try to build it all at once, you will fail. You will end up late and some other language that is small will take your place. It would be great if there were some small programming language that felt large, the way Basic English is small but feels large in some ways. But I don’t know how to do it and I have good cause to doubt that it can be done at all. In its day, APL was a small language that felt large; but our needs have grown and APL did not have a good pattern for growth. So I think the sole way to win is to plan for growth with help from users. This is a win for you because you have help. This is a win for the users because they get to have their say and get to bend the growth to their needs. But you need to have one or more persons, too, or one or more groups, to take on the task of judging and testing and sifting what the users do and say, and adding what they think best to the big pile of code, in the hope that other users will trust what they say and not have to go to all the work to test and judge and sift each new claim or each new piece of code, each for his own self. Parts of the language must be designed to help the task of growth. A good set of types, ways for a user to define new types, to add new words and new rules to the language, to define and use all sorts of patterns—all these are needed. The designer should not, for example, define twenty kinds of number types in the language. But there will be users who, all told, beg for twenty kinds of numbers. The language should have a way for the user to define number types that work well with each other, and with plus signs and other such signs, and with the many ways of pushing bits in and out of the computer. One might define just one or two number types at the start, to show how it ought to be done. Then leave the rest to the users. Help them all as best you can to work side by side (and not nose to nose). You may find that you need to add warts as part of the design, so that you can get it out the door fast, with the goal of taking out the warts at a later time. Now, there are warts and then there are warts! With care, one can design a wart so that it will not be too hard to take out or patch up later on. But if you do not take care at the start, you may be stuck for years to come with a wart you did not plan. Some warts are not bad things you put in, but good things you leave out. Have a plan to add those good things at a later time, if you should choose to do so, and make sure that other parts of your design don’t cut you off from adding those good things when the time is right.
I hope to bring these thoughts to bear on the Java programming language. The Java programming language has done as well as it has up to now because it started small. It was not hard to learn and it was not hard to port. It has grown quite a bit since then. If the design of the Java programming language as it is now had been put forth three years ago, it would have failed—of that I am sure. Programmers would have cried, “Too big! Too much hair! I can’t deal with all that!” But in real life it has worked out fine because the users have grown with the language and learned it piece by piece, and they buy in to it because they have had some say in how to change the language. And the Java programming language needs to grow yet some more—but, I hope, not a lot more. At least, I think only a few more rules are needed—the rest can be done with libraries, most of them built by users and not by Sun. If we add hundreds of new things to the Java programming language, we will have a huge language, but it will take a long time to get there. But if we add just a few things—generic types, overloaded operators, and user defined types of light weight, for use as numbers and small vectors and such—that are designed to let users make and add things for their own use, I think we can go a long way, and much faster. We need to put tools for language growth in the hands of the users. I hope that we can, in this way or some other way, design a programming language where we don’t seem to spend most of our time talking and writing in words of just one syllable. One of the good things I can say for short words is that they make for short talks. With long words, this talk would run an hour and a half; but I have used less than an hour. I would like to tell you what I have learned from the task of designing this talk. In choosing to give up the many long words that I have come to know since I was a child, words that have many fine shades of meaning, I made this task much harder than it needed to be. I hope that you have not found it too hard on your ears. But I found that sticking to this rule made me think. I had to take time to think through how to phrase each thought. And there was this choice for each new word: is it worth the work to define it, or should I just stick with the words I have? Should I do the work of defining a new word such as mirror, or should I just say “looking glass” each time I want to speak of one? (As an example, I was tempted more than once to state the “ly” rule for making new words that change what verbs mean, but in the end I chose to cast all such words to one side and make do. And I came that close to defining the word without, but each time, for better or for worse, I found some other way to phrase my thought.) I learned in my youth, from the books of such great teachers of writing as Strunk and White [13], that it is better to choose short words when I can. I should not choose long, hard words just to make other persons think that I know a lot. I should try to make my thoughts clear; if they are clear and right, then other persons can judge my work as it ought to be judged. From the work of planning this talk, in which I have tried to go with this rule much more far than in the past, I found that for the most part they were right. Short words work well, if I choose them well. Thus I think that programming languages need to be more like the languages we speak—but it might be good, too, if we were to use the languages we speak more in the way that we now use programming languages.ls All in all, I think it might be a good thing if those who rule our lives—those in high places who do the work of state, those who judge what we do, and most of all those who
