Scalable computer programming languages


There will always be things we wish to say in our programs that in all known languages can only be said poorly.

    -- Alan Perlis

NOTE

If you're going to send me email comments about this article, please keep a few things in mind:
  1. I wrote this article a long time ago (in 2001, if memory serves), and frankly I'm not that interested in discussing it anymore. These days I'd rather write programs and teach programming languages than write opinion pieces.

  2. The article accurately reflects my opinions as of the time I wrote it, but may not accurately reflect my current opinions.

  3. I didn't post this article to reddit.com or to any other site. I have no interest in drumming up traffic to this page.

  4. I've received some surprisingly hostile emails from people who disagree with me. To those people: please realize that this is a rant, an opinion piece. If you don't agree with me, feel free to write your own opinion piece. But name-calling and trolling just makes you look bad and undercuts your own arguments. I won't respond to trolls, even if they also have some good points to make. If you can't talk civilly, talk to someone else. I am continually astonished by how angry comparative discussions of programming languages can get. Can't we all get along? There are more important things to get mad about, like cancer, world hunger, nuclear proliferation, etc.

  5. That said, I welcome constructive comments. In fact, I've received a number of insightful comments, and I've added some notes at the end summarizing some of the feedback I've received as well as some of the ways my views have shifted since I originally wrote this.

Introduction

Computer programmers often use the word scalable to describe a desirable feature of a program or algorithm. The idea is that something that is scalable not only works for very small scale jobs but is equally effective (or nearly as effective) when the job in question is much larger (or, more commonly, when the job grows from being small to being large). A trivial example of poor scalability might be an inefficient algorithm used in a prototype implementation of a program. It works fine as long as only very small data sets are being used, but it breaks down when larger data sets are used because the computation takes so much time that the program effectively grinds to a halt. This kind of scalability is well understood by any decent programmer. A different kind of scalability is represented by the architecture of a computer program. A program that can't easily be extended to cope with new requirements is called brittle, which is the opposite of scalable. Since effectively all programs that are successful grow new parts, a nonscalable program usually will require a total rewrite when the requirements change, which is very wasteful. The usual reason for this kind of non-scalability is poor design abstraction; too many fundamental design decisions have been hard-wired into the code in so many places that it is difficult to change them all without introducing lots of bugs. Many, many books have been written about good program design; a classic is Abelson and Sussman's Structure and Interpretation of Computer Programs, which covers this and much more interesting material as well and is highly recommended. But that's not what I want to talk about here. What I want to talk about is what makes computer programming languages scalable or not. I want to show that the notion of scalability is every bit as valid when applied to programming languages as it is when applied to programs or algorithms. I'll also discuss several well-known and not so well-known programming languages from this perspective and give some concrete recommendations, as well as discuss some of the social factors which hinder progress in this field.

What is a scalable computer language?

Simply put, a scalable computer language is a language that one can write very large programs in (and extend very large programs that have already been written) without feeling an undue amount of pain. In a scalable programming language, the difficulty of managing the complexity of the program goes up roughly linearly with the size of the program. Conversely, a nonscalable computer language is one in which increasing the size and scope of a problem tends to make the program much harder to manage (i.e. the complexity of the program goes up much more than linearly with its size).

Aspects of a programming language that affect scalability

Garbage collection: very, very good

Garbage collection (GC) means that the computer language (literally, the computer language's runtime system) automatically manages the reclamation ("freeing") of memory that is no longer being used. This is a huge win for the programmer, and can dramatically increase the scalability of the language. The reason is simple. It's usually obvious where one has to allocate memory. It can be very difficult to know exactly when it's safe to free it. This is especially true when references to the allocated memory are passed around, returned from functions, stored in multiple data structures that are later deleted (or not), etc. etc. The effects of this are very insidious. Languages without GC implicitly discourage the programmer from using all but the simplest data structures, because more often than not, the memory management problem quickly becomes intractable when using more complex data structures. What usually happens in practice is that the programmer rolls his/her own (bad) garbage collector (maybe a reference counter), with performance that is usually worse than what you would get if you used a language with GC built in.

I saw a rather stark example of this recently. One of the things I do professionally is teach the C programming language to Caltech undergraduates. I emphasized how important it was to always free memory that had been allocated. However, many of my students simply ignored me, and their code was littered with memory leaks. I got so tired of writing "this code has a memory leak here" on their assignments that I wrote a very simple memory leak checker. They are now required to write code that passes through the memory leak checker without any reported leaks before they submit their assignments. However, I was somewhat dismayed to find that my own answers to the assignments had a couple of subtle memory leaks as well! Since I have more than ten years of C programming experience, and have worked on several very large projects, this suggests to me that manual memory management is much harder than I'd previously supposed it to be.

There is a cost to GC, both in time and space efficiency. Well-designed garbage collectors (especially generational GC) can be extremely efficient (more efficient, for instance, than naive approaches such as reference counting). However, in order to do this they tend to have significantly greater space usages than programs without GC (I've heard estimates on the order of 50% more total space used). On the other hand, a program that leaks memory has the greatest space usage of all. I've wasted way too much of my life hunting down memory leaks in large C programs, and I have no interest in continuing to do so.

In conclusion, I would say that of all the items I'm discussing here, GC is the single most important one to ensure that a programming language is scalable. This is why programmers who move from a language without GC (say C++) to one of roughly equivalent abstractive power but with GC (say Java) invariably say how much happier they are now that they don't have to worry about memory management and can concentrate on the algorithms they're trying to write. Personally, I'd rather pull my own teeth out than write a large project in a language without GC.

Direct access to memory and pointer arithmetic: very bad

Some computer languages, notably C and C++, allow the programmer to directly interact with memory addresses through pointers, as well as allowing pointer arithmetic (incrementing and decrementing pointer variables). This kind of low-level programming is sometimes necessary (e.g. when writing device drivers) and sometimes simply useful (e.g. when micro-optimizing code that has to run as fast as possible). However, my (imperfect) understanding of the issue is that programming with pointers makes precise garbage collection impossible, or nearly so. There is a kind of GC called "conservative" garbage collection (here is a free implementation of the Boehm-Demers conservative GC) which can work with languages like C and C++. It's certainly better than nothing, but there are no guarantees that all memory will be managed correctly (i.e. memory leaks are unlikely, but possible). In practice, this supposedly isn't a problem, but I note with some interest that very little C/C++ code that I've seen actually uses conservative GC, and those that do are usually in code that is implementing a language that includes GC.

The scalability cost of pointers goes far beyond just making GC harder. Pointers (and especially pointer arithmetic) tend to destroy any safety guarantees you might want to be able to make about a program. It's not hard to see why. When you have a pointer to (say) an integer, and you can add 1,000,000 to that pointer and dereference some random region of memory which may or may not be part of your program's run-time image, all hell can break loose. If you're lucky, you'll just get a core dump and your program will terminate. If you're not so lucky, some part of the program memory will be corrupted, leading to mysterious bugs that are extremely difficult to track down, because they manifest themselves far away from where the original problem was. This leads to a huge increase in debugging times, which dramatically hurts programmer productivity. As the program gets larger, the opportunities for this kind of problem increase, which represents a significant barrier to scalability.

The usual argument in favor of direct pointer manipulations is that they make it possible to write faster code. This is often true; I've seen cases where using pointer arithmetic judiciously increased the speed of a program by a factor of five. However, the reverse is also often true; many program optimizations (normally performed automatically by the compiler) are rendered much more difficult or impossible in code that uses pointers. In other words, languages that enable micro-optimizations often make macro-optimizations impossible.

The author of the Eiffel language, Bertrand Meyer, has said that (I'm paraphrasing) "you can have pointer arithmetic, or you can have correct programs, but you can't have both". I agree with him. I think that direct memory access through pointers is the single biggest barrier to programming language scalability.

None of this is meant to imply that pointers and pointer arithmetic don't have their place; they are crucial for low-level close-to-the-metal programming. However, large programs written at a low level are simply not scalable. The right way to use languages like C is to implement small, focused low-level components of applications written primarily in higher-level languages. In fact, this is also the right way to use C++; you write some low-level classes that use pointers in some of their methods and then encapsulate these methods so you never have to expose the pointer manipulations to a user of the class. This would be nearly ideal, except that the compiler has no way to enforce this; you can always use pointers if you want to.

By the way, there is an interesting language called Cyclone which is essentially a "safe" variant of C. It has three different types of pointers, some of which allow pointer arithmetic and some of which don't. Pointer arithmetic in cyclone is always checked for safety. This language is thus much safer than C; the run-time cost varies from negligible to substantial. However, cyclone doesn't correct the other problems with C (described below), so I wouldn't describe it as particularly scalable.

Static type checking: mostly very good

A statically typed language is one where each data item has a specific type which cannot be changed for the duration of the program. In addition, variables have specific types, and you can't assign a value of a different type to that variable. There are a number of issues related to static typing, and there is a lot of confusion about these issues, so I'll try to summarize the main points here.

Advantages of static type checking

The primary scalability advantage of static type checking is that a great number of errors are caught at compile time, rather than at run time. Thus, in a statically typed language with a well-designed type system, most of the trivial errors will be caught by the compiler, which leaves the programmer free to concentrate on the more interesting parts of the program. This increases productivity as well as programmer happiness ;-) Furthermore, if the type system extends to the module system (see below), then any use of an imported module will be type-checked as well. This rules out large classes of errors in the use of external modules written by others just as it does in the code one writes for oneself. This, in turn, makes it easier to re-use other people's code with confidence, which makes the language significantly more scalable.

In addition, static type checking leads to significantly faster code than dynamic (run-time) type checking. If the compiler knows that 'a' and 'b' both represent small integers, and it needs to compute 'a + b', then it can insert the code for addition of small integers right into the program. If the only thing that's known about 'a' and 'b' is that they represent objects that might be integers, or floats, or strings, then the decision on what to do has to be deferred until run time. Type checking at run time is expensive. Therefore, in addition to the other advantages of static typing, you also get faster code.

Type declarations vs. type inference

The usual reason why many programmers don't like static type checking is that type declarations are verbose and detract from the purity of the algorithm. This is certainly the case in most statically typed languages; a particularly egregious example is Java, where you can have statements like this:

Foo foo = new Foo();  // Declare a new object of type Foo.

You have to use the word "foo" three times to get the message across! What most programmers don't realize is that there is an alternative. Languages with type inference can figure out the types of almost all variables from their context, which means that the programmer doesn't have to type in declarations. A good example of this is the objective CAML (Ocaml) programming language. This leads to code that is as concise as code written in dynamically typed languages like Scheme or Python but with all the benefits of static typing.

Type casting

Unfortunately for scalability, some languages (notably C and C++, again) have mechanisms for type casting, which is basically an escape hatch that allows you to tell the compiler "I know I said that this value was of type X, but from now on you can pretend that it's actually of type Y, and I'll take the responsibility". This feature is necessary in C because the type system is extremely weak (it's not nearly as necessary in C++ and is generally considered bad programming style). It's also possible (and often necessary) to cast between types in Java (almost always from a superclass to a subclass), but if the cast fails then an exception is thrown, whereas in C, the cast never fails (and anything can happen if you cast an object to the wrong type). In conclusion, unchecked type casting is bad, and languages that require a lot of it are less scalable than those that don't. Note that the majority of the type casting in Java (which is checked) is simply to get around the lack of parameterized types, so even checked type casting may indicate a weakness in the language.

Static vs. dynamic type checking

The relative virtues of static versus dynamic type checking are one of the great holy wars amongst computer language researchers and users. In contrast to static type checking (described above), in dynamic type systems objects, not variables, have types. A variable in these system is simply a binding to an object, and it can be bound to (for instance) an integer one moment, a string another moment, and a list another moment. This means that operations which require particular types for their arguments (e.g. addition) have to check these types at run-time instead of at compile time. This leads to significant performance penalties, as well as a loss of safety; a type error is not normally caught until run-time. On the other hand, dynamic typing is incredibly expressive, much more so than any static type system. This is a big topic that I don't intend to explore here (it would be an essay all by itself). Suffice it to say that a lot of the flexibility of dynamic types can be recovered in a statically typed language if the type system is powerful enough. A good example of a statically typed language with a very powerful type system is, once again, Ocaml (get the idea that I like that language yet? ;-)).

Another (weak) argument in favor of dynamic type checking is that dynamically typed languages tend to be much less verbose than statically typed ones, which means that the algorithm can often be expressed more clearly and succinctly. However, statically typed languages with type inference like (ahem) Ocaml circumvent this problem, as mentioned above.

If static type checking is just too restrictive for your tastes, then dynamically typed languages like Lisp, Scheme, Python, Ruby or Smalltalk are quite pleasant to program in, although my view is that the lack of static type checking hurts the scalability of these languages substantially. What typically happens in large projects written in these languages is that extensive unit tests are written to catch type errors as well as logical errors (the Smalltalk community has been particularly aggressive about promoting this approach; see Kent Beck's extreme programming site for much more on this, which is not limited to Smalltalk or to dynamically typed languages).

Another interesting approach is "soft typing", which is a cross between static type checking and dynamic type checking. Roughly speaking, it allows the same expressiveness as dynamic typing, but will statically check the types of everything that it can, and if it can't decide whether some program element is correctly typed at compile time it will check it at run time. This approach is used in the Dylan language (and also in Common Lisp, to a greater or lesser extent depending on the compiler) and is being actively developed from both the theoretical and practical standpoints by the PLT Scheme team of computer language researchers/implementors. I look forward to seeing what they come up with.

Exception handling: good

Exception handling is a useful feature for making code more robust and also cleaner. The basic idea is that some operations can fail for various reasons, making it impossible to return a desired value. For instance, reading a line from a text file is impossible if the file pointer is already at the end of the file. In primitive languages like C, the only solution is to return an error code and laboriously test for it whenever the function is called. However, since a C function can only return one value, the real return value of the function has to be kludged as a writable parameter of the function, which is very messy and bug-prone (since it requires that the user manipulate pointers). With exception handling, when an exceptional condition occurs an exception is thrown, and it unwinds the call stack until it reaches a suitable exception handler. This feature is so useful that almost all new languages incorporate it. Interestingly, exceptions also work much better in the presence of garbage collection; avoiding memory leaks in a language like C++ that has exception handling but has no GC is quite tricky (see Scott Meyers' books Effective C++ and More Effective C++ for an extensive description of this issue). This is yet another argument for garbage collection (as if we needed one).

Run-time error checking: good

Dynamically typed languages do all their error checking at run time. Most of this error checking is to ensure that operations are called with correctly typed arguments. This is costly and somewhat error-prone when compared to languages that can do all their type checking at compile time. However, even in statically-typed languages, there arise situations which can't be handled by the type system alone. More precisely, there arise situations where any reasonable (decidable, efficient) type system can't know at compile time that an error is going to happen. In order to deal with this you need run-time error checking. Here are some typical examples.

Array bounds violations

If an attempt is made to access an array outside of its bounds (e.g. trying to access the 100th element of a 10-element array), then an array bounds violation occurs. Scalable languages will always catch this error and will usually throw an exception so that either the program can deal with it or the person running the program knows that something bad happened (ideally a stack trace will be printed in the latter case). Checking for array bounds violations has a significant cost, however, so ideally the compiler will have an option that permits the user to disable this check if speed is more important than safety. Note that array bounds violations are never caught in C programs; instead, they cause fun problems like core dumps and memory corruption.

Arithmetic errors

Another class of errors that can normally only be caught efficiently at run time are arithmetic errors. The classic example of this is integer division by zero. Again, scalable languages will throw an exception in this case. Other integer arithmetic errors such as overflows also typically raise exceptions in many languages. In contrast, floating point errors such as 1.0 / 0.0 (= infinity), -1.0 / 0.0 (= -infinity) and 0.0 / 0.0 (= not-a-number or NaN) do not normally raise exceptions even in otherwise very safe languages (such as Java or Ocaml). The reason for this is not clear to me (apparently it's written into the IEEE floating-point arithmetic standards and nobody wants to challenge it), and frankly, I think that this is a mistake. Like array bounds checking, it's good to have a compiler option to disable arithmetic error checking if speed is more important than safety for a given application. This is a generally useful principle: make the language safe by default, and give the programmer the option of trading safety for speed, ideally without requiring him/her to rewrite any code.

Assertions and contracts: very good

Most languages (even C) provide an "assert" feature, which allows the programmer to state that at a given point in a program a particular relationship must be true. For instance, in a C program you might say:
assert(i == 100);
meaning "at this point in the program, 'i' should have the value 100, and if it doesn't something is wrong." This is a kind of built-in sanity checking for your program. It is possible to disable assertion checks during compilation as well. Typically, asserts are enabled while developing a program and disabled after the program is completed in order to speed up the program. Some languages (notably Eiffel) have a much more elaborate assertion system known as "Design by Contract" which includes separate kinds of assertion checks for preconditions of functions, postconditions of functions, class invariants, loop invariants and variants, and much more, all incorporated into the object system of the language. If used correctly, this is a huge win. Bugs will tend to show up much closer to where they occurred, which makes debugging vastly easier. This in turn makes the language much more scalable.

Support for abstractions

In general, the more support a language provides for developing abstractions, the better. Good abstraction capabilities mean you can say more with less code, and less code means fewer bugs and shorter development times, which leads to better scalability. There are lots of worthwhile language features I won't discuss here; instead, I'll single out the ones that have the most effect on language scalability.

Module systems: very good

Frankly, a language without a good module system is useless for developing large programs. A module system allows the programmer to develop and test parts of a program in isolation, and then combine these parts at a later time. Modules are usually used to implement reusable code libraries containing data structures and functions representing some interesting aspect of the problem domain (for instance, operations on linked lists). Once a module is written and debugged, any other program can then import and use the code in that module. This is a huge win, since fundamental data structures and operations don't have to be re-written for every program (which is called "reinventing the wheel" in programmer jargon). This allows the programmer to write code much more rapidly and with a much greater confidence of success, and is thus a vital component of scalable computer languages.

Object-oriented programming: good

Object-oriented programming (OOP) is often presented as the cure to all programming problems. It is not. However, some kinds of applications benefit greatly from the object-oriented paradigm. The basic idea in OOP is that programs are decomposed into "objects" which have "methods". Methods are functions that have access to the internal state of the object. Only the methods corresponding to an object can access that internal state, so if the state changes in an unexpected way, one of the methods must have been responsible for it. This is called "data hiding" or "encapsulation". Also, in OOP it is possible to create new classes of objects by inheritance; this means that the new object class is a version of the old object class, but with some new methods and/or new data. Finally, and most importantly, calling object methods is "polymorphic"; this basically means that the appropriate method to run on an object is chosen at run time rather than at compile time. Certain kinds of programming tasks (such as simulations) are well suited to the OO paradigm, while other tasks (such as compilers) don't really need OO. In general, it's good if a language supports OO, but I prefer languages that don't force you to use OO for all programs. It is often claimed that programs written in the OO style are easier to extend and modify than non-OO programs. I think this is an exaggeration. If you have to subclass a class every time you want to add a new method, your program will start to look very kludgy very fast. Nevertheless, OO is a useful abstraction technique to have available.

Functional programming: good

The "dual" to the OOP paradigm, if you will, is the functional programming (FP) paradigm. Instead of decomposing a program into a group of objects, functional programs decompose a program into a group of functions. Like OO, functional programming requires a language that supports certain language features. The primary feature is that in a functional language, functions are "first-class", which means that they can be treated as data (i.e. they can be passed as arguments to a function, returned from a function, and/or created on the fly inside a function). Another crucial feature is the ability to define recursive functions efficiently (technically, there has to be tail-recursion optimization). It turns out that if you have these capabilities, then you can do without loop statements (e.g. for and while loops in C), instead using recursion to implement the loop. In addition, functional programs typically avoid using mutable data (assignment statements) as much as possible. They also tend to use a lot of very small functions instead of a few large functions. The advantages of FP include: I don't have nearly enough space here to go into this in detail. If you're interested, you should read Abelson and Sussman's Structure and Interpretation of Computer Programs, which goes into great detail on this subject. Suffice it to say that having support for the functional style can make a computer language much more scalable. Unfortunately, very few languages support this style (examples include Lisp, Scheme, Ocaml, Standard ML, and Haskell), apparently because average programmers find the functional style "hard to understand" and because FP has a (largely but not entirely undeserved) reputation for inefficiency (in fact, the efficiency of FP depends enormously on the details of the language implementation).

Macros: mostly good

By "macros" I mean structural macros like in Lisp, not textual substitution macros like in C. Structural macros can be a very useful abstraction device; they allow the programmer to encapsulate programming idioms that are often repeated and which cannot be expressed as higher-order functions. A good example of this would be macros to implement control constructs. In most languages, you are stuck with the control constructs (if, for, while) that are provided by the language, but in Lisp you can define your own using macros. Macros are very interesting (see Paul Graham's book On Lisp for much more on them). They have some problems as well: Nonetheless, when used correctly macros can increase the abstraction level of a program dramatically, which is good for scalability.

Components

"Components" is a vague word in the context of computer languages, but here I'm using it to represent a single unit of functionality that can be composed with other parts of a program. It's sort of like a library, but more self-contained. There are a number of component architectures around (Java beans, CORBA, Microsoft COM); some, like Java beans, only support one language, while others support a whole range of languages. One big attraction of components is that they can be language-independent and even location-independent. However, if a language specifically supports one (or many) component architectures it's a big win for scalability because then you can re-use components written by another developer (and maybe in another language). This is usually just a library issue, but some languages (C# being a notable example) are designed specifically to make component development easier. This is a good thing, but I don't have the space to go into it here.

Syntax and readability

Syntax is not the most interesting aspect of computer languages; in fact, it's probably the least interesting aspect, which is odd considering that it seems to be the only aspect that most programmers ever learn ;-) Syntax does make a difference to scalability, however. In my opinion, it's not that important that a syntax be "familiar" (which in practice means "like C syntax"); rather, the important issue is how consistent it is. Very baroque and inconsistent syntaxes with dozens of strange exceptions to general rules (C++ and (especially) Perl are the worst offenders here) make programs very hard to understand and maintain, simply because nobody ever remembers all the special cases. At the other extreme, very minimalistic and simple syntaxes like Lisp syntax are difficult for many people to get used to (though personally, I quite like Lisp's syntax). Python is a good example of a language that has a very friendly syntax both for new programmers and experts.

The scalability of different programming languages

In this section I'll make more specific comments about how some programming languages stack up in terms of their scalability. I can't cover every programming language, but I'll try to cover a representative sample.

C

C combines all the power of assembly language with all the ease of use of assembly language.

    -- unknown

I'd just like to take this moment to point out that C has all the expressive power of two dixie cups and a string.

    -- Jamie Zawinski, in the source code for xkeycaps

There is a point in your life when you realize that you have written enough destructors, and have spent enough time tracking down a memory leak, and you have spend enough time tracking down memory corruption, and you have spent enough time using low-level insecure functions, and you have implemented way too many linked lists.

    -- Miguel de Icaza

Greenspun's Tenth Rule of Programming: "Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified bug-ridden slow implementation of half of Common Lisp."

    -- Philip Greenspun

Your superior intellect is no match for our puny weapons.

    -- unknown, via Aaron Stern

I'll be blunt: C is a horrible language for developing large projects in. C contains almost all of the bad language features described above and almost none of the good ones. Furthermore, some of the good features it does have (like static type checking) are so corrupted by type casting that they are much less useful than they would otherwise be. C also offers essentially no abstraction capabilities whatsoever except for arrays (which are really just pointers in disguise) and structs (ditto).

Does that mean that C is a useless language? Far from it. It has its place, and its place is a vital one. C is an excellent language for writing code that has to interface directly with the machine ("bare-metal" programming). It is also a good language for implementing better languages in ;-) The Ocaml runtime engine, for instance, is written in C (with some help from assembly language). C is useful for writing the 1% of your application that absolutely, positively, has to run as fast as possible. However, if you're trying to write a large application entirely in C you are in for a miserable time. Instead, you're better off picking a good language that has a C foreign-function interface (FFI), so you can "drop down" into C when you really need to (which hopefully won't be that often).

Some people (particularly novice programmers who have no idea what they're talking about) are under the illusion that because C offers such direct control over the machine, it's the only "real" programming language for "true hackers". If that were true, all "true hackers" (whatever that means) would be writing code in assembly language, which is much closer to the machine than C is. I hope some of the quotes above, some of which are from world-famous open-source hackers, will help to dispel this myth. If not, then I recommend that you try to write a really large program (> 100,000 lines of code) in C, and tell me what you think at the end of that experience. I think you'll find my arguments much more persuasive.

C++

C++ : an octopus made by nailing extra legs onto a dog.

    -- off smalltalk.org

Think of C++ as an object-oriented assembly language.

    -- off the guile Scheme mailing list

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

    -- Bjarne Stroustrup

Programming in C++ is premature optimization.

    -- off comp.lang.python

C++ adds object-oriented and generic programming features to C, along with a host of other features. This can significantly improve the abstraction level of C++ programs relative to C programs, and this should make the language more scalable. Unfortunately, the presence of pointers and the absence of GC, in my opinion, undoes all the benefits of the useful features and makes C++ non-scalable (for instance, NONE of the standard template library (STL) container classes use GC). In addition, C++ is so mind-bogglingly complex, with so many odd features that interact in peculiar ways, that it's almost impossible to master. I will readily admit that I'd rather write a large application in C++ than in C, but that's like saying I'd rather eat rotting meat than swallow sulfuric acid ;-)

To be fair, I have to point out that there is some really neat stuff possible in C++ using templates (typically lumped under the name "template metaprogramming"; see the book Modern C++ Design by Andrei Alexandrescu for more information.). This is very much wizard-level programming (which is OK), but to my mind it doesn't even come close to compensating for the lack of GC. What would make C++ a more scalable language is (a) including a garbage collector as part of the standard library, and (b) having some way to restrict the use of pointers to particular modules that need direct pointer access (such as very low-level data structures). I'm not very optimistic that this will ever happen, but I hope it does.

Java and C#

Java and C# are more recent languages which have learned from the mistakes of the past. They both have GC, neither have pointers, and they both support object-oriented programming. This makes programming in these languages comparatively non-painful, and they scale well. Both also have interesting mechanisms for achieving platform independence (which is beyond the scope of the present discussion). C# is part of the .NET framework, which (in theory) permits a considerable amount of inter-language interaction; this is in my opinion a huge win, but is also beyond the scope of the present discussion. Both languages tend to be quite verbose, which is off-putting to many people (including me), but that's not that big an issue. C# also contains an interesting mechanism for using unsafe code (i.e. code which uses pointers and doesn't use GC) but encapsulated in modules specifically marked "unsafe" (an idea borrowed from the Modula-3 language, which is now sadly all but defunct). If you feel you can't live without the ability to program with pointers, but you don't need them for most of your code, that's the right way to go.

The abstraction level of both C# and Java is mediocre; it's much better than C, somewhat weaker (!) than C++, and not nearly as good as languages that support both object-oriented and functional programming (such as Lisp and Ocaml). Therefore, I find programming in these languages to be pretty boring and tedious. Many of the scalability features in these languages are not, strictly speaking, part of the languages at all but of the environment(s) built up around the languages. For instance, the support for components, versioning, packaging and documentation generation are all features of the environments. I hope we will soon start to see these kinds of meta-features in better languages than Java or C#.

Eiffel

Eiffel is a very cleanly-designed object-oriented language that has been specifically designed to be scalable. It has a number of interesting features, including: There are many other features as well. Eiffel's author, Bertrand Meyer, has written a number of books about the language and the ideas behind it, of which the most comprehensive is Object-oriented Software Construction.

Eiffel has a few deficiencies. The syntax is very verbose (there are something like 70 keywords) and also quite unfamiliar. The theory behind Eiffel rules out some language features that we have come to expect, like multiple exit points from loops. Many familiar features are found in a completely unfamiliar form. Classes are the only module units, which I think is abusing the notion of class (and which leads to the excessive use of multiple inheritance in situations in which it isn't really needed). These deficiencies are not show-stoppers, however.

More significantly, functional programming isn't supported (although the last time I checked something analogous to function pointers was in the process of being added to Eiffel; I'm not sure what the current situation is). Also, the type system has some holes in it that require link-time analysis to make sure that a program is correctly typed. This is a problem if you want to create shared libraries (although I admit I don't know all the details).

In conclusion, I would say that Eiffel is quite a scalable language, certainly more so than C++, Java, or C#. I personally find the lack of support for functional programming quite stifling, but I'd rather program in Eiffel than in Java, C++, or C# any day.

Python and Smalltalk

I'm lumping together Python and Smalltalk, two quite different languages, because they are both dynamically typed languages which support object-oriented programming. One difference is that it's possible to program in a non-OO manner in Python, whereas Smalltalk is nothing but objects, all the way down. From a scalability perspective, both languages are safe, excellent for prototyping, and generally enjoyable to work in. On the other hand, the lack of static type checking hurts their scalability (not to mention their efficiency), and to compensate for this most serious Python and Smalltalk programmers spend a lot of time writing test suites (which is a good idea, but the test suites can be much smaller in statically typed languages since you get type safety for free). I have much more experience with Python than with Smalltalk, and I can say unequivocally that it's a great language for writing small programs, but when the program source code is longer than about a thousand lines I start wishing I had a type checker.

Perl

Python: executable pseudocode. Perl: executable line noise.

    -- off comp.lang.python

I am not a fan of Perl. Basically, I think that Perl is simply Python with an incredibly obfuscated syntax and a few extra features that nobody really needs. Perl is incredibly non-scalable; I dare you to try to understand any Perl program of more than a hundred lines or so. The fault is not just the syntax; the semantics of the language are full of little oddities (e.g. overloading on the return type of a function), and frankly, I recommend that you just stay away from Perl. Maybe Perl 6 will not be as painful; but then, maybe it won't. I'll check again when Perl 6 actually happens.

Parenthetically, one cool thing that has come out (actually, that is in the process of coming out) of the Perl 6 effort is an amazingly cool project called Parrot, which is a virtual machine targeted at dynamic languages. The goal is to have a common virtual machine for running Perl, Python, Ruby, Scheme etc. I like this project very much, so please check it out.

Common Lisp and Scheme

Most people are just too damn dumb to recognise how great Lisp is.

    -- off slashdot

[Lisp] is the only computer language that is beautiful.

    -- Neal Stephenson

The journey of a thousand miles begins with an open parenthesis.

    -- Rainer Joswig

Will write code that writes code that writes code for food.

    -- Martin Rodgers

Those who do not understand lisp are doomed to re-implement it.

    -- attributed to Erik Naggum, off comp.lang.lisp

Lisp is, quite simply, a brilliant computer language. There are two forms of Lisp that are still alive today (three if you count Dylan, but I discuss that below): Common Lisp and Scheme. Their differences are mostly irrelevant for the discussion here, so I'll lump them together for the most part.

Here are some of Lisp's interesting features.

One of the consequences of all these features is that it is extremely easy to implement new programming paradigms within the Lisp language. For instance, you can implement a full object-oriented system (or several different incompatible OO systems) within the Lisp language itself. For this reason, Lisp is sometimes referred to as a "language laboratory". Because of this, Lisp programmers often write their programs bottom-up by effectively writing a new program-specific language for the application and then writing the application in that language. Paul Graham's book On Lisp discusses this approach in great detail. Lisp is also an incredibly dynamic and flexible language; code can be compiled on the fly and modules can be re-loaded as needed. Despite this, Lisp compilers exist that are very efficient and can routinely produce code that is only (say) 1.5 times slower than optimized C code.

The only serious drawback that Lisp has is that it relies mainly on dynamic typing. This is great for interactive exploration, but becomes an obstacle as the size of a program gets larger. Common Lisp at least allows type declarations, which the compiler is free to check for consistency (or to ignore). However, it's not easy to get the kind of absolute type safety in a Lisp program that you'd get in (say) an Ocaml program. Still, I really like Lisp and I think that every serious programmer needs to learn it. Also, at least one implementation of Scheme (the PLT Scheme implementation) promises to provide a usable type checker in the near future, which I look forward to.

Dylan

Dylan is a dialect of Lisp that uses a more conventional infix syntax and that has a number of features that improve the language's scalability relative to Lisp. Most notably, it allows (and encourages) the use of type declarations in code. If these type declarations can be checked at compile time, they are; otherwise they're checked at run time. This is a real win for the programmer. Unfortunately, dylan hasn't yet caught on, but there is a team of open source hackers at Gwydion Dylan that are hard at work trying to bring Dylan back from obscurity. I hope they succeed, because Dylan has the potential to be a very scalable language, and is perhaps the only language that combines the best features of scripting languages and scalable languages.

Objective CAML (Ocaml) and Standard ML

There is a joke about American engineers and French engineers. The American team brings a prototype to the French team. The French team's response is: "Well, it works fine in practice; but how will it hold up in theory?"

    -- unknown

Perl, C, FORTRAN, Pascal and AWK are purely procedural (not even OO) languages. Functional languages are to large reflecting telescopes what these languages are to binoculars.

    -- off slashdot

The best and the brightest in the programming languages field are working on functional programming languages.

    -- Bjarne Stroustrup (paraphrased from an interview)

I've saved the best for last ;-) As you've probably gathered from my comments above, I am a big fan of the Objective CAML language (usually referred to as Ocaml). Ocaml is being developed by a team of researchers at INRIA, a computer science research institute in France. The curious acronym CAML stands for "Categorical Abstract Machine Language" and is basically a historical artifact; I won't discuss it further. Ocaml is closely related to another language called Standard ML. However, since Ocaml provides almost all of the features of Standard ML and many more, I won't discuss Standard ML further either.

What does Ocaml provide that makes it so great, and in particular makes it such a scalable language?

I don't expect you to understand all of these features, but I also don't have the patience to explain them all here; go to the web site for more information. The point is that Ocaml has almost all of the typical features that I've identified as giving a computer language more scalability, and several other features that are found almost nowhere else that also enhance the scalability. It's also incredibly efficient for such a sophisticated language; a lot of Ocaml code is quite competitive with C code that does the same task (sometimes the Ocaml code is even faster). For this reason, Ocaml is by far my favorite computer language and the one I consider first when contemplating writing a new program.

Having said that, I have to add that Ocaml is a long way from perfect. The syntax is a little weird in many places. The language has a fairly long learning curve. There is not enough good documentation and far too few books. The standard library is much smaller than what you'd find in (say) Java. On the other hand, none of these problems are show-stoppers, and I predict that Ocaml is going to become a major force in the world of programming languages in the next ten years, especially among very high-level hackers.

Social considerations

Given all the comments I've made above, the obvious question is: why do people continue to use lousy non-scalable programming languages when there are good alternatives that will make their lives much easier? In this section I'd like to look into the factors that prevent programmers from learning and using scalable programming languages.

Ignorance

Most programmers, even professional programmers, know next to nothing about programming languages. Most programmers have probably never even heard of any languages other than C, C++, Java, C#, Visual Basic, Fortran, and (maybe) Perl. That's too bad, because there isn't a great language in that entire list. What this shows is that people who develop and use good languages have got to get better at getting the word out. For my part, I would recommend that anyone reading this go out and learn Ocaml (especially!), Lisp and/or Scheme, Python, Smalltalk, and Eiffel, especially if they think that "all languages are the same".

The learning curve

As somebody who loves to learn new programming languages and paradigms, I hate to admit this, but one of the biggest reasons bad languages persist is that most people hate learning new programming languages. They would rather stick to a shitty language that they more-or-less understand than take the one month or so to become familiar with a new language. This is an example of a very, very general phenomenon both in computing and elsewhere, which is that nobody ever wants to learn anything new. I think this is because learning is very painful for most people (I say I think so because I've always really enjoyed learning and so I can't relate to this at all). This would make sense if learning a new language wasn't worthwhile (let's face it, it's a lot of work), but as I've tried to show above, there are huge differences between good languages and bad ones, and if the only language you know is a bad one, then learning a good one is most definitely worthwhile.

In addition, the learning problem becomes much, much worse when the new language embodies a different programming paradigm (e.g. functional programming), even when the language also supports more familiar paradigms. I see this even in Caltech undergrads, who supposedly have the highest average SAT scores of any group of students in the world. Most of them already know how to program in C when they get here, and we teach them Scheme in a mostly-functional style that is totally unfamiliar to them. It's remarkable how much difficulty they have with this, big SAT scores notwithstanding.

Finally, it's true (but absolutely amazing to me) that small differences in syntax between computer languages can make it incredibly difficult for someone who has mastered one language to learn another. This is why Java and C# (correctly) adopted a C-like syntax; it's not that it's better, it's just more familiar and thus is less likely to be rejected. Programmers are an odd lot; they will often reject a language based on the most trivial criteria (e.g. whether or not the language uses semicolons to separate statements).

Misconceptions

There are a lot of stupid misconceptions about programming languages that make people not want to learn new ones. The most common ones are: I've also heard some truly incredible howlers by people who should know better, such as "in Lisp, lists are the only data type" (this was written in the book Generative Programming, which was published in 2000, and is completely, utterly false).

I hope I've helped to dispel some of these misconceptions in the material I've written above.

Sacrificing everything for speed

One serious obstacle to the adoption of good programming languages is the notion that everything has to be sacrificed for speed. In computer languages as in life, speed kills. It's not that I like slow programs, but the notion that speed is the only important attribute of a program (as opposed to correctness, maintainability, extensibility, readability etc.) is simply wrong. In fact, most programs only have a small core (maybe 20%) that absolutely needs to be fast surrounded by a much larger infrastructure of code that doesn't need to run all that quickly. If you're going to write your entire application in a non-scalable language like C because only 20% of the program requires it, then that's a real shame. It's especially ironic because most good languages have a foreign function interface (FFI) into C which allows you to write that 20% in C and use the better language for the rest of the code. However, most programmers can't be bothered learning an FFI any more than they can be bothered learning a new computer language. In addition, most programmers who are obsessed with speed never even bother to profile their code to find out where the bottlenecks are, presumably because they can't be bothered to learn how to use a profiler either. Do you detect a pattern here?

Fortunately, the rise of Java and scripting languages, and the success of projects that have used those languages, have made the speed obsession much less of a factor than it used to be.

Sacrificing everything for bit-level control

Another factor hindering the adoption of scalable programming languages is the idea that a programming language must give you intimate, bit-level control over every data structure or it isn't any good. If you believe this, think back on all the programs you've ever written. How many of them actually needed this kind of bit-level control? In my case, the answer is zero, and unless you spend a lot of time writing device drivers, your answer is probably pretty low too. Nevertheless, bit-level control is a nice feature, and that's precisely what C is good at. This is why any good language worth its salt has a C FFI.

Sacrificing everything for the lowest common denominator

Another major factor hindering the acceptance of new computer languages is the lowest common denominator. Programmers overwhelmingly prefer to learn a language that everybody else knows. There are some good reasons for this: However, this is a chicken-and-egg situation. If nobody ever learns a new language, then no progress is going to be made in the field. In fact, there are only two successful models of language adoption that I know of: Since I can hardly expect large corporations to espouse genuinely good technology, I think that the only way for scalable programming languages to catch on is through the grass-roots method. I believe that Ocaml is starting to gain market share this way. And, truth be told, Java and C# aren't that bad even though they are corporate-sponsored. They are reasonably scalable languages with a mediocre abstraction capability. But we can do much better.

Conclusions and recommendations

I've tried to show that there are many aspects of computer languages that can have large effects on their scalability, and I've discussed how different languages compare in this regard. To recap, a scalable language would ideally have:

If you've read this far and agree with many of my observations, then my recommendation to you is simple: learn some new languages. In particular, learn all of the languages that I've discussed above, even the ones I put down. Write sizable programs in all of these languages. This will not only be fun, but it will teach more about what makes a computer language scalable than you will ever learn by reading. And if you only want to learn one programming language out of all the ones I mentioned, please do yourself a favor and learn Ocaml. It's not perfect, but it's the best language I've ever used. If you have the time to learn two languages, the other language should be Common Lisp or Scheme.

Have fun!

Epilogue: May 2003

Since writing the original version of this article, I've had some experiences which have modified some of my views. Most importantly, I am not as high on Ocaml as I was then. While I still think Ocaml is an outstanding language in many respects, there are significant things I don't like about it. I wanted to use Ocaml as the language for a large project I wanted to work on, but found very quickly that I couldn't use it the way I wanted. The object system doesn't support multimethods (which is no surprise; very few languages do), and there were certain features I wanted that called for multimethods. In C++ or Java, you can fake multimethods because you have run-time type identification (RTTI). But the Ocaml developers refuse to add this feature to Ocaml on quasi-religious grounds; the idea of having programs which cannot be proven to be type-safe at compile time goes against everything they believe in. No matter if the type violations give rise to a safe exception just like dividing by zero does; they won't hear of it. So I'm not using Ocaml for that project.

I've also had more experience with both Java and C++ since writing the first version of this article. I haven't changed my opinion of Java much; the best thing about it is that it comes with a huge variety of useful libraries. The upcoming "Tiger" release of Java will add several new language features which will make programming in Java much less painful (notably generics and autoboxing), which I welcome. I think Java is a fine language for many types of applications, but it doesn't have the qualities of coolness that make me fall in love with a language, and the abstraction level is still less than many other languages, including C++. Speaking of C++, I now understand better why C++ is the way it is. Even though C++ is monstrously complex, the ability to write code that moves smoothly from a bit-level of abstraction to a fairly high level of abstraction is extremely valuable for many projects, especially ones where efficiency is paramount. I intend to do more C++ programming and also to use the Boehm-Demers conservative GC to see how effective it is in combination with C++'s other features.

Martin Rodgers (author of one of the quotes above) sent me an email where he said that his main criterion for scalability is that a language not put a glass ceiling on abstraction. This is the classic Lisp programmer's viewpoint, and I am very sympathetic to it. One of the big challenges in programming language design is to get languages with the flexibility of Lisp but with more static guarantees. The ML family of languages are a compromise between static type safety and Lisp-like flexibility. I look forward to future developments in this area.

Epilogue 2: July 2006

This article got posted to reddit.com and probably some other places, so I've been getting a lot of comments on it. Here is my response to some of the ones I found most interesting.

Julian Morrison sent me this email:

There's a very important feature you missed, and it's the real explanation for the success of Java: separable, atomic, pre-packaged, versioned functionality. Jarballs. Those, more than anything else, make reusability real. Java programming is about plugging together ready-built parts. Nothing else comes close.

I have to agree with him on this, and it's a major omission from the discussion above (the component stuff is sort of related to this). I don't find Java to be a very inspiring language, but I like the Java infrastructure a lot (the same comment applies to C#). With Java, you can download packages and have pretty good confidence that everything will work as it should (with some caveats that I'll mention below). There are a bunch of features of the Java infrastructure that make this possible: bytecode, having the same virtual platform on every real platform, versioning, metadata, etc. but they all result in a chunk of code which (ideally) "just works". In fact, it doesn't always "just work" but it "just works" more often than in most other languages I've used. To give an example, I tried to install a podcasting program which was written in Python, but which used Python interfaces to a number of libraries written in C or C++. I couldn't get it to work (and I've been doing this for a long time) because of all the version skew problems. This wasn't Python's fault as a language; it was the fact that the Python packages didn't track the C/C++ versions that were being used well enough. This is much less likely to happen in Java, for the reasons mentioned above and also because using code written in C/C++ as part of a Java program (i.e. through JNI) is less necessary (here's where JIT-compiling pays off big time).

On the other hand, my colleague Donnie Pinkston, who has forgotten more about Java than I'll ever know, has pointed out that it's very easy to get into classloader hell for projects that define their own classloaders (which apparently is often necessary), so it's not all a bed of roses. But I think there is a point to be made that Java gets some of the large-scale issues less wrong than most languages. These issues tend to be boring non-sexy things that don't excite my computer language sense, but they are absolutely essential in practice. If nobody can install your program, nobody can use it. And yes, I think I've spent as much time typing "configure;make;make install" as anyone, and I don't think that's good enough. Material for another rant.

One person mentioned that Common Lisp has multimethods, which is true (it's part of CLOS, the Common Lisp Object System). Multimethods are very cool (some languages, like Goo, build them in right from the start); my impression, though, is that they're hard to implement efficiently. One day I'll build a language with multimethods to find out for myself.

Someone argued that I'd been too hard on Ocaml for claiming that it's not able to fake multimethods. All I can say is: try it, and see how easy you think it is. I still think Ocaml is a great language, by the way.

One person argued for Lisp's scalability on a number of levels, which essentially comes down to Paul Graham's argument that in Lisp, you extend the language towards the problem instead of coding the problem down to the language (which I mention above). Two key features that enable this are a uniform syntax and syntactic macros. I'm definitely a fan of uniform syntax (for one thing, it lessens the conceptual load of learning the language) and of syntactic macros. However, macros have interesting and complex interactions with module systems, and many people feel that Common Lisp didn't get this right. Matthew Flatt's paper Compilable and Composable Macros goes into the PLT Scheme macro/module system, which is an interesting new approach to this problem that tries to be the best of both worlds. Also, there has been a lot of recent work on "multi-stage programming", which generalizes the notion of compile-time computation to an arbitrary number of stages (compile-time, run-time, link-time, whatever) and also can incorporate static type checking. This seems extremely promising to me. Tim Sheard has a number of papers on this and related subjects, as well as links to other work.

Julian Noble (an ex-Caltecher!) asked me about Forth, a language that he is intimately familiar with (he wrote a book called Scientific Forth on (you guessed it) scientific computing in Forth). Forth was my first love among programming languages (I read Leo Brodie's Starting Forth a long time ago), and I've had kind of a love/hate relationship with it ever since. You can do cool things in Forth that are nearly impossible to do in other languages (like change the lexical syntax), but it always seems to me that things that are fairly easy to do in other languages are hard to do in Forth. I could write a very long article explaining this in detail, and I probably will one day. Also, Forth has the "write-only" quality that I dislike about Perl, only even more so (though mitigated somewhat by the very uniform syntactic and semantic model). I've written two experimental languages to try to get at the essence of what I like about Forth without the things I don't like; I haven't gotten there yet. (By the way, writing Forth-like languages seems to be a rite of passage for computer language geeks.) I agree with Julian that Forth is a very easy language to debug, and also that it's a language that everyone with a serious interest in computer languages should study.

I pretty much stand by my statements on C++ that I expressed in the first epilogue. C++ is a terrifically powerful language, but it's extremely complex and there are so many ways to shoot yourself in the foot that it's hard to use. I've started to think of C++ as a giant macro system around C, which makes a lot of the design decisions easier to understand. Stan Lippman's book Inside the C++ Object Model is essential reading if you want to understand C++ better, though the book is somewhat out of date.

Since writing the last epilogue, I've gotten very enamored of Haskell, which I've been interested in for a long time. It was actually a couple of my students (Brandon Moore and Aaron Plattner) who turned me (back) on to Haskell. Haskell has come a long way in the last few years, and is now starting to be used for serious applications. I think Haskell has the potential to be the most scalable language ever, but the learning curve is monstrous. The great thing about Haskell is that its purely-functional nature allows you to combine components arbitrarily, and they always work the way you expect! Al Barr once described this to me as the "snap-together" quality of a language, and Haskell code snaps together very nicely. However, there's a cost; dealing with I/O and mutable state requires more work than in most languages (though the type system does give you nice guarantees about which functions can do I/O or state manipulation and which ones can't). Another interesting thing: the "point-free" style of programming in Haskell reminds me a lot of Forth! In Forth you create new functions by concatenating old functions; in Haskell you do the same thing, but with a function composition operator between the functions (of course, it's not _quite_ that simple, but it's close). So maybe I've come full circle: Haskell is Forth, lambda calculus is SKI combinators, and I need to up my medication ;-)

References

  1. Structure and Interpretation of Computer Programs, by Hal Abelson, Gerry Sussman and Julie Sussman (the full text is online, but you really should buy a copy ;-)).

  2. The Boehm-Demers conservative garbage collector (Hans Boehm's page).

  3. The Eiffel programming language.

  4. The Cyclone programming language.

  5. The objective CAML (Ocaml) programming language.

  6. The extreme programming home page.

  7. The Gwydion Dylan home page. GD is an open-source implementation of the Dylan language.

  8. The PLT Scheme home page. PLT Scheme is an outstanding implementation of the Scheme programming language, with many innovative features.

  9. Scott Meyers, Effective C++, 2nd Ed. and More Effective C++. Addison-Wesley, 1997 and 1995 (respectively).

  10. Andrei Alexandrescu, Modern C++ Design. Addison-Wesley, 2001.

  11. Paul Graham, On Lisp.

  12. Bertrand Meyer, Object-oriented Software Construction. Prentice-Hall, 2000.

  13. The Gwydion Dylan implementation of the Dylan language.

  14. The Standard ML of New Jersey (SML/NJ) implementation of the Standard ML language.

  15. The Haskell home page.

Go back to my home page. Last updated October 13, 2014

Mike Vanier (mvanier@cs.caltech.edu)