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Intercal -- the Language From Hell



Abandon all sanity, ye who enter here!



Through me you pass into the city of woe:
Through me you pass into eternal pain:
Through me among the people lost for aye.

Justice the founder of my fabric mov'd:
To rear me was the task of power divine,
Supremest wisdom, and primeval love.

Before me things create were none, save things
Eternal, and eternal I endure.
All hope abandon ye who enter here.

 --- Dante [Divine comedy] (1814 translation into English by the Reverend H. F. Cary)

(The section above is of course Dante's description of the inscription to the gates of Hell)

Computers work in essentially the same way, executing instructions, moving data around, etc. Programming languages are mere abstractions allowing us to tell the same computer how to do the same things using different "words and methods". These abstractions provided by languages like C, C++ or even Java, GoLang or LISP were created on the back of many years of research and when we learn to program we seldom spend the time to understand WHY the language works the way that it does, focussing rather on HOW to use it.

Some concepts are best explored by imagining what life would be like if the language we were using to program looked very different. For example every embedded programmer I ask has told me that GOTO is bad, but very seldom could someone actually explain why, and it would be even more rare to find someone who was familiar with Dijkstra's paper on the subject. (see this page for that one and many more examples). I have struggled to find a better explanation of what Dijkstra was trying to convey than the "COME FROM" statement in INTERCAL!

The following article appeared in Computer Shopper (the British magazine of that name, not the Ziff-Davis title) around September 1992.

Intercal -- the Language From Hell

Where would we be without effective programming languages?

Wait, don't turn the page; that was a rhetorical question. (The answer isn't: word-processing on slide rules.) Every once in a while it's worth thinking about the matter. Most of us (the lucky ones, that is) don't come within spitting distance of programming from one day to the next. Those who do, usually don't give any thought to them: it's time to write that function so do it, in C or Cobol or dBase whatever comes to hand. We don't waste time contemplating the deeper significance of the tools we use to do the job. But what would we do without well-designed, efficient programming languages?

Go back to slide rules, maybe.

A computer, in a very real sense, is its language. The processor speaks in a native tongue, its own machine code: unreadable by humans and next to impossible to program in, but nevertheless essential. It's the existence of this language which makes the computer such a general purpose tool; a hardware platform upon which we layer the abstractions, the virtual machines, of our operating systems. Which are, when you stop to think about it, simply a set of grammatically correct expressions in the computer's internal language. Take the humble 386 PC for example. Feed it one set of instructions and it pretends to be a cute little Windows machine; feed it another set, and something hairy happens -- it turns into a big, bad file server and starts demanding passwords and issuing threats.

This flexibility is not an accident. Almost from the start, the feature which distinguished computers from their complex calculating predecessors was the fact that they were programmable -- indeed, universally programmable, capable of tackling any computationally tractable problem. Strip away the programmability of the machine and you lose the universality. Not to mention the usefulness.

Programming languages, as any fule kno, originated in the nineteen-fifties as a partial solution to a major difficulty; machine code is difficult to write and maintain and next to impossible to move (or port) from one computer to another of a different type. Assemblers, which allowed the programmers to write machine code using relatively simple mnemonic commands (which were then "assembled" into machine code) were an early innovation, but still failed to address the portability issue. The solution that finally took off was the concept of an abstract "high level" language: and the first to achieve widespread use was Fortran.

A high level language is an artificial language with a rigidly specified grammar, which can be translated into machine code via a program called a "compiler". Statements in the high level language may correspond to individual statements, or whole series of statements, in the machine language ofthe computer. Indeed, the only thing that makes compilation practical is the fact that all computer language systems are effectively equivalent; any algorithm which can be expressed in one language can be solved in any other, in principle. So why are there so many languages?

A load of old cobol'ers

There are several answers to the question of language proliferation. Besides the facetious (some of us don't like Cobol) and the obvious (designing languages looks like a Fun Thing to a certain type of warped personality), there's the pragmatic reason. Simply put, some languages are excellent for expressing certain types of problem, but are lousy at dealing with other situations. Take Prolog, for example. Prolog is a brilliant language for resolving formal logic propositions expressable in the first order predicate calculus, but you'd need your head examining if you tried to write an operating system in it. (The Japanese MITI tried to do something similar with their Fifth Generation Project, and when was the last time you heard of them? Right.) Alternatively, take C. C is a wonderful language. It combines the flexibility and speed of raw machine code with the readability of ... er. Yes, you can re-jig a C compiler to run on anything. You can fine-tune it to produce tight, fast machine code that will execute on your toaster. Yes, you can write wonderful device drivers in it. But, again, you'd need your head examining if you set out to write a humongous database application in it. That's what Cobol is for; or SQL. So to the point of this article ... INTERCAL.

An icky mess

INTERCAL is a programming language which is to other languages as elephants are to deep-sea fishing -- it has nothing whatsoever to do with them. Nothing ... except that it's a programming language. And it serves as a wonderful example of the fact that, despite their theoretical, abstract interchangeability, not all languages are born equal. Here's a potted history of the offender:

INTERCAL is short for "Computer Language With No Readily Pronouncable Acronym". It was not so much designed as perpetrated at Princeton University, on the morning of May 26th, 1972, by Donald R. Woods and James M. Lyon. They have been trying to live it down ever since. The current implementation, C-INTERCAL, was written by Eric S. Raymond (also the author of The New Hacker's Dictionary), and -- god help us -- runs on anything that supports C (and the C programming tools lex and yacc).

INTERCAL is, in its own terms, elegant, simple and concise. It is also flexible and forgiving; if the compiler (called, appropriately enough, ick) encounters something it doesn't understand it simply ignores it and carries on. In order to insert a comment, just type in something that ick thinks is wrong; but be careful not to embed any valid INTERCAL code in your comments, or ick will get all excited and compile it. There are only two variable types: 16-bit integers, and 32-bit integers, denoted by .1 (for the 16-bit variable called "1") or :1 (for the 32-bit variable named "1"); note that .1 is not equivalent to :1 and definitely has nothing to do with 0.1 (unless it happens to be storing a value of 0.1).

INTERCAL supports two unique bitwise operators, interleave (or mingle) and select, denoted by the "^" and "~" symbols respectively. You interleave two variables by alternating the bits in the two operands; you select two variables by taking from the first operand whichever bits correspond to 1's in the second operand, then pack these bits to the right in the result. There are also numerous unary bitwise operators, and in order to resolve matters of precedence the pedantic may use sparks (') or rabbit-ears (") to group expressions (Don't blame me for the silly names: INTERCAL has a character set which is best described as creative.) It is not surprising that these operators are unique to INTERCAL; the parlous readability of C would not be enhanced by the addition of syntax like:

PLEASE DO IGNORE .1 <-".1^C'&:51~"#V1^C!12~;&75SUB"V'V.1~

Like any other language, INTERCAL has flow-of-control statements and input and output statements. To write something or other into a variable, you need the WRITE IN list statement, where list is a string of variables and/or array elements. The format of the input data should be as numbers, the digits of which are spelt out in english in the range ZERO (or OH) to FOUR TWO NINE FOUR NINE SIX SEVEN TWO NINE FIVE. To output some information, you need the READ OUT list statement, where list again consists of variables. Numbers are printed, by default, in the form of "extended" Roman numerals (the syntax of which I will draw a merciful veil over), although the scholarly may make use of the standard library, which contains routines for formatting output in Sanskrit.

Like FORTRAN, INTERCAL uses line numbers which are optional and follow in no particular order. Unlike FORTRAN, INTERCAL has no evil, unspeakable GOTO command, and not even an IF statement. However, you would be wrong to ascribe this to INTERCAL being designed for structured programming; it is actually because C-INTERCAL is the only known language that implements the legendary COME FROM ... control statement (originally described by R. L. Clark in "A Linguistic contribution to GOTO-less programming", Comm. ACM 27 (1984), pp. 349-350). For many years the GOTO statement -- once the primary means of controlling the sequence of execution of a program -- has been reviled as contributing to unreadable, unpredictable code that is virtually impossible to follow because it jumps about the place like a kangaroo on amphetamines. The COME FROM statement enables INTERCAL to do away with nasty GOTO's, while still preserving for the programmer that sense of warm self-esteem and achievement that comes from successfully writing a really nasty piece of self-modifying code involving computed GOTO's in FORTRAN (Or, for the terminally hip and hacker-ish, involving a triple-indirect pointer to a union of UNIX kernel data structures in C. Capisce?) Basically, the COME FROM statement specifies a line, or lines, which -- when the program executes them -- will jump to the COME FROM: effectively the opposite of GOTO.

Because INTERCAL contains no equivalent of the NEXT statement for controlling whether or not some statement is executed, it provides a creative, endearing and unique substitute; abstention. For example, you can abstain from executing a given line of code with the ABSTAIN FROM (label) form of the command. Alternatively, and more uselessly, you can abstain from executing all statements of a specified type; for example, you can say




The abstention command is revoked by the REINSTATE statement. It is a Bad Idea to ABSTAIN FROM REINSTATING. It is also worth noting that the ABSTAIN syntax is rather confusing; for example, DO ABSTAIN FROM GIVING UP is not accepted, although a valid synonym for this is DON'T GIVE UP. (GIVE UP is the statement that terminates execution of a program. You are encouraged to use this statement at every opportunity. It is your only hope of escape.)

The designers of the INTERCAL language believed that source code should be easy to understand or, failing that, friendly; in extremis, good old-fashioned politeness will do. Consequently, the syntax of the language looks a bit odd at first. After the line label (if any) there should be a statement identifier; this can be one of DO, PLEASE, or PLEASE DO. Programs which are insufficiently polite to the compiler may be rejected with the error message PROGRAMMER IS INSUFFICIENTLY POLITE; likewise, programs which spend too much time grovelling to the compiler will be terminated with extreme prejudice. The DO, PLEASE or PLEASE DO is then followed by (optionally) one of NOT, N'T, or %n, then a statement: the NOT or N'T's meaning should be self-evident, while the %n is merely the percentage probability that the following statement will be executed.

Of course, with only two binary and three unary operators, it is rather difficult for programmers to get to grips with the language. Therefore, in a fit of quite uncharacteristic generosity, the designers have supplied -- and even partially documented -- a library of rather outre subroutines that carry out such esoteric operations as addition, subtraction, logical comparison, and generation of random numbers. The subroutines will be charmingly familiar to those PDP-11 assembly-language hackers among you who are also fluent in FORTRAN and SPITBOL.

Why you should program in Intercal

INTERCAL, despite being afflicted with these unique features (which, arguably, should remain that way) has survived for twenty years and is indeed thriving. The relatively recent C-INTERCAL dialect (which introduced the COME FROM statement, among other things) has spread it far beyond its original domain; the infamy continues, transmitted like a malign influence across the Internet. It is a matter of record that no computer language has ever succeeded in gaining widespread acceptance on the basis of elegance, comprehensibility or necessity. Look at FORTRAN, Cobol or C; all of them spread despite the fact that better alternatives were available. In fact, there are reasons to believe that INTERCAL is the language of the future.

Firstly, INTERCAL creates jobs. Yes, it's true. In general, if a particular programming tool is unfriendly, ugly, and absolutely essential, the response of management is to throw more programmers at it instead of replacing it. INTERCAL is so monumentally difficult to use for anything sensible that it is surely destined to be the cause of full employment, massive increases in the DP budget of every company where it is used, and general prosperity for programmers.

Secondly, once you have learned INTERCAL you will be able to win friends and influence people. As the authors point out, if you were to state that the simplest way to store a value of 65536 in a 32-bit variable is

DO :1 <- #0$#256

any sensible programmer would say that this was absurd. Since this is indeed the simplest way of doing this, they'd be made to look like a fool in front of their boss (who, by Murphy's law, would have turned up at just that moment). This will have a devastating effect on their ego and simultaneously make you look brilliant (until they realise that you cribbed this example from the manual, like me. Deep shame.)

Thirdly, INTERCAL helps sell hardware. There's been a lot of fuss recently about big corporations inventing gigantic, bloated windowing and operating systems that require monstrously powerful computers to run on; some cynics suggest that this is because personal computers are now so powerful that they can already do anything any reasonable individual would want them to, and it's necessary to invent warm furry buttons that soak up millions of processor cycles in order to sell new equipment. With INTERCAL, you no longer need Windows or OS/2 to slow your 486 PC to a crawl! A Sieve of Eratosthenes benchmark (that computes all the prime numbers less than 65536), when coded in INTERCAL, clocked over seventeen hours on a SPARCStation-1. The same program, in C, took less than 0.5 seconds -- thus proving, quite clearly, that INTERCAL software is so slow that you absolutely must buy a CRAY-3 or equivalent in order to have any hope of getting any work out of it.

Consequently, it is quite clear that INTERCAL represents a major alternative to conventional programming languages. Anyone who learns this language is going to go far -- at least as far as the nearest psychiatric institution. Rumour has it that INTERCAL was not entirely unrelated to the collapse of the Soviet Union and the success of the Apollo missions. It is even reported to improve your sex life and restore hair loss! So we warmly advise you to take this utterly unbiased report at face value and go forth and get your friendly neighbourhood software company to switch to INTERCAL, wherever or whatever they may be using right now. You know it makes sense ...


The INTERCAL Resources Page

INTERCAL on WikiPedia

Intercal on Rosettacode

The INTERCAL reference manual



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