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Copyright 1984 by Gary M. Rader ╔═════════════════════════════════════════════════════════════════════════════╗ ║ Why Not Lisp? ║ ║ ║ ║ by ║ ║ ║ ║ Gary M. Rader ║ ╚═════════════════════════════════════════════════════════════════════════════╝ ┌──────────────────────────────────┐ │ Introduction │ └──────────────────────────────────┘ Browsing through almost any personal computer journal will yield at least one (if not many) programs computer hobbyists can use on their personal computer. These programs are invariably written in BASIC. Occasionally one finds programs written in Pascal, Assembly or, in what seems to be the current favorite of software developers, C. Other languages are mentioned in articles, often without programs - Fortran, Cobol, Forth, APL, Modula-2, Ada, and even Logo. The language that never seems to appear is Lisp. Why? The impression is that Lisp is either subhuman (subcomputer?), superhuman, or beyond comprehension. This article examines some of the strengths and weaknesses of Lisp, and compares it to other microcomputer languages. ┌──────────────────────────────────┐ │ History │ └──────────────────────────────────┘ Lisp was one of the first high level languages developed for digital computers. Fortran and Cobol were developed in the same time frame. Unlike those languages, Lisp has never been standardized, and several dialects exist. MacLisp from the Massachusetts Institute of Technology (MIT) and InterLisp from Stanford University are two main dialects. However, one dialect can generally imitate another so lack of standardization usually causes no serious problem. Lisp was initially developed by John McCarthy while at MIT. Traditionally, it has been used mainly for symbol manipulation, primarily in the area of artificial intelligence. Symbol manipulation is the complement of arithmetic. It deals with structures as opposed to numbers. Probably the most familiar symbol manipulation application is a word processing program in which words (symbols) are manipulated. Other examples include compilers, interpreters, data base systems, computer games, and business graphics. In the area of natural language processing, pluralizing a noun such as "key" to "keys" is an example of symbol manipulation. Historically, Lisp systems were slow with minimal arithmetic capabilities. These deficiencies have been largely overcome, but the image persists and Lisp is rarely found outside university environments. Another reason for its nonacceptance was the lack of good tutorial material. The arrival of Winston and Horn's excellent book LISP (Addison Wesley, 1981, paper) has rectified this situation. (EDITOR'S NOTE: Apparently, laziness is the only valid remaining reason not to learn Lisp.) This is the book of choice for learning Lisp. ┌──────────────────────────────────┐ │ What Distinguishes Lisp? │ └──────────────────────────────────┘ What Distinguishes Lisp from other Languages? 1. First, Lisp is interactive, like BASIC and APL. This greatly speeds program development and debugging. Like APL (and partly like BASIC) variables are not declared. Developed programs can be compiled for increased speed. 2. One of Lisp's most powerful features is its ability to generate code and execute it. For example, to satisfy a user's query, a data base program might first generate an appropriate program and then execute (or "evaluate" as we say in Lisp) it. APL has a similar capability. This is possible because Lisp programs and Lisp data have identical structures. 3. Lisp compilers are unique in that they can handle a mixture of both compiled and interpreted code. That is, under Lisp a system of interpreted AND compiled programs can be run! Some of these programs can be compiled, and the others will be interpreted. This allows a program to have the speed of a compiled program together with the above mentioned flexibility of "evaluating" generated code. 4. Various types of macros facilitate translations and redefinition of the Lisp language. New languages can be built from Lisp relatively easily using macros. The original Logo was initially created this way. 5. Another powerful feature is that of function types in which the number of arguments can be variable or fixed. These arguments can be either evaluated or unevaluated. Most languages require the number of arguments to be fixed. APL allows a variable number (as long as it is either 0, 1 or 2). The Lisp function PLUS is a built-in function which takes a variable number of arguments. It can be used to add an indefinite number of numbers, not just a pair of numbers. The evaluated/unevaluated feature is similar to call-by- value and call-by-name used in other languages. 6. "Bignums" permit unlimited precision integer arithmetic thereby eliminating the problem of round-off errors. In certain engineering and applied mathematics situations the problem of round-off can be very serious. 7. Strings, functions, and variable names can be of unlimited length. This allows the creation of completely descriptive names without the need for abbreviations. 8. The main Lisp data structure is the linked list, which allows the development of general structures, even circular graphs. The links (pointers) are transparent to the user. Other languages such as C allow this capability through the explicit manipulation of pointers. Linked lists allow certain operations to be performed much more efficiently than when arrays are used. Consider an array of numbers, say X(1), X(2), X(3), etc. To insert a new third element in this array we shift each element to the next element in the array and set X(3) to the new number. In Lisp, this is accomplished by linking X(2) to the new element, and then linking the new element to X(3). For long sequences the linked list approach is significantly faster than using arrays. 9. Lisp is a functional language - it uses function calls, and this makes it structured, almost naturally. All Lisp programs are systems of functions with a top level function calling subfunctions and so on. New functions can be defined in isolation and used as if they were primitive Lisp operations. 10. Lisp has perhaps the most powerful set of debugging features of any language. Break points can be set and the environment examined when a break occurs. The input arguments and results of executing specified functions can be displayed selectively during program execution. You can create unique and powerful debugging facilities by appropriately redefining the "evaluate function" mechanism of Lisp. ┌──────────────────────────────────┐ │ Drawbacks │ └──────────────────────────────────┘ 1. One drawback of present day Lisp is the reclamation of memory. Referred to as "garbage collecting," it occurs automatically when no free memory is available for creating new lists or strings (BASIC has a similar drawback). Processing stops (for a few seconds) until all unused memory has been reclaimed. This can happen at any time during the execution of a program. In many cases it will be unnoticed, but it can happen while a line is being displayed on the screen! This creates quite a shock and can lead new users to believe the system has crashed. This also presents problems for real time programming. The problem can be reduced when necessary by explicitly calling the garbage collector at points in the program where the user will not notice, and by pursuing "garbage collectionless programming" (by minimizing the creation of new lists). Research on incremental garbage collectors which reclaim a little bit of memory with each new usage of memory will, hopefully, solve this problem. Compiling programs reduces the length of garbage collections as compiled code does not need to be processed. Despite the negative impression created above, automatic memory management is also one of Lisp's great strengths. It allows the programmer to concentrate on solving the problem at hand without being distracted by conceptually irrelevant details concerning memory (e.g., variable declaration, requesting memory from the operating system, and returning memory when no longer needed). This undoubtedly is an important reason for Lisp's preeminence in the world of artificial intelligence. 2. Using lists instead of arrays requires more memory, as space for links must generally be provided. 3. As data and programs can be widely dispersed in memory, using Lisp in a virtual memory environment can cause speed degradation. While following links to a desired data item, the page containing the actual item might be swapped out in the course of tracking it down! Of course, this can happen in other languages such as C in which pointers can point to other pointers, which point to other pointers. 4. Many people complain that Lisp has too many parentheses. This is a prejudice, not a drawback. Usually, if functions and data are suitably formatted ("pretty printed"), this objection disappears. For those preferring fewer parentheses, the Lisp driver can be redefined to provide it. (This was done for the Logo syntax.) ┌──────────────────────────────────┐ │ Lisp on the IBM PC │ └──────────────────────────────────┘ Several Lisps are currently available for the IBM PC. The Soft Warehouse sells muLISP-83 (also sold by Microsoft), Integral Quality markets IQLISP, and Norell Data Systems offers LISP/88. I have used muLISP-83 for almost half a year and have been pleasantly impressed by its performance. I have not used IQLISP but from Integral Quality's brochure it seems that it is a fully functional Lisp. LISP/88 has just become available for distribution, and I have no experience with this product. muLISP enforces completely structured programming by not having a GOTO command.(However, exits from loops can be efficiently performed using the THROW/CATCH combination.) In addition to the expected functionality, muLISP provides access to the PC's BIOS through its REGISTER, INTERRUPT, and PORTIO commands. This is a useful feature for it allows programmers to easily incorporate graphics, sound, game paddle communication, and more in their programs. Random I/O is available. muLISP does not provide arrays or floating point numbers. (However the Soft Warehouse is currently implementing floating slash - i.e., floating point in which the precision is user-defined. This prevents the round-off problem from resurfacing.) A program development system and a debugging system are included with muLISP along with utility files containing libraries of MacLisp and InterLisp functions. An on-line tutorial for learning Lisp is also included. (The Soft Warehouse had developed a tutorial correlated with Winston and Horn's LISP book but Addison Wesley, the publisher, demanded a royalty!) A nice feature for program developers is that application programs written in muLISP do not require customers to own the full muLISP system. A small royalty is paid for use of the muLISP runtime system. Programs are distributed as .COM files. IQLISP uses all of the PC's memory while muLISP uses a maximum of 256K bytes. Arrays and floating point numbers are provided as is 8087 support. IQLISP does not have random I/O. Both Lisps allow user-defined machine language routines to be called from within programs. Both include a "string" data type. muLISP is allegedly 4 to 7 times faster than IQLISP. One reason is that muLISP functions are compiled into "distilled" code (similar to Pascal's p-code) before interpretation. Another is that IQLISP accommodates a larger address space. Both companies state they will be offering a Lisp compiler. When I contacted Integral Quality last Spring, they indicated that a Lisp compiler would be available by now. To my knowledge, it is not yet available. The compiler from the Soft Warehouse will be ready in late 1984. LISP/88 may or may not be in the same league as these two. However, it definitely has one notable feature - a price of $49.95! ┌──────────────────────────────────┐ │ Conclusion │ └──────────────────────────────────┘ Unjustified prejudices continue to restrain microcomputer users from learning and using Lisp. Several quality microcomputer implementations are available along with an excellent book for learning Lisp. Lisp provides powerful capabilities, many of which are unavailable in other languages. Three Lisps are currently available for the IBM PC - muLISP, IQLISP, and LISP/88. A review of muLISP, and IQLISP appears in PC Magazine (December, 1983). ┌───────────────────────────────────────┐ │ Products mentioned in this article │ ├───────────────────────────────────────┤ │ │ │ IQLISP ($175) │ │ Integral Quality │ │ P. O. Box 31970 │ │ Seattle, Washington 98103 │ │ (206) 527-2918 │ │ │ ├───────────────────────────────────────┤ │ │ │ LISP/88 ($49.95) │ │ Norell Data Systems │ │ 3400 Wilshire Blvd. │ │ P. O. Box 70127 │ │ Los Angeles, California 90010 │ │ (213) 257-2026 │ │ │ ├───────────────────────────────────────┤ │ │ │ muLISP-83 ($250) │ │ The Soft Warehouse │ │ P. O. Box 11174 │ │ Honolulu, Hawaii 96828-0174 │ │ (808) 734-5801 │ │ │ └───────────────────────────────────────┘ ┌──────────────────────────────────┐ │ File Name: ██ lisp1.txt ██ │ └──────────────────────────────────┘ ,Tû%───┐ parentheses, the Lisp driver can be redefined to provide it. (This was done for the Logo syntax.) ┌──────────────────────────────────┐ │ Lisp on the IBM PC │ └──────────────────────────────────┘ Several Lisps are currently available