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AUTOMATA DESIGN ASSOCIATES A.D.A PROLOG Documentation Version 1.95C for the Educational and Public Domain Versions June 21, 1987 Copyright Robert Morein and Automata Design Associates 1570 Arran Way Dresher, Pa. 19025 (215)-646-4894 News 1 Copyright Notice 2 Introduction 3 Product Descriptions 4 Upgrade Policy 11 Technical Support 11 Installation 12 Running the Demonstration Programs 12 The Special Offer 14 Writing Programs for PD and Ed PROLOG 15 Running the Interpreter 16 A Reference of Note 18 Memory Size Configuration 19 The Built In Predicate Library 20 Description of the Modifications 22 Description of the New Predicates 26 Memory Metric Predicates 29 IBM PC Video Display Predicates 30 The Interrupt Menu 32 Conserving Memory Space 34 Prolog Tutorial 36 Reading List 49 News A.D.A. offers the first Prolog with a Cyclic Structure Unifier for the IBM PC. If you have CGA graphics capability, be sure to run "The Knight's Tour", by Tim Elliot, to be found in GAMES.ARC. It's a graphic representation of a classic puzzle: how to make the "knight" piece of chess, which has a rather peculiar movement rule, visit all the squares of a chess board. Javier Salazar has contributed a wang algorithm, which determines the correctness of predicate calculus formulae, and a logic tutor. Jim Adams of Bendix Aerospace has contributed a skolemizer program (look for the file "LOGIC.ARC". This nifty thing takes statements written in the formalism of the predicate calculus and converts them to prolog Horn clauses. You should refer to chapter 10 of Clocksin and Mellish for explanation. Take a look at the chess endgame program in "games." Simon Blackwell's "PIE" Truth Maintenance System is presented in revised, debugged, and enlarged form. This system is found in the directory "expert" and augments the strictly deductive capabilities of raw Prolog with additional forms of reasoning. PIE has a syntax that resembles colloquial English. Wait till you see the backwards quote marks! The predicates "batch" and "nobatch" are introduced to allow execution of batch files without confusing messages and prompts appearing on the screen. I've put one in Simon's "KOPS" file. Copyright Notice The public domain PD PROLOG system has been contributed to the public domain for unrestricted use with one exception: the object code may not be disassembled or modified. Electronic bulletin boards and SIG groups are urged to aid in giving this software the widest possible distribution. This documentation may be reproduced freely, but it may not be included in any other documentation without the permission of the author. Introduction We hope that you'll get some fun out of this PROLOG. It will afford you exposure to THE fifth generation language at the cost only of some intellectual effort. The motive is perfectly explicable: We want you to think of Automata Design Associates for fifth generation software. It also gives us a nice warm feeling. The minimum memory requirement is 200 k of transient program area, plus whatever space is needed to execute programs from within PROLOG. DOS or MSDOS 2.0 are required. The program does not require IBM PC compatibility to run, although the screen access routines do require compatibility. Products by Automata Design Associates Automata Design Associates specializes in software for artificial intelligence and robotic applications. A PROLOG language system is available in various configurations. A LISP interpreter will be introduced in March of 1985. 1. LISP systems PD LISP PD LISP is a public domain Common LISP subset. It comes with a 150 page instruction manual on disk, and is available from us for $10.00. .UNX LISP UNX LISP, written by Dave Morein, is a Common LISP subset. It has nifty things like SAVE/RESTORE, which let you stop a job in the middle, save it, and start it up again, and tree structured lexical scoping. It is available from us for $69.95. I.PROLOG Systems There are six versions of PROLOG available from Automata Design Associates. The systems run under the MSDOS or PCDOS. All of them access all available memory up to 1 megabyte. .Public Domain PROLOG This serves to further the general awareness of the public about PROLOG. It also is an excellent adjunct to anyone learning the language. Most of the core PROLOG described by Clocksin and Mellish in the book Programming In PROLOG is implemented. A complete IBM PC video screen support library is included in this and all other A.D.A. prologs. Trace predicates are not. This version is available from us for $10.00 postage paid. .Educational PROLOG At extremely modest cost this affords an educational institution or individual a PROLOG system which provides the maximum available programming area available within the 8086 small programming model. Tracing, a debugging aid, allows monitoring a program as it runs. User settable spy points selectively allow this. Exhaustive tracing is also available. I/O redirection gives some file ability. An "exec" function allows the execution of a program or editor from within PROLOG, thus encouraging an interactive environment. An "interrupt" menu is added, permitting the control of tracing, toggling the printer, and screen printing. Definite clause grammar support is now included. The cost of Educational PROLOG is $29.95. .FS PROLOG A small increment in price adds full random access file capability. Character and structure I/O are allowed. The "asserta and "assertz" predicates are expanded and work with a clause indexing ability. One can assert clauses anywhere in the database under precise pattern matching control. A tree structured lexical scoping system and floating point arithmetic are other enhancements. The cost of FSM PROLOG is $49.95 .VMI PROLOG -- Virtual Memory (Replaces type VMS) At reasonable cost the addition of virtual memory gives an expansion of capabilities of an order of magnitude. The database on disk is treated transparently. No special provisions need be made by the user. Virtual and resident databases may be mixed. A unique updating algorithim preserves the format of the database as typed by the user while making only those changes necessary to make it equivalent to the database in central memory. The cost of VMI PROLOG is $99.95 .VML PROLOG Large model Virtual Memory System A.D.A. PROLOG is a remarkable fifth generation developement tool for the implementation of intelligent strategies and optimized control. It is both the kernel for applications of virtually unlimited scope and a sophisticated developement tool that multiplies the productivity of the programmer many times. Conventional Prolog is based on the first order predicate calculus. VML emulates the second order calculus, with unconstrained manipulation of cyclic, self referential structures. The cyclic unifier is selectable by a mode switch, with no loss of compatibility. Perhaps only one other Prolog system, written by Alain Colmareur, permits manipulation of cyclic structures. Permitted by the second order predicate calculus, but not the first, cyclic structures are self referential, or recursively defined. A conventional Prolog system typically crashes when a cyclic structure is found, because such a structure appears to be infinite. But with our Cyclic Unifier, available in types VML and VMA Prolog, cyclic structures can be handled with impunity. Many structures in nature are cyclic, and we suggest that many applications will appear to the user. For example, consider the benzene ring, which chemists refer to as a cyclic aromatic hydrocarbon: H H \ / C ===== C / \ / \ H --- C C --- H \\ // \\ // C ----- C / \ H H This might be represented in Prolog by a cyclic structure as follows: X = [ [[c,h]],[c,h],[[c,h]],[c,h],[[c,h]],[c,h]|X] It would appear that groups and topological properties can be succinctly described as cyclic structures. A simple group can be expressed as follows: X = [a,b,c|X]. Then the cyclic nature of the group can be observed with the goal "member( M, X )", which gives: a,b,c,a,b,c,a,b,c,....(ad infinitum) With a cost/performance ratio exceeding that of any other product and a compatibility insured by compliance to the Edinburgh syntax, performance is enhanced by numerous extensions, many of them invisible to the user. A quick overview of some of the features discloses: 1) This system now incorporates a cyclic structure unifier. The only other Prolog system capable of unifying cyclic structures is PrologII, written by the inventor of Prolog, Alain Colmerauer. 2) Invisible compilation to a semantic network preserves the flexibility of the interpreted mode and the speed of a compiler. The programmer can compile and recompile any portion of a module at any time. The edit/compile/test cycle is short and free of strain. An interface is provided to an editor of choice. 3) Floating point arithmetic with a full complement of input and output methods, transcendental and conversion functions. 4) Virtual memory. Module size and number are unrestricted, with a total capacity of several hundred megabytes. Resident and virtual modules may be co-resident. Compilation is incremental. The cache algorithim is sophisticated. Changes made in the database can be updated to disk by a single command. 5) A powerful exec function and acceptance of stream input make integration into applications practical. 6) Global polymorphic variables retain information that would otherwise require the "assertion" of facts. 7) A quoted variable class, borrowed from LISP, permits referencing variables as objects as well as by value. 8) Multidimensional arrays, dynamically created and destroyed, efficiently store numeric and nonnumeric structures. Arrays are ideal for representing spatial and ordinal relationships. 9) Debugging facilities let you see your program run without any additional generation steps. 10) Totally invisible and incremental garbage collection. There is NEVER any wait for this function. 11) A tree structured, dynamically configurable lexical scoping system. The work of many programmers can be coupled together in the form of libraries and nested domains. Each lexically scoped domain is a hidden space which communicates with the parent domain via exports and imports. Domains can be linked together under program control to achieve any desired configuration. 12) The Grammar Rule Notation is an integral feature. 13) Keyword redefinition makes porting code easy. The cost of this system is $200 for the MSDOS version. .VMA PROLOG Large model Virtual Memory System This system has additional forms of virtual memory. It was intended for deep reasoning problems. Contact us for more information. Prolog Runtime Systems The A.D.A. Runtime Systems permit distribution of debugged code in source form without royalties. A Runtime System provides all of the facilities of the corresponding Development System except for: 1) Tracing. 2) The console command loop. A Runtime System is invoked from the DOS command line with an argument specifying a stream file that replaces the console. The stream file is a UNIX* innovation that allows applications to be connected by "pipes", in a convenient, high level manner. All the other development facilities remain available, including multiple modules and virtual memory. Since update to disk is included, the ability of the system to learn is not impaired in any way. Compare this to our competitors, whose hard compiled code results in fixed goals and inability to change the distribution on a dynamic basis. Four versions are available. Each is priced the same as the corresponding development system, and can be distributed without royalty by the license holder with his applications: FS Runtime - $50.00 VMI Runtime - $100.00 VML Runtime - $200.00 VMA Runtime - $250.00 The above systems are in stock for immediate delivery. * trademark AT&T Upgrade Policy Half the cost of any A.D.A. PROLOG system may be credited to the purchase of a higher level version. The full cost of VMS prolog may be applied to the purchase of VMI or VML PROLOG. Updates to a particular level product vary from $15.00 to $35.00. Technical Information Technical information may be obtained at (215) - 646- 4894. This is available provided that you have read the entire documentation supplied with this disk, and at least the first third of Programming In Prolog. We will politely inquiries from people who just want to see the sample programs run. Perhaps we can answer the following questions in advance: There is no support for: APPLE II, Atari, Commodore, or CPM 80 . Other machines from these manufactures may be supported in the future. The MSDOS products are available on "5" IBM compatible diskettes. To Place Your Order: You may place your order at the following number: (215)-646-4894 - day and night. Returns The software may be returned within 30 days of purchase for a full refund. This applies whether or not the software has been used. We do ask that manuals, disks and packaging be returned in excellent condition. Installation You will need an IBM PC compatible machine with a minimum of 256 kbytes of memory. ED PROLOG benefits from all available memory up to the 1 megabyte INTEL limit. To determine the amount of TPA your machine has, run the "chkdsk" program which is supplied with DOS. The last line reads: XXXXXX bytes free where XXXXXX is a six digit number. If this number is greater than 200000, ED PROLOG will have reduced workspace. If it's over 245000, the amount of memory is optimal. If this is not the case, there are two possibilities: 1) The machine doesn't have enough memory. 2) Something else is removing memory from TPA, such as a co- resident program, a ramdisk, a large dos driver, or a large number of file or disk buffers. If you're short of memory, make sure that no other programs, ramdisks, or drivers besides DOS are running in the machine. You may find it helps to eliminate (by renaming) the config.sys file when you intend to run ED PROLOG. How to run the Demonstration Programs without Knowing What You're Doing We strongly advise that you purchase the book Programming in PROLOG by Clocksin and Mellish, publisher Springer Verlag, 1981. For the impatient we give some advice. Type the demonstration program you wish to run. There must be at least one entry point within the program. Note: Please understand that these are demonstrations programs. Regarding user interface, they are poorly written. You will probably have to read Clocksin and Mellish to appreciate that the following examples of input are not equivalent: "yes." , "yes" . The program fragment "console" shows how you may improve the console input routines of any of these programs. The Hematology Diagnosis Program - "hemat" Although the logical structure is not as sophisticated as that of "animals", it is interesting for several reasons: 1) The program evaluates numerical data to arrive at a diagnosis. 2) Although inaccurate, it demonstrates that useful question answering systems are not difficult to write in PROLOG. 3) There are some mistakes in the program, which only slightly impede its usefulness. This program uses structure input. Terminate all your answers with a period, as in "y.<CR>", or "no.<CR>". The starting point is "signs.<CR>". PROLOG will prompt you for signs of anemia. The program attempts to diagnose two varieties of a hemolytic anemia. The program could use a good working over by a hematologist and we would be delighted to collaborate. Prime Number Generator - "sieve" This program demonstrates that anything can be programed in PROLOG if one tries hard enough. Asking the question "primes( 50, L ).<CR>" causes a list of prime numbers less than 50 to be printed out. "Sieve" is heavily recursive and quickly exhausts the stack space of the small model interpreters. Grrules This is an example of the use of the definite clause grammer notation. PD PROLOG does not have this translation facility, but ED PROLOG and all of our other versions do. It is possible to perform the translations by hand if you have thoroughly read C & M. Then you would have the pleasure of asking: ?-sentence( X, [every,man,loves,a,woman], [] ). and having the meaning elucidated as a statment in the predicate calculus. Special Offer # 1 For some inexplicable reason, demonstration programs are hard to come by. We are too busy writing PROLOG fill this gap. We will reward the contribution of "cute" sample programs. Here are the guidlines as of 9/3/86: 1) We've got enough games, puzzles and geneology programs. 2) We need: a) Alternate forms of reasoning, such as probabalistic (Bayesian, fuzzy logic) forward chaining, cover sets. b) Programs which use the graphics. c) 3) Expert systems. You must be certain that your contribution does not infringe on anyone's copyright. The reward is the following: A copy of VMI virtual memory PROLOG. There will be a charge of $20.00 to cover our costs. That way, we won't go broke rewarding sample programs. The sample program will be published as an intact file together with whatever comments or advertisments the author may see fit to include, on our distribution disks. Exceptional contributions may merit a copy of type VML large model virtual memory PROLOG which now incorporates a UNIX1 style tree structured domain system. Special Offer # 2 If you are a hardware manufacturer and would like a PROLOG language for your system, the solution is simple. Just send us one of your machines! Provided your system implements a "C" compiler, it will be ported in no time flat. ______ 1. Trademark of AT & T. Writing Programs For ED PROLOG You do not type in programs at the "?-" prompt. There is no built-in editor. The command "consult( user )" is accepted but does not cause PROLOG to enter an editing mode. We feel that the most universally acceptable editing method is for the user to use a text editor of choice, which can be invoked from within PROLOG by the "exec" predicate. Use Wordstar or your customary editor to write a program. Then run PD PROLOG and use the consult function to load the program. In all cases except PD PROLOG, you can run your editor without leaving PROLOG by use of the "exec" predicate. Running the Interpreter COMMANDS: Give commands in lower case. TO RUN: Invoke PROLOG.EXE. After the "?-" prompt appears, you may give Prolog goals to be executed, followed by a period, and a carriage return. TO ENTER A FACT: Don't do it except with the "assert" predicates. This is the most frequently misunderstood aspect of A.D.A. Prolog. If you want to enter a bunch of facts, put them in a file and "consult" them using the "consult" predicate. TO ASK A QUESTION: At the prompt, type "<expression>.<CR>", where <expression> is a question as described by Clocksin and Mellish. Be sure to terminate the question with a period. The question may be up to 500 characters long. TO INPUT A STRUCTURE AT THE KEYBOARD: The structure may be up to 500 characters in length. Be sure to terminate with a period. TO ASK FOR ANOTHER SOLUTION: If a solution has been provided, the PROLOG interpreter will ask "More? (Y/N):". Only if a "y" is typed will the interpreter perform a search. TO ABORT A SEARCH: Simply type the escape key. The interpreter will respond with "Interrrupted.", and return to the command prompt. TO LOAD DATABASE: Type "consult(<filename>).<CR>" The file name must have the extension ".PRO". It is not necessary to include the extension in the argument of consult. The file name as given must not be the same as a predicate name in the file or any file which will be loaded. TO TRACE: When the system displays the prompt "?-", type "trace.<CR>". The display will likely move too rapidly for you to read. To stop the display, type Control S. To restart the display, type Control S. To turn the trace display off, type "notrace<CR>" at the prompt "?-". The interrupt menu contains additional options, such as sending all trace output to a file, as well as display at the console. TO INTERRUPT A PROGRAM: See the section entitled "The Interrupt Menu" for a description of the flexible options. Basically, one types ESC to terminate a program, while Control V or Control I interrupt a program. TO RECONSULT A FILE: The predicate "recon" is identical to the Edinburgh predicate "reconsult." TO REMOVE A DATABASE: Type "forget(<filename>).<CR>" TO REMOVE ALL ASSERTIONS MADE BY PROGRAMS OR YOU: Type "forget( user ).<CR>" TO EXIT TO THE OPERATING SYSTEM: Type "exitsys.<CR>" The system is totally interactive; any commands the operator gives are and must be valid program statements. Statements must terminate with a period. All commands which take a file name also accept a path name. Any name which is not a valid PROLOG atom (refer to C & M) must be enclosed in single quotes. Thus one could say consult( expert ) but one would need single quotes with consult( 'b:\samples\subtype\expert' ). To exit the system, type "exitsys.<CR>" Atoms may contain MSDOS pathnames if they are enclosed by single quotes, ie., '\b:\samples\animal' . You may consult more than one file at a time. However, all names are public and name conflicts must be avoided. The order in which modules are loaded may, in cases of poor program design, affect the behavior. Command Line Arguments ED and PD PROLOG accept one command line argument, which is the name of a "stream" which replaces the console for input. The "stream" in MSDOS is a pipe or file which supplies input until end-of-file is reached. Control then reverts back to the console. To avoid noisy parser error messages when end-of-file is reached, the last command in the file should be "see( user )." See Simon Blackwell's PIE program for an example of this. A Reference of Note With minor exceptions, the syntax is a superset of that described by Clocksin and Mellish in the book Programming in Prolog by W.F. Clocksin and C.S. Mellish, published by Springer Verlag in Berlin, Heidelberg, New York, and Tokyo. We shall refer to this book as "C & M". A book review section is included at the end of this documentation. There are very few syntactical differences, mostly unrecognized and/or minor. When an operator is declared using the "op" statement, the operator must be enclosed in single quotes in the "op" statement itself, if it would not otherwise be a legal Edinburgh functor. Subsequently, however, the parser will recognize it for what it is, except in the "unop" statement, where it must again be enclosed in single quotes. Variable numbers of functor paramaters are supported. A goal may be represented by a variable, which is less restrictive than the C & M requirement that all goals be functors. The variable must be instantiated to a functor when that goal is pursued. Rules which appear inside other expressions must be enclosed in parenthesis if the "," operator is to be recognized as a logical connective. All infix operators described by C & M, and user defined infix, prefix, and postfix operators with variable associativity and precedence are supported exactly as in C & M. Memory Size Configuration Type ED can access the full memory of the machine, but previously required configuation by A.D.A. The supplied program "prconf.exe" allows the user to configure the amount of memory used by the PROLOG system. The reason why this is important is that it is the vacant memory above the system which is used by the "exec" function to load and execute other programs. The amount of memory used is configured into the PROLOG.EXE file and cannot be changed when PROLOG is actually running. The configuration is carried out with PRCONF.EXE and PROLOG.EXE on the same disk. The user runs PRCONF and answers the sole question Enter the amount of workspace required in decimal Kbytes: PRCONF then modifies the PROLOG.EXE file and writes the changes out to disk. This may be done as many times as required and experimentation is beneficial. There are several factors competing for memory and they must be balanced: 1) The amount of workspace must be sufficient to run the prolog programs. The total amount of memory used by PROLOG given by the following formula: size of PROLOG.EXE +size of workspace selected +size of space needed to "exec" other programs, if used +size of "COMMAND.COM" if "exec" is used ---------------------------------------- = Total used by Prolog system. "Total" must be less than the available transient memory. To get this, run the "chkdsk" program, supplied with DOS, and read the last line of the printout. 2) There must be sufficient memory to "exec" the desired programs. 3) Drivers and auxiliary programs installed by the user such as "sidekick" or "superkey" take memory in a preemptive fashion. You may have some difficulty in determining how much free memory there really is. After PRCONF has been run, load PROLOG and note that the copyright notice displays the highest memory location used by PROLOG. If the selected amount of workspace is greater than the memory of the machine allows, PROLOG simply acquires all available memory for its own use. This has only one bad effect - the "exec" function won't function. The Built In Predicate Library Available Operators in PD and ED PROLOG Column 1 gives the function symbol. Column 2 gives the precedence. The range of precedence is 1 to 255. A zero in the precedence column indicates the symbol is parsed as a functor, and precedence is meaningless in this case. Column 3 gives the associativity. A zero in the associativity column indicates the symbol is parsed as a functor, and associativity is meaningless in this case. Column 4 indicates which version the function is available in. Unless otherwise noted, the function is available in all versions. Nonstandard predicates are indicated by "NS". op/pred precedence associativity availability "!" 0 0 "|" 0 0 "=" 40, XFX "==" 40, XFX "\\=" 40, XFX "\\==" 40, XFX "/" 21, YFX "@=" 40, XFX ">=" 40, XFX "=<" 40, XFX ">" 40, XFX "<" 40, XFX "-" 31, YFX "*" 21, YFX "+" 31, YFX "=.." 40, XFX "-->" 255, YFY (not in PD PROLOG) "?-" 255, FY "arg" 0, 0, "asserta" 0, 0, "assertz" 0, 0, "atom" 0, 0, "atomic" 0, 0, "batch" 0, 0 "clause" 0, 0, "clearops" 0, 0, "cls" 0, 0, NS "concat" 0, 0, "consult" 8, FX, "crtgmode" 0, 0, NS "crtset" 0, 0, NS "curset" 0, 0, NS "curwh" 0, 0, NS "debugging 0, 0, "dir" 0, 0, "display" 0, 0, "dotcolor" 0, 0, NS "drawchar" 0, 0, NS "drawdot" 0, 0, NS "drawline" 0, 0, NS "exec" 0, 0, "exitsys" 0, 0, NS "forget" 0, 0, NS "functor" 0, 0, "get0" 8, FX, "integer" 0, 0, "is" 40, XFX, "listing" 0, 0, "memleft" 0, 0, NS "mod" 11, XFX, "name" 0, 0, "nl" 0, 0, "nodebug" 0, 0, (not in PD PROLOG) "nonvar" 0, 0, "nospy" 50, FX, (not in PD PROLOG) "not" 60 FX "notrace" 0, 0, (not in PD PROLOG) "op" 0, 0, "popoff" 0, 0, NS "popoffd" 0, 0, NS "popon" 0, 0, NS "popond" 0, 0, NS "print" 0, 0, "prtscr" 0, 0, NS "put" 0, 0, "ratom" 0, 0, "read" 0, 0, "recon" 0, 0, (Note: this is "reconsult") "repeat" 0, 0, "retract" 0, 0 "rnum" 0, 0, "see" 0, 0, "seeing" 0, 0, "seen" 0, 0, "skip" 0, 0, (not in PD PROLOG) "spy" 50, FX, "tab" 0, 0, "tell" 0, 0, "telling" 0, 0, "told" 0, 0, "trace" 0, 0, (not in PD PROLOG) "true" 0, 0, "unop" 0, 0, "var" 0, 0, "write" 0, 0, Description of the Modifications I/O Redirection I/O redirection is a feature described by Clocksin and Mellish. The predicates "see", "seeing", "seen", "tell", "telling", and "told" are used to select the streams used for input and output. The predicates "seen" and "told" require as arguments the name of the stream that is to be closed. This enables the system to remember the indices of several streams and switch back and forth between them. The predicate "batch", when inserted at the beginning of a stream file, has the following properties: 1) The normal prompt, "?-", and advisory messages do not appear at the screen. 2) It is self cancelling if the input stream is reassigned to the console. 3) It may also be cancelled by the predicate "batch". call( <goal> ) The predicate as defined in C & M is obsolete. The purpose was to permit a goal search where the goal name was a variable instantiated to some functor name. A.D.A. permits writing of goals with such names, so the mediation of the "call" clause is no longer necessary. The "call" predicate may be trivially implemented for compatibility with the PROLOG definition call( X ) :- X. clause The function clause( X, Y ) has the new optional form clause( X, Y, I ). If the third variable is written, it is instantiated to the current address of a clause in memory. The only use of the result is with succeeding assertfa and assertfz statements. debugging "Debugging" prints a list of the current spypoints. After each name a sequence of numbers may appear, indicating the number of arguments that is a condition of the trace. The word "all" appears if the number of arguments is not a condition of the trace. op( <prec>, <assoc>, <functor> ) Defines the user definable grammar of a functor. The definition conforms to that in C & M. We mention here a minor but important point. If <functor> is not a valid PROLOG atom it must be enclosed in single quotes when declared in the "op" declaration. It is not necessary or legal to do this when the functor is actually being used as an operator. In version 1.6, a declared or built-in operator can be used either as an operator or as a functor. For example, +(2,3) = 2 + 3. is a true statement. Declared operators are annotated in the directory display with their precedence and associativity. Output predicates display write print put These functions have been modified to accept multiple arguments in the form: print( <arg1>, <arg2>, <arg3>,... ) Thus, "put( a, b, c )" would result in the display of "abc". The names of some PROLOG atoms that may occur are not accepted by the PROLOG scanner unless surrounded by single quotes. This only applies when such an atom is read in, not when it is internally generated. Nevertheless, this presents us with a problem: We would like to be capable of writing valid PROLOG terms to a file. In some cases, it is necessary to add the single quotes. In other cases, such as human oriented output, they are an irritant. The modified definitions of the following predicates are an attempt at a solution: display Operator precedence is ignored, all functors are printed prefix and single quotes are printed if needed or they were supplied if and when the atom was originally input. write Operator precedence is taken into account and operators are printed according to precedence. Single quotes are printed under the same conditions as for "display." print Operator precedence is taken into account and operators are printed according to precedence. Single quotes are never displayed. This is a human oriented form of output and should never be used for writing of terms for the PROLOG scanner. get0 read The functions "get0" and "read" have been extended to support input from a stream other than the one currently selected by "see". To direct output to a file or other stream, an optional argument is used. For example, "get0( char, <file name> )" or "get0( char, user )" would cause input to come from <file name> or the console. If the file has not already been opened, "get0" will fail. Atoms enclosed by single quotest, eg. '\nthis is a new line' can contain the escape sequences '\n', '\r', '\t' and '\''. If these atoms are printed by "display" or "write" they are printed just as they are. If they are printed by the "print" clause they are translated as follows: '\n' results in the printing of a carriage return and a line feed. '\r' results in the printing of a carriage return only. '\t' results in the printing of a tab character. '\'' allows the printing of a single quote within a quoted atom. The "portray" feature is not presently implemented. Description of the New Predicates batch, nobatch This predicate now makes it possible to avoid the appearance of messages and prompts on the screen when input has been redirected to come from a file. The predicate formerly had a different effect which no longer exists. When placed at the head of a stream file which is "seen" the system prompt is not repetetively displayed as the stream is executed. Advisory messages are not displayed either. The command "batch" is self cancelling if control reverts to the console at any time. It may also be cancelled by invoking the predicate "nobatch." A related change is that when the Prolog system is invoked with a command line argument for a stream, the system prompt is not initially displayed. So if the first command in the stream file is "batch.", it is possible to use i/o redirection without the appearance of any system messages except aor the signon message. clearops Nullify the operator status of every operator in the database. concat( (<variable> | <functor>), <List> ) A list of functors or operators is concatenated into one string, which becomes the name of a new atom to which <variable> or <functor> must match or be instantiated. dir( option ) Provide an alphabetized listing to the console of atoms, constants, or open files. Without options, simply type "dir.<CR>". Options are: dir( p ) - list clause names only. dir( c ) - list consulted files only. Consulted files are prefixed by "S:". exec( arg1, arg2, ... argn ) Execute the program named as arg1, provided that there is sufficient memory. arg2 through argn must be functors which are supplied as the command line argument, separated by one space from neighbors. The PCDOS or MSDOS operating system program "COMMAND.COM" must be reachable by the DOS search path, because using "exec" invokes "COMMAND.COM", which is the DOS shell. You can temporarily leave PROLOG and execute the DOS shell with the invocation "exec( command )." The DOS prompt should appear, and you can run programs bearing in mind that PROLOG is still resident and occupying the bulk of memory. To return to PROLOG, type "exit" at the DOS prompt. This function is restricted in the case of the type PD to executing a program named "prologed", which may be an editor supplied by you. This function will not work properly unless the system is configured for memory. exitsys Exit to the operating system. forget( <file name> ) Make a database unavailable for use and reclaim the storage it occupied. ratom( <arg>, <stream> ) Read an atom from the input stream, to which <arg> matches or is instantiated. <stream> is optional. If <stream> is not given, the input stream defaults to the standar input. Input is terminated by a CR or LF, which are not included in the stream. Arithmetic Capabilities Integer arithmetic is supported. Numbers are 32 bit signed quantities. The following arithmetic operators are supported: "+", "-", "*", "/", <, <=, >, >=, mod. Arithmetic operators must never be used as goals, although they may be part of structures. It is legal to write: X = a + b which results in the instantiation of X to the struture (a + b). But the following is not legal: alpha( X, Y ) :- X + Y, beta( Y ). Evaluation of an arithemtic expression is mediated by the "is" and inequality predicates. For instance, the following would be correct: alpha( X, Y, Z ) :- Z is X + Y. beta( X, Y ) :- X + 2 < Y + 3. Memory Metric Predicate The purpose of this predicate is to give the prolog system a sense of how much memory remains so that expensive search strategies can be controlled. It is not possible to exactly quantify how much memory remains. At the lowest level, there are two types of memory - the stack and the heap. The stack expands down from high memory, while the heap tends to expand at unpredictable intervals upwards. If the stack and heap meet, the prolog system must abort the search and return to the prompt. Judicious use of the memory metric predicates reduces the probability of this happening. The stack is easy to quantify because it expands downwards in a predictable way with recursion. The symbol space is a "heap". For those interested, the structure of the heap is determined by the C compiler under which Prolog was compiled. There is a function internal to Prolog known as the allocator searches the heap for enough contiguous memory to create a new symbol. The heap resembles a piece of Swiss cheese; the holes represent symbols and already allocated memory while the remained is searched by the allocator for a piece of contiguous memory large enough to satisfy a request. If one cannot be found, the uppermost bound of the heap is expanded upwards, and that bound is the number which we measure for an estimate of remaining memory. The sqace between the top of the heap, and the top of the stack, which we call "gap", serves as a rough guide to how much memory remains. The demands of the system are not entirely predictable, however. For example, the creation of a new symbol larger than "gap" would cause an abort. The user must use the numbers supplied by these functions as a heuristic guide, tempered by experience, to minimize the possibility of an unexpected abort. "Gap" is measured in 256 byte pages. memleft( X ) If the argument is an integer, this is satisfied if the size of "gap" is greater than "X". If the argument is a variable, it is instantiated to the amount of "gap" remaining. IBM PC Video Display Predicates A high level method is provided for drawing and displaying on the screen of IBM PC and compatible computers. cls Clear the screen and position the cursor at the upper left hand corner. crtgmode( X ) Matches the argument to the mode byte of the display which is defined as follows: mode meaning 0 40 x 25 BW (default) 1 40 x 25 COLOR 2 80 x 25 BW 3 80 x 25 COLOR 4 320 x 200 COLOR 5 320 x 200 BW 6 640 x 200 BW 7 80 x 25 monochrome display card crtset( X ) This sets the mode of the display. The argument must be one of the modes given above. curset <row>, <column>, <page> ) Sets the cursor to the given row, column, and page. The arguments must be integers. curwh <row>, <column> ) Reports the current position of the cursor. The argument must be an integer or variable. The format is: 1) page zero is assumed. 2) The row is in the range 0 to 79, left to right. 3) The column is in the range 0 to 24, bottom to top. dotcolor( <row>, <column>, <color> ) The argument <color> is matched to the color of the specified dot. The monitor must be in graphics mode. drawchar( <character>, <attribute> ) Put a character at the current cursor position with the specified attribute. The arguments <character> and <attribute> must be integers. Consult the IBM technical reference manual regarding attributes. drawdot( <row>, <column>, <color> ) Put a dot at the specified position. The monitor must be in the graphics mode. The arguments must be integer. The argument <color> is mapped to integers by default in the following manner: drawline( <X1>, <Y1>, <X2>, <Y2>, <color> ) Draw a line on the between the coordinate pairs. The monitor must be in the graphics mode and the arguments are integer. prtscr Print the screen as it currently appears. Be sure that the printer is on line and ready before invoking this predicate, since otherwise, the system may lock up or abort. The integer argument <color> referred to in the above predicates is represented as follows: COLOR PALETTE 0 PALETTE 1 0 background background 1 green cyan 2 red magenta 3 brown white To change the palette and the background, see the IBM Technical Reference Bios listings for more information. The Interrupt Menu (type ED only) This menu has been modified. It was formerly called the ESCAPE menu, but the meaning of the ESCAPE key has been redefined. It is no longer necessary to display the menu to perform one of the menu functions. This reduces the amount of display which is lost by scrolling off the screen. Version 1.9 offers the ability to attempt a goal from the interrupt menu, while not disturbing the ongoing computation. At any time while searching, printing, or accepting keyboard input, you can break to this menu. It is generally not possible to do this during disk access, since control passes to the operating system at this time. Two keys cause this break to occur: ^V: The menu is displayed and a command is accepted at the prompt "INTERRUPT>". After a command, the menu is redisplayed until the user selects a command which causes an exit. ^I: The menu is not displayed. Command is accepted at the prompt "INTERRUPT>" until the user selects a command which causes an exit. ESC: Typing this key causes a termination of the PROLOG search and control returns to the user command level with a prompt of "?-". Notice that previously, the ESC key invoked this menu. As the resulting menu indicates, the following functions are possible: A: Abort the search and return to the prompt. O Open a trace file. The user is prompted for the file name. The file receives all trace output. If a file is already opened it is closed with all output preserved. C Close the trace file if one is open. Otherwise there is no effect. ^C: Immediately exit PROLOG without closing files. This is not advised. ^P: Typing <Control>P toggles the printer. If the printer is on, all input and output will also be routed to the printer. S: If the machine in use is an IBM PC compatible machine, the currently displayed screen will be printed. If the machine is not an IBM PC compatible, do not use this function. T: If trace is in use, most of the trace output can be temporarily turned off by use of this function, which is a toggle. G: Satisfy a goal. The system prompt will appear. Do not be confused into thinking that the system has reverted to the prompt. After the user has entered the goal in the usual manner, satisfaction of the goal is immediately attempted. This need not affect the running computation in any way, although it is certainly possible to do so. After the goal is attempted, control returns to the INTERRUPT menu. The purpose is to facilitate detailed analysis of a running system. R: Entering another ESC causes a return to the current activity (keyboard input or search) with no residual effect from the interruption. Conserving memory space Success popping is controlled by the predicates "popond", "popoffd", "popon", and "popoff". Success popping is means of reclaiming storage which is used on backtracking to reconstruct how a particular goal was satisfied. If it is obvious that there is no alternative solution to a goal this PROLOG system is smart enough to reclaim that storage. In this system, succees popping is an expensive operation. Therefore, there is a tradeoff of memory versus time. On the other hand, discrete use of success popping can actually speed up a program by recreating structures in a more accessible form. The definitions of the control predicates is given in this manual and their use is totally optional. The modulation of success popping has no effect on program logic (read solution.) The "cut" can save substantial time and computational overhead as well as storage. Although the execution of the cut costs time, you can design your program to use cuts in critical places to avoid unnecessary backtracking. Thus the execution speed of the program can actually increase. Anyone who has read Clocksin and Mellish is aware, of course, that the "cut" has a powerful logical impact which is not always desirable. popoff See the below definition. popon The inference engine does complete success popping for goals which appear after "popon". Consider this example: goal :- a, popon, b, c, popoff, d. If no alternative solutions exist for b, then success popping will reclaim storage by removing unnecessary records describing how "b" was satisfied. If the Prolog system cannot rule out possible additional solutions, success popping will never occur, regardless of your use of "popon". Since goal "d" occurs after "popoff", success popping will never occur. popoffd If no "popon" or "popoff" declarations occur in a clause, the default action is determined by "popoffd" and "popond". If "popoffd" has been invoked, the default is that success popping will not occur. popond The inverse of "popoffd". Turns on default success popping. printf( <stream>, <term1>,<term2>,... ) Prolog Tutorial Introduction Probably you have heard of the language PROLOG within the last year or so. You probably wondered the following things: 1) What does the name stand for? Names of computer languages are almost always acronyms. 2) What is it good for? 3) Why now? 4) Can I get a copy to play with? Congratulations! You obviously know the answer to the fourth question. We now respond to the other three. 1) The name stands for "programming in logic." This we shall elaborate on in depth later on. 2) PROLOG is good for writing question answering systems. It is also good for writing programs that perform complicated strategies that compute the best or worst way to accomplish a task, or avoid an undesirable result. 3) PROLOG was virtually unknown in this country until researchers in Japan announced that it was to be the core language of that country's fifth generation computer project. This is the project with which Japan hopes to achieve a domainant position in the world information industry of the 1990's. PROLOG is one of the most unusual computer languages ever invented. It cannot be compared to FORTRAN, PASCAL, "C", or BASIC. The facilities complement, rather than replace those of conventional languages. Although it has great potential for database work, it has nothing in common with the database languages used on microcomputers. Perhaps the best point to make is that while conventional languages are prescriptive, PROLOG is descriptive. A statement in a conventional language might read: if( car_wheels = TRUE ) then begin (some sort of procedure) X = X + 1; end A statment in PROLOG could just be a statment of fact about cars and wheels. There are many relationships that hold. For instance, has( car, wheels ). has( car, quant(wheels, four) ). round( wheels ). Each of these statments is an independent fact relating cars, wheels, and the characteristics of wheels. Because they are independent, they can be put into a PROLOG program by programmers working separately. The man who is a specialist on car bodies can say his thing, the wheel specialist can have his say, and the participants can work with relative independence. And this brings to light a major advantage of PROLOG: PARALLEL PROGRAMMING!!! With conventional programming languages projects can still be "chunked", or divided between programmers. But efficiency on a team project drops drastically below that of the individual programmer wrapped up in his own trance. As the number of participants grows the need for communication grows geometrically. The time spent communicating can exceed that spent programming! Although PROLOG does not eliminate the need for task coordination, the problem is considerably simplified. It also provides the ability to answer questions in a "ready to eat form." Consider your favorite BASIC interpreter. Based upon the statements about cars and wheels previously given, could you ask it the following question? has( car, X ), round( X ). Does a car have anything which is round? The question instructs the PROLOG interpreter to consider all the objects that it knows are possessed by a car and find those which are round. Perhaps you are beginning to guess that PROLOG has the abilities of a smart database searcher. It can not only find the facts but selectively find them and interpret them. Consider the problem of a fault tree, as exemplified by this abbreviated one: {Car won't start} | | [Engine turns over](No) --> [Battery voltage](no)-\ (Yes) v | {Check battery} | [Smell gasoline](yes) --> {Try full throttle cranking} | (failure) /--------/ | (details omitted) The fault tree is easily programmed in BASIC. Later we shall show that PROLOG supplies a superior replacement for the fault tree. Though the tree is capable of diagnosing only the problem for which it was designed, PROLOG dynamically constructs the appropriate tree from facts and rules you have provided. PROLOG is not limited to answering one specific question. Given enough information, it will attempt to find all deductive solutions to any problem. PROLOG PRIMER I. Rules and Facts This is where you should start if you know nothing about PROLOG. Let us consider a simple statment in PROLOG, such as: 1) has( car, wheels ). This statement is a "fact. The word "has" in this statment is known either as a functor or predicate. It is a name for the relationship within the parenthesis. It implies that a car has wheels. But the order of the words inside the bracket is arbitrary and established by you. You could just as easily say: 2) has( wheels, car ). and if you wrote this way consistently, all would be well. The words has, wheels, and car are all PROLOG atoms. "Wheels" and "car" are constants. A database of facts can illustrate the data retrieval capabilities of PROLOG. For instance: 3) has( car, wheels ). has( car, frame ). has( car, windshield ). has( car, engine ). You could then ask PROLOG the question: 4) has( car, Part ). The capital "P" of Part means that Part is a variable. PROLOG will make Part equal to whatever constant is required to make the question match one of the facts in the database. Thus PROLOG will respond: Part = wheels. More?(Y/N): If you type "y" the next answer will appear: Part = frame. More?(Y/N): If you continue, PROLOG will produce the answers Part = windshield and Part = engine. Finally, you will see: More?(Y/N):y No. indicating that PROLOG has exhausted the database. Incidentally, when a variable is set equal to a constant or other variable, it is said to be instantiated to that object. Notice that PROLOG searches the database forwards and in this case, from the beginning. The forward search path is built into PROLOG and cannot be changed. An author of a program written in a prescriptive language is quite conscious of the order of execution of his program, while in PROLOG it is not directly under his control. The other major element is the rule which is a fact which is conditionally true. In logic this is called a Horn clause: 5) has( X, wheels ) :- iscar( X ). The fact iscar( car ) and the above rule are equivalent to 6) has( car, wheels). The symbol :- is the "rule sign." The expression on the left of :-is the "head" and on the right is the body. The variable X has scope of the rule, which means that it has meaning only within the rule. For instance, we could have two rules in the database using identically named variables. 7) has( X, transportation ) :- has( X, car ), has( license, X ). 8) has( X, elephant ) :- istrainer( X ), hasjob( X ). The variables X in the two expressions are completely distinct and have nothing to do with each other. The comma between has( X, car ) and has( license, X ) means "and" or logical conjuction. The rule will not be true unless both the clauses has(X, car) and has( license, X ) are true. On the other hand if there is a rule 9) has( license, X ) :- passedexam( X ). consider what PROLOG will do in response to the question: 10) has( harry, transportation ). (Notice that harry has not been capitalized because we do not want it taken as a variable. We could, however, say 'Harry' enclosed in single quotes.) It will scan the database and use (7), in which X will be instantiated to harry. The rule generates two new questions: 11) has( harry, car ). 12) has( license, harry ). Assuming that harry has a car, the first clause of (7) is satisfied and the database is scanned for a match to (12). PROLOG picks up rule (9) in which X is instantiated to harry and the question is now posed: 13) passedexam( harry ). If there is a fact: passedexam( harry ). in the database then all is well and harry has transportation. If there is not, then PROLOG will succinctly tell you: No. But suppose Harry has money and can hire a chauffer as any good programmer can. That could be made part of the program in the following way. The rule which PROLOG tried to use was: 14) has( X, transportation ) :- has( X, car ), has( license, X ). At any point following it there could be included another rule: 15) has( X, transportation ) :- has( X, money ). or simply the bald fact: 16) has( harry, transportation ). These additional rules or facts would be used in two circumstances. If at any point a rule does not yield a solution, PROLOG will scan forward from that rule to find another applicable one. This process is known as "backtracking search" and can be quite time consuming. If in response to the "More?" prompt you answer "y" PROLOG will search for an additional distinct solution. It will attempt to find an alternate rule or fact for the last rule or fact used. If that fails, it will back up to the antecedent rule and try to find an alternate antecedent. And it will continue to back up until it arrives at the question you asked, at which point it will say: No. "Antecedent" to a rule means that it gave rise to its' use. For example, (7) is the antecedent of (9) in the context of the question (16). II. Grammar It is a boring subject, but it must be discussed. All PROLOG statements are composed of valid terms, possibly a rule sign (":- "), commas representing conjunction ("and"), and a period at the very end. A term is a structure, constant, variable, or number. What is a structure? It is a kind of grouping: 1) Structures consist of a functor, and a set of objects or structures in parenthesis. 2) Objects are constants, variables, numbers, or lists, which we have not discussed yet. 3) A constant or functor must be a string of letters and numbers, beginning with a lower case letter, unless you choose to enclose it in single quotes ( 'howdy pardner' ), in which case you are freed from these restrictions. 4) A variable must be a string of letters and numbers beginning with a capital letter. 5) A functor may optionally have arguments enclosed in parenthesis , as in: hascar( X ) or hascar. An example: "has( X, transportation )." is a structure. III. Input / Output You now know enough to write simple databases and interrogate them profitably. But before we examine more sophisticated examples, it will be necessary to add input and output to the language. There are built in functions which appear as rules which are satisfied once. Thus the statment: write( 'Hello world.' ). can be included on the right side of a rule: greetings( X ) :- ishuman( X ), write( 'Hello world.' ). You can also write "write( X )" where X is some variable. Note that 'Hello world.' is not enclosed in double quotes. Single quotes, which denote a constant, are required. Double quotes would denote a list, which is another thing entirely. Provided that a match to "ishuman" can be found, the builtin function "write" is executed and the message printed to the screen. The builtin read( X ) reads a "structure" that you input from the keyboard. More formally, we have read( <variable> or <constant> ) write( <variable> or <constant> ) If you write read( Input ), then the variable "keyboard" will be assigned to whatever is typed at the keyboard, provided that the input is a valid PROLOG structure. The builtin "read" will fail if instead of Keyboard we wrote read( baloney ), where "baloney" is a constant, and the user at the keyboard did not type exactly "baloney." When you input a structure in response to a "read" statement, be sure to end it with a period and an <ENTER>. There is a convenient way of putting the cursor on a new line. This is the builtin "nl". For example: showme :- write( 'line 1' ), nl, write( 'line 2' ). would result in: line 1 line 2 There is also a primitive form of input/output for single characters. It will be discussed later. IV. A Fault Tree Example Consider the "won't start" fault tree for an automobile: {Car won't start} | | [Engine turns over](No) --> [Battery voltage](no)-\ (Yes) v | {Check battery} | [Smell gasoline](yes) --> {Try full throttle cranking} | (failure) /--------/ | | /------------------------/ | | | | | [Check for fuel line leaks](yes)-->{Replace fuel line} | (no) | | | | | [Check for defective carburator](yes)--\ | (no) v | {Repair carburator} \----\ | | [Is spark present](no)-->[Do points open and close](no)-\ | (yes) v /----/ | {Adjust points} | /------------------------/ | | | [Pull distributor wire, observe spark](blue)--\ | (orange) v | | {Check plug wires & cap} | | | [Measure voltage on coil primary](not 12V)--\ | (12V) v | | {Check wiring, ballast resistor} | | | [Check condenser with ohmmeter](conducts)--\ | (no conduction) v | | {Replace condenser} | | | [Open and close points](voltage not 0 - 12)--\ | (voltage swings 0 - 12) v | | {Fix primary circuit} | | | {Consider hidden fault, swap components] | | \-------{Call a tow truck!!} A PROLOG program to implement this is simple. Each statment represents a decision point fragment of the tree. The PROLOG interpreter dynamically assembles the tree as it attempts a solution. 'car wont start' :- write( 'Is the battery voltage low?' ), affirm, nl, write( 'Check battery' ). 'car wont start' :- write( 'Smell gasoline?' ), affirm, nl, 'fuel system'. 'fuel system' :- write( 'Try full throttle cranking' ). 'fuel system' :- write( 'Are there fuel line leaks?' ), affirm, nl, write( 'Replace fuel line.' ). 'fuel system' :- write( 'Check carburator' ). 'car wont start' :- write( 'Is spark present?' ), not( affirm ), nl, 'no spark'. 'no spark' :- write( 'Do points open and close?' ), not( affirm ), nl, write( 'Adjust or replace points.' ). 'no spark' :- write( 'Is the spark off the coil good?' ), affirm, write( 'Check plug wires and cap.' ). 'no spark' :- write( 'What is the voltage on the primary of the coil: ' ), read( Volts ), Volts < 10, nl, write('Check wiring and ballast resistor.'). 'no spark' :- write( 'Does the capacitor leak?' ), affirm, write( 'Replace the capacitor.' ). 'no spark' :- not( 'primary circuit' ). 'primary circuit' :- write( 'Open the points. Voltage across coil?:'), nl, read( Openvolts ), Openvolts < 1, write( 'Close the points. Voltage across coil?:'), read( Closevolts ), Closevolts > 10, nl, write( 'Primary circuit is OK.' ). 'no spark' :- write( 'Consider a hidden fault. Swap cap, rotor,points,capacitor.' ). 'Car wont start' :- write( 'Get a tow truck!!' ). --End program-- The above is a simple example of an expert system. A sophisticated system would tell you exactly the method by which it has reached a conclusion. It would communicate by a "shell" program written in PROLOG which would accept a wider range of input than the "valid structure" required by the PROLOG interpreter directly. V. Lists Consider a shopping list given you by your wife. It is a piece of paper with items written on it in an order that probably symbolizes their importance. At the top it may say EGGS!!!, followed by carrots, hamburger, and finally a flea collar for the dog, if you can find one. In PROLOG such a list would be written: 1) [eggs, carrots, hamburger, fleacollar] The order of a list is important so that eggs and carrots cannot be reversed and PROLOG be uncaring. Let us put the list in a structure: shopping( [eggs, carrots, hamburger, fleacollar] ). Then if you wished to isolate the head of the list you could ask the question: shopping( [ Mostimportant | Rest ] ). and PROLOG would respond: Mostimportant = eggs, Rest = [carrots, hamburger, fleacollar]. The vertical bar "|" is crucial here. It is the string extraction operator, which performs a combination of the CDR and CAR functions of LISP. When it appears in the context [X|Y] it can separate the head of the list from the rest, or tail. You may have gained the impression that PROLOG is a rather static language capable of answering simple questions, but it is far more powerful than that. The string extraction operator is the key. It permits PROLOG to whittle a complex expression down to the bare remainder. If the rules you have given it permit it to whittle the remainder down to nothing, then success is achieved. An example of this is the definition of "append." Let us suppose you have not yet done yesterday's shopping, let alone today's. You pull it out of your wallet and sootch tape it to the list your wife just gave you. Yesterday's list was: [tomatoes, onions, ketchup] Combined with [eggs, carrots, hamburger, fleacollar] we obtain [eggs,carrots,hamburger,fleacollar,tomatoes,onions,garlic]. To take one list and to attach it to the tail of another list is to "append" the first to the second. The PROLOG definition of append is: Rule1: append( [], L, L ). Rule2: append( [X|List1], List2, [X|List3] ) :- append( List1, List2, List3 ]. The general scheme is this: The definition consists of one rule and one fact. The rule will be used over and over again until what little is left matches the fact. The [] stands for empty list, which is like a bag without anything in it. This is an example of a recursive definition. Suppose we ask: append( [a,b,c], [d,e,f], Whatgives ). 1. Rule 2 is invoked with arguments ( [a,b,c], [d,e,f], Whatgives ). 2. Rule 2 is invoked again with arguments: ( [b,c], [d,e,f], List3 ). 3. Rule 2 is invoked again with arguments: ( [b], [d,e,f], List3 ). 4. The arguments are now ([], [d,e,f], List3 ). Rule 1 now matches. End. How does this cause a list to be constructed? The key is to watch the third argument. Supplied by the user, it is named "Whatgives". The inference engine matches it to [X|List3] in rule 2. Now lets trace this as rule two is successivly invoked: Whatgives | | | v Rule2: [X|List3] (List1 = [b,c]) | \ | \ | \ v \ Rule2: a [X'|List3'] (List1' = [c]) | \ | \ | \ v \ Rule2: b [X''|List3''] (List1'' = [], ie., empty set.) | \ | \ | \ Rule1: c L ( in Rule1 = [d,e,f] ) End. L in rule 1 is [d,e,f] for the following reason: Notice that rule 2 never alters List2. It supplies it to whatever rule it invokes. So L in rule 1 is the original List2, or [a,b,c]. This example would not have worked if the order of rules one and two were reversed. The PROLOG inference engine always attempts to use the the first rule encountered. You could imagine it as always reading your program from the top down in attempting to find an appropriate rule. Since rule 2 would always satisfy, an unpleasant thing would have happened if the order were reversed. The program would loop forever. A Reading List I hope that this tiny introduction to PROLOG whets your appetite. But if you want to go on, there is no one book which I could recommend as a complete reference. This is a reflection on the depth of the subject. If you don't consider yourself very technical, or you're a manager wishing to learn about Prolog, or you simply want as much assistance as possible, a good book is: Introduction to Prolog Bharath, Ramachandran TAB books, 1985 I don't recommend it to those seriously interested, or as a precursor to the more advanced texts. The "standard text", which throws piles of source code fragments at you is: Programming In Prolog W.F. Clocksin and C.S. Mellish Springer - Verlag Berlin,Heidelberg,New York. 1981,1984 It can be purchased directly from Springer Verlag in New York. The authors apparently believe in the Montessori method of instruction. The importance of this book cannot be underrated, however, when you consider that it established a standard, core Prolog dialect simply by virtue of its publication. The first several chapters are eminently readable. When you get bogged down, however, it might be time to switch. If you think that what you want is detailed explanation, we recommend: Prolog for Programmers Kluzniak, Feliks and Szpakowicz, Stanislaw APIC Studies in Data Processing No. 24 Academic Press, 1985 This is a detailed text which rewards careful study. Most likely, you'll decide you didn't want detailed explanation afterall. It has the theoretical foundation for a university course in the subject. Slightly less theoretical and very comprehensive is: The Art of Prolog Sterling and Shapiro MIT Press, 1986 One book presents interesting "AI" applications such as game playing, strategy, and shell systems in considerable detail. You may be interested in seeing "AI" applications such as game playing, strategy, and shell programs in considerable detail. The following book has alot of code that with minor modification can be made to work (watch the range of precedences for operators, which is 0 to 255 in C & M Prolog). Prolog Programming for Artificial Intelligence Ivan Bratko Addison-Wesley, 1986 Bratko's book can take one from the very fundamentals to rather advanced data structures. However, as an introduction, it may be too fast paced, requiring supplementation. If one purchased all of the books, one would undoubtedly find reward in switching from one to the other, as the need arose. The state of the art is well characterized by the following texts: Logic Programming Edited by K.L. Clark and S.-A. Tarnlund Academic Press, 1982 Academic papers describing shortfalls and extensions to the language. A potpourri. Logic Programming and Its Applications Edited by Michel Van Caneghem and David H. D. Warren Ablex, 1986 Inspirational short-takes of state of the art applications. Implementations of Prolog Edited by J.A. Campbell Ellis Horwood, 1984 Chronicles all the mistakes and successes of those trying to produce a nice implementation. Leaves out the Warren machine, which is a pity, however, since it is the father of all other compilation techniques. The above three texts are difficult, but ideal if you want to get into research. A purely mathematical treatise, I recommend this text for the pure mathematician: Foundations of Logic Programming J.W. Lloyd Springer - Verlag 1984 It is a proof-theoretic derivation of the automatic theorem prover used by PROLOG. Index A Reference of Note, 18 Append, 47 Arithmetic Capabilities, 28 Batch, 22, 26 Call, 22 Chkdsk, 12 Clause, 22 Clearops, 26 Clocksin and Mellish, 12 Cls, 30 Co-resident program, 12 Command Line Arguments, 17 Concat, 26 Conserving memory space, 34 Crtgmode, 30 Crtset, 30 Curset, 30 Curwh, 30 Cut, 34 Cyclic Unifier, 7 Debugging, 23 Definite clause grammer, 13 Description of the Modifications, 22 Dir, 26 Disk buffers, 12 Display, 24 Dos driver, 12 Dotcolor, 31 Drawchar, 31 Drawdot, 31 Drawline, 31 Educational PROLOG, 5 Exec, 27 Exitsys, 27 Forget, 27 FS PROLOG, 5 Get0, 25 Grrules, 13 I/O Redirection, 22 IBM PC Video Display Predicates, 30 Input / Output, 43 Installation, 12 LISP PD LISP, UNX LISP, 4 Lists, 47 Megabyte INTEL limit, 12 Memleft( X ), 29 Memory Metric Predicate, 29 Memory Size Configuration, 19 Memory used by PROLOG, 19 Nested goals, 33 New Predicates, 26 Nobatch, 26 Op, 23 Output predicates, 24 Popoff, 34 Popoffd, 34, 35 Popon, 34 Popond, 34, 35 PRCONF.EXE, 19 Prime Number Generator, 13 Print, 24 Print screen, 33 Printer toggle, 33 Programming In PROLOG, 5, 12 Prolog Runtime Systems, 10 PROLOG Systems, 5 Prolog Tutorial, 36 PROLOG Versions, 5 Prtscr, 31 Public Domain PROLOG, 5 Put, 24 Ramdisk, 12 Ratom, 27 Read, 25 Reclaim the storage, 27 Reconsult, 17 Reference, 49 Returns, 11 Running the Interpreter, 16 Short of memory, 12 Sieve, 13 Special Offer, 14 Springer Verlag, 12 Succees popping, 34 Technical Information, 11 The Built In Predicate Library, 20 The Interrupt Menu, 32 Trace file, 33 Trace toggle, 33 Upgrade Policy, 11 VMA PROLOG Large model Virtual Memory System, 9 VMI PROLOG, 6 VML PROLOG, 7 Write, 24