Turbo Prolog

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Use of Fail Turbo Prolog contains a standard predicate that forces backtracking-fail. The effect of fail corresponds to the effect of 2=3. We'll use Program 15 to illustrate the use of this predicate. 1* Program 15 *1 domains name = symbol predicates father(name,name) everybody clauses father(leonard,katherine). father(carl,jason). father(carl,marilyn). everybody if father(X,y) and write(X,1I is II,Y,II's father\n ll ) and fail. The goal father(X, Y) could be used in two different situations: • As an inquiry to the Turbo Prolog system (an external goaO • On the right side of a rule (an internal goaO, as in: grandfather(X,B) if father(X,y) and father(Y,B). With father(X, Y) as an external goal, Turbo Prolog will write out all possible solutions in the usual way: X= .... , Y= ... . X= .... , Y= ... . .... solutions With father(X, Y) as an internal goal, Turbo Prolog will continue with the next subgoal once it has been satisfied and will display only one solution. However, the predicate everybody in Program 15 uses the fail predicate to disturb the usual mechanism. The object of the predicate everybody is to produce neater responses from program runs. Compare the answers to the two goals Goal: father_to(X,y) X=leonard, Y = katherine X=carl, Y=jason X=carl, Y=marilyn 3 solutions Goal: everybody leonard is katherine's father carl is jason's father carl is marilyn's father No solution The predicate everybody makes use of backtracking to generate more solutions for father(X, Y) by trying to satisfy the right side of everybody: father(X,Y) and write(X,1I is lI,y,llIs father\n ll ) and fail. Tutorial III: Turbo Prolog's Relentless Search for Solutions 57
These subgoals must be satisfied from left to right. The first father(X,y) can be satisfied with X = leonard and Y = katherine, so that Turbo Prolog continues to the next subgoal, the standard predicate write. It fulfills its task by writing some values, and then continues to the last subgoal, the standard predicate fail. Fail can never be satisfied, so Turbo Prolog is forced to backtrack. write cannot be resatisfied (offer new solutions), so Turbo Prolog must backtrack again to the first subgoal. A new solution, namely X = carl and Y = sam, is found. Turbo Prolog can now continue to the next subgoal, where the values are written out, and finally reaches the last subgoal-fail-which once again initiates backtracking, and so on. Exercise Type in Program 14 and evaluate the following goals: father(X,y). and everybody. Why are the solutions to everybody terminated by False? For a clue, append: everybody as a second clause to the definition of predicate everybody and reevaluate the goal. PREVENTING BACKTRACKING: THE CUT ELEMENT Turbo Prolog contains an element that prevents backtracking under certain circumstances. The element is called the cut and is written as an exclamation mark ( !). Its effect is simple: It is impossible to backtrack past a cut There are two main uses of the cut: • When you know in advance that certain possibilities will never give rise to meaningful solutions, so it is a waste of time and storage space to backtrack over them. By using a cut in this situation, the resulting program will run quicker and use less memory. • When the logic of a program demands the cut. In the following examples, we will use several schematic Turbo Prolog rules rl, r2, r3 which all describe the same predicate r, plus several subgoals a,b,c, etc. Using the Cut to Prevent Backtracking to a Previous Subgoal in a Rule r1 if a and band ! and c. This is a way of telling Turbo Prolog that we are satisfied with the first solution of subgoals a and b. As a concrete example, consider Program 16. It is based on the idea that two people might like one another if they have at least one interest in common. 58 Turbo Prolog Owner's Handbook
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Turbo Prolog Owner's Handbook Copyr
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Chapter 4 Tutorial II: A Closer Loo
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Dynamic Databases in Turbo Prolog D
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fail field_attr field_str filepos f
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I 1-2 Sample Diagnostic Display ...
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The tutorials cover all aspects of
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Prolog is a full-fledged compiler w
Page 17 and 18: Turbo Prolog has a built-in pattern
Page 19: In addition to the version of Turbo
Page 27 and 28: Saving a Window Layout Select Save
Page 29 and 30: The clauses section contains a coll
Page 31: Objects and Relations In Turbo Prol
Page 35: Since Person I and Person2 are both
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Page 45: 1* Program b *1 domains person, hob
Page 48: Program 7 is a complete Turbo Prolo
Page 57: To declare a domain for lists of in
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Page 66 and 67: The clause Turbo Prolog chooses to
Page 72 and 73: Exercise Suppose an average taxpaye
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Page 80: and A= -0.756802 tes t2 ( L; , B) .
Page 85: DEBUGGING AND TRACING Before a prog
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Page 90 and 91: makewindow( ... ) shiftwindow( ...
Page 92 and 93: You can change the attribute of the
Page 97: A WORD GUESSING GAME USING WINDOWS
Page 100 and 101: goal 1* Program 32 *1 makewindow(1,
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Page 106: 1 Program 37 1/ predicates inspiral
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Page 111: To access a file, it must first be
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Program 50 uses random to select th
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110 1* Program 51*1 predicates anim
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The other rule for positive will as
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Try the program with the goal route
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track as soon as one solution has b
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* Program 55 */ domains loc = right
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string word ("" , [ 1 ) : -! . stri
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Program 59 makes use of the readkey
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11 Programmer's Guide This chapter
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Reserved Names The following are re
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2. mylist = elementDom* This is a c
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symbol (\) followed by an ASCII cod
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For example, given trace domains li
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save("mydata.dba") the file mydata.
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domains file name, address age sex
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Tail recursion elimination is a tec
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Determinism, Non-determinism and Ho
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• The DOS linker, LlNK.EXE, must
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Compiling and Linking the Modules B
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so that the data segment addressed
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clauses dosver(VERSION): AX=L;8*2
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The Options Menu The Options pull-d
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Save Configuration S saves the curr
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ARITHMETIC FUNCTIONS AND PREDICATES
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• list of the possible flow patte
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date(Year, Month, Day ) (integer,in
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contained inside that window, the v
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less than or greater than zero, a b
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eadchar( CharVariable) (char) : (0)
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storage(StackSize,HeapSize, TrailSi
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f - Reals in fixed decimal notation
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Predicate and Database Section
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SYSTEM liMITS «arithmet
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ASCII Character Set (continued) Spe
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B Error Messages Following is a com
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600 Too many domain names 601 Too m
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C PLINK USE OF THE FILE PLINK.BAT P
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D PROWG.SYS The table below describ
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E Using Turbo Prolog with Turbo Pas
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F Glossary anonymous variable The v
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list A special sort of object consi
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Index A Anonymous Variable, 22-23 A
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p Pascal procedures. calling from T
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SlnBiCI The Organizer For The Compu
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Learn Pascal From The Folks Who Cre
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2VRBO PASCAt EDJ2OR7tDr.BOX It's Al
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NOTES
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NOTES
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Turbo Prolog

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