IMPLEMENTING PASCAL-LIKE CONSTRUCTS: 
AN EXERCISE IN PROLONG PROGRAMMING 
INFORMATICA 2/87 
UDK 681.3.06:519.682 PROLOG 
Bogdan Filipič 
Department of Computer Science and Informatics 
»Jožef Štefan« Institute, Ljubljana 
ABSTRACT. Due to tho declarativo meaning ,of programa. 
Prolog is a poworful programming language. Howevor, in 
practice it turna out that numerous tasks within a oertain 
kind of programs aro quite prooedural. The paper describes 
a simple implementation of some Pascal-like construots in 
Prolog as a practical solution to this problem. 
IMPLEMENTACIJA PASCALSKIH KONSTBUKTOV KOT VAJA IZ PBOGBA-
HIKAMJA V PKOL060. Prolog je zaradi deklarativnega pomena 
programov močan programski jezik, vendar se v praksi izkaSe, 
da so mnoga opravila znotraj določenih programov povsem 
proceduralnega znaOaja. V članku je kot praktična reSitev 
tega problema prikazana implementacija nekaterih pasoalskih 
konstruktov v Prologu, 
INTRODUCTION IF-THEH AND IF-THEH-ELSE PROCEDURES 
Due to the declarative meaning of programs, 
Prolog is a poverful programming language 
[1,3]. It is especially well suited for 
solving non-numerical problems. From the 
programmer's point of view, programming in 
Prolog is very effiolont. On the other hand. 
Prolog implementations suffer from the lack of 
supporting the prooedural programming 
approach. In practice it namely turns out 
that numerous tasks within a certain kind of 
programs are quite prooedural. One may, for 
example, wish to perform an aotion 
conditionally, repeat a procedure until a 
certain oondition is satisfied or execute a 
seguence of actions a given number of times. 
To do this in Prolog, we usually use outs and 
repeat-fail loops. Unfortunatelly, improving 
efficienoy through these inechanisms often 
weakens the clarity and the conciseness of a 
Prolog program. 
Some Prolog implementations support condi-
tional by default. Remember, for example, 
P -> Q t R from 0ECsystem-lO Prolog [2], 
C-Prolog [5] or Suintus Prolog [6]. Arity 
Prolog [7] even supports ifthen/2 and 
iftheneIse/3 predicates. Here are common 
definitions of these procedures: 
i-f_then(P,Q) 
if_then(P,Q). 
calKP), .', calKO). 
if_then_else(P,Q,R) t- calI(P>, 
if_then_else(P,Q,R) s-calKR). 
calKO). 
Using operators, we may simply define an 
appropriate predicate if/1 that is determined 
as a Principal funotor. Its definition 
includes both above alternativos (see 
implementation in the appendix). We may now 
code conditionals of both forms; 
The problem can be solved neater by using 
explicit definitions for the oontrol of 
execution. Including Pascal-like construots 
into Prolog may be seen as a practical 
solution to this problem. 
2f P then Q. 
if P then O eise R. 
where P, Q and R denote single Prolog goals or 
sequences (i.e. conjunctions or disjunctions) 
of goals. 
V=. 

28 
CAS£ PROCEDURE 
REPEAT-umiL PROCEDURE 
Onoe the if predicate has been implemented, we 
may define čase as a seguenoe of if procedures 
trying to matoh a certain condition value and 
to sati8fy related goal(s), We introduce the 
Prolog čase construct 
čase X o-f C XlsQl, X7sQ2, ..., XnsQn J. 
that is interpreted as 
if X = XI then Ql 
else if X = X2 then Q2 
else if 
Here we introduce the repeat/l predicate that 
will have similar effect as the built-in 
repeat/0. You must keep in mind that proposed 
repeat O until P. 
is just a Pascal-like notation for the 
procedure that will be executed in the Prolog 
sense, i.e. failure of O will result in 
backtracking. The execution may be viewed as 
Pascal-liko when backtracking is not possible, 
for exainple: 
repeat ( urite< 'Filename? '), read(F) ) 
until exists<FJ. 
else if X " Xn then On. 
and 
čase X of 
C XJsQl, X2:Q2, , Xnj<3n otheruise R J. 
that stands for 
Note that, when O stands for a sequence of 
goals, they must be put into brackets and the 
left braoket must be separatod from the 
Principal functor repeat by blank. Otherwise 
the entire expression denotes a predicate of 
an arity greater than 1 whioh will fail, of 
course. Similarly, this syntax restriction 
must be considered when using if. 
if X = XI then Ql 
else if X = X2 then 02 
else if 
FOK PROCEDURE 
e7$e if X 
else R. 
Xn then Qn 
Note that our implementation does not support 
more than one value being assigned vrith each 
alternative. This can be done by Joining 
possible values into a list and substltuting 
condition X = Xi with testing the list 
membership. The following is an example of 
using the čase construct: 
aanu_seIection s-
nrite( 'Selection? ' }, 
read_line( Ansuer ), 
čase Ansuer of 
C a s opt ioni ( ... }, 
b I option2{ ... }, 
C s option3( ,-, }, 
h s ( help, aenu_selection), 
X s exit_menu 
otheruise 
( teepf 
Nritef 'Illegal ansuer' ), nI, 
mBnu_selection 
) 
J. 
J 
read^line( AnsNer ) ;— 
get< ASCII >, 
name( Ansuer, CASCIIJ ). 
In order to implement the far loop, let us 
first introduce a utility for managing global 
counters [4]. Suppose we have certain valuee 
in our program that can be passed to or 
modified by any part of the program. 
Modifying these values may be understood as 
assigning values to global variables in 
procedural languages. Suoh variables are 
particularly suitable as counters. Each 
counter Is spooified by its name, a key, and 
the related integer value. The counter 
managing predicates below siiBply use retract 
and assert to assure the current value of a 
counter to be recorded in the database. 
Based on previously defined repeat procedure 
and the global counters, the for loop has the 
following two forms: 
for CCount,IJ 
for CCoant,11 
II to 12 do a. 
II donnto 12 do Q. 
beep s- put<7>. 
Count identifies the procedure and should be 
instantiated to a Prolog constant. Further-
more, the global counter with the key Count Is 
activated when executing the procedure. Its 
current value is instantiated to I. The 
following example illustrates hov to use the 
loop: 
for Ci,l3 f i to 10 do 
( nrite(l), nI ). 

29 
figain, the exeoution is Pascal-like only when 
the goal(s) appearinfl in tho loop can be 
satisfied in no more than ono way. 
REFERENCES 
[1] Bratko I.: Prolog Programming for Arti-
fioial'Intelligence, Addison-Wesley, 1986 
CONCLUSION 
The papar presentB the implementation of some 
Pascal-like constructs in Prolog. We found 
them useful for vriting procedural segments of 
programs, such as input and output procedures. 
As already stated, our purpose is not to 
reduoe the Importanoo of standard Prolog 
concepts, but just to add some convenient 
procedural foaturos. In our opinion, 
combination of both declarative and procedural 
approaches is the right solution • when 
vondering about hov to code a oomplex task 
offeotively. 
And finally, the, reador might have noticod 
that we said nothing about the repeat 
procedure when discussing Pascal-like 
featuros. The reader is invited to imploment 
it himself as an exercise. 
[2] Byrd L., Pereira F., Harren D.: A Guide 
to Version 3 of DEC-10 Prolog, Department 
of Artificial Intelligence, University of 
Edinburgh, 1883 
[3] Clocksin H.F., Mellish C.S.: Programming 
in Prolog, Springer-Verlag, 1984 
[4] Filipie B., Mozetie I.: A Library of 
Prolog Utilities, Keport IJS DP-4466, 
JoZef Štefan Institute, Ljubljana, 1986 
[5] Pereira F.: C-Prolog Oser's Manual, 
Univorsity of Edinburgh, Department of 
Computer Aided Architectural Design, 1984 
[6] Quintus Prolog User's Guide and Reference 
Manual, Quintus Computer Systems Inc., 
Palo Alto, 1986 
[7] The Arity/Prclog Programming Languago, 
Arity Corporation, Conoord, 1986 
APPENDIX: THE PROGRAM 
% file PROCED s 
% Implementing Pascal-like constructs 
X , 
.-- op( 900, fx, if ). 
}- op( 850,. xfx, then ). 
s- op< 800, xfx, else ). 
t- op( 900, fx, čase >. 
t- op( 850, xfx, of J. 
r- op<'800, xfx, otherMJse ), 
s- op( 750, xfx, 's' >. 
s- op( 900, fx, repeat ). ; 
»- op( 850, xix, until ). 
s- op( 900, fx, -for ). 
z- op( 850, xfx', to ). 
i— op( 850, x-fx, donnto ). 
s- op( 800, xfx, i3o }. 
s— op( 750, x1x, 'f' }. 
CcountsJ, X Counter managemertt. 
i1 P then Q :~ 
if_then_else( 
Q - (R else S), 
if_ttien_else(P,R,S) , 
if_then(P,Q) 
) . 
% 11 'else' found 
Z then if_then_else 
Z else if then. 
if_then<P,Q) s- calKP), .', calKO). 
if_then(^,_>. 
if_then_else(P,Q,_> s- calKP), .', calKO). 
if_then_else(_,_,R) s- calKR). 
čase X of C XntO othertiise R J s-
if X=Xn then Q else R, .'. 
čase X of C XniQ ] s-
<if X=Xn then O, /. 
čase X of C XisQ ! Others J i-
if X=Xi then Q else čase X of Others. 
repeat Q until P s-
repeat, calK <Q, .') ), call(P). 
for C_,_Js'Il to 12 do _ :-
ll>12, .'. 
for CCount,I]s=Il to 12 do Q :-
ctr _set(Count,11), 
repeat C ctr_inc<Count,1), Q > 
until 1=12, 
ctr_remove(Count), /. 
for C_,_Jt=ll doNnto 12 do _ s~ 
IK12, /. 
for CCount,IJs = Il donnto 12 do Q i-
ctr_set(Count,Il>, 
repeat ( ctr_dec(Count,1}, Q ) 
until 1=12, 
ctr_reaove(Count>, /. 
O 

30 
2 File COUHTS i 
Z Hanaging global countsrs 
% 
Z ctr_set/2 sets a counter to the 
X desired nuaber. 
Z ctr_dec/2 decrements a counter ond 
Z roturns its pravious value. 
ctr_dec(KeY,a} i-
retract( countor(KBy,H} ), 
m is H - 1, 
assertC counter(Koy,HI J ), /. 
ctr_set(Hey,h> s-
integar (M) , 
ctr_raoovo(Key)p 
assort( counter (Koy,H) ), .'. 
Z ctr_inc/2 increaents a counter and 
Z returns its pravious valuo. 
ctr_inc(KBy,N) M-
ratractf countor(Koy,M} ), 
Ml is » * 1, 
assartf countor(KaY^MI> )f f> 
% ctr_is/2 returns the current vaiua 
Z of a counter. 
ctr_is(Key,H) t-
counter (Key,ti) , /. 
Z ctr_remova/t resovoa a counter 
Z froE^ tho databaso, 
ctr _rBaove(Key) t — 
retract( counter(Key,_) ), fail. 
ctr reBove( > .