ISO/IEC 13211-1:1995

INTERNATIONAL
ISO/IEC
STANDARD
13211-1
First edition
1995-06-01
Information technology - Programming
languages - Prolog -
Part 1:
General core
Technologies de I’informa tion
- Langages de programmation -
Prolog -
Partie 1: Noyau g&Gral

---------------------- Page: 1 ----------------------
ISO/IEC 13211-1 : 1995(E)
Page
Contents
. . .
~111
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1 Scope .
1
..........................................
1.1 Notes
1
2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.............................. 10
4 Symbols and abbreviations
........................................ 10
4.1 Notation
....................... 10
4.1.1 Basic mathematical types
................... 10
4.1.2 Mathematical and set Operators
.............................. 10
4.1.3 Other functions
............................ 11
Abstract data type: Stack
4.2
.......................... 11
4.3 Abstract data type: mapping
......................................... 11
5 Compliance
.................................. 11
5.1 Prolog processor
...................................... 12
5.2 Prolog text
...................................... 12
Prolog goal
5.3
12
...................................
5.4 Documentation.
....................................... 12
5.5 Extensions
.................................... 12
5.5.1 Syntax
.......................... 12
5.5.2 Predefined Operators
.................... 12
5.5.3 Character-conversion mapping
.................................... 12
5.5.4 Types.
.................................. 13
5.5.5 Directives
................................ 13
5.5.6 Side effects
............................ 13
5.5.7 Control constructs
..................................... 13
5.5.8 Flags
............................ 13
5.5.9 Built-in predicates
@ ISO/IEC 1995
Unless otherwise specified, no part of this publication may be
All rights reserved.
reproduced or utilized in any form or by any means, electronie or mechanical, including
photocopying and microfilm, without Permission in writing from the publisher.
ISO/IEC Copyright Office l Case Postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland
ii

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ISO/IEC 132114 : 1995(E)
@ ISO/IEC 1995
5.510 Evaluable functors . 13
55.11 Reserved atoms. . 13
13
6 Syntax .
........................................ 13
6.1 Notation
........................... 13
6.1.1 Backus Naur Form
.......................... 14
6.1.2 Abstract term Syntax
............................... 15
6.2 Prolog text and data.
................................. 15
6.2.1 Prolog text
................................ 15
6.2.2 Prolog data.
.......................................... 15
6.3 Terms
............................... 16
6.3.1 Atomic terms
..................................
6.3.2 Variables 16
6.3.3 Compound terms - functional notation . 16
17
6.3.4 Compound terms - Operator notation .
6.3.5 Compound terms - list notation . 19
............ 20
6.3.6 Compound terms - curly bracketed term
............... 20
6.3.7 Terms - double quoted list notation
......................................... 20
6.4 Tokens
................................ 21
6.4.1 Layout text.
.................................... 21
6.4.2 Names
.................................. 23
6.4.3 Variables
............................. 23
6.4.4 Integer numbers
6.4.5 Floating Point numbers . 23
........................... 24
6.4.6 Double quoted lists
........................... 24
6.4.7 Back quoted strings
................................ 24
6.4.8 Other tokens
.............................. 24
6.5 Processor Character set
............................ 25
6.5.1 Graphit characters
6.5.2 Alphanumeric characters . 25
6.5.3 Solo characters . 25
6.5.4 Layout characters . 25
6.5.5 Meta characters . 26
6.6 Collating sequence . 26
......................... 26
7 Language concepts and semantics
.......................................... 27
7.1 Types
................................... 27
7.1.1 Variable
.................................... 27
7.1.2 Integer
............................... 28
7.1.3 Floating Point
7.1.4 Atom . 29
7.1.5 Compound term . 29
7.1.6 Related terms . 29
...................................... 30
7.2 Term Order
7.2.1 Variable . 31
...............................
7.2.2 Floating Point 31
.................................... 31
7.2.3 Integer
..................................... 31
7.2.4 Atom
7.2.5 Compound . 31
......................................
7.3 Unification 31
..................... 31
7.3.1 The mathematical definition
...........................
7.3.2 Herbrand algorithm 31
7.3.3 Subject to occurs-check (STO) and not subject to occurs-
33
check (NSTO) .
.................... 33
7.3.4 Normal unification in Prolog
...................................... 33
7.4 Prolog text
7.4.1 Undefined features . 34
. . .
111

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@ ISO/IEC 1995
ISO/IEC 13211-1 : 1995(E)
34
..................................
7.4.2 Directives
................................... 35
7.4.3 Clauses
........................................ 36
7.5 Database
.............. 36
7.5.1 Preparing a Prolog text for execution
36
...................
7.5.2 Static and dynamic procedures
36
....................
Private and public procedures
7.5.3
37
......................
A logical database update
7.5.4
........ 37
7.6 Converting a term to a clause, and a clause to a term
........... 37
7.6.1 Converting a term to the head of a clause
.......... 37
7.6.2 Converting a term to the body of a clause.
........... 37
7.6.3 Converting the head of a clause to a term
.......... 38
7.6.4 Converting the body of a clause to a term.
............................. 38
7.7 Executing a Prolog goal
.................................. 38
7.7.1 Execution
............... 38
7.7.2 Data types for the execution model-
................................ 39
7.7.3 Initialization
............................. 39
7.7.4 A goal succeeds
................................ 39
7.7.5 A goal fails
........................... 39
7.7.6 Re-executing a goal
.................. 40
7.7.7 Selecting a clause for execution
................................ 40
7.7.8 Backtracking
................................ 40
7.7.9 Side effects
................ 40
7.7.10 Executing a user-defined procedure
42
7.7.11 Executing a user-defined procedure with no more clauses
.................... 42
7.7.12 Executing a built-in predicate
................................. 43
7.8 Control constructs
.................................... 43
7.8.1 truel0.
..................................... 43
7.8.2 fail/O
.....................................
44
7.8.3 callll
45
7.8.4 !/O - tut .
47
..........................
7.8.5 (‘,‘)/2 - conjunction
............................ 47
(;)/2 - disjunction
7.8.6
.............................. 49
(->)/2 - if-then
7.8.7
............................ 50
7.8.8 (;)/2 - if-then-else
.................................... 51
7.8.9 catch/3
................................... 53
7.8.10 throwfl
54
.............................
7.9 Evaluating an expression
................................. 54
7.9.1 Description
.................................... 54
7.9.2 Errors
...................................... 54
7.10 Input/output
............................ 54
7.10.1 Sources and sinks
................................... 55
7.10.2 Streams
............................. 58
7.10.3 Read-Options list
............................ 58
7.10.4 Write-Options list
.............................. 59
7.10.5 Writing a term
.......................................... 60
7.11 Flags
.................... 60
7.11.1 Flags defining integer type 1
................................. 61
7.11.2 Other flags
.......................................... 61
7.12 Errors
......................... 62
7.12.1 The effect of an error
........................... 62
7.12.2 Error classification
.................................... 63
8 Built-in predicates
................
63
8.1 The format of built-in predicate definitions
................................. 63
8.1.1 Description
.......................... 64
S.l.2 Template and modes
.................................... 64
8.1.3 Errors
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@ ISO/IEC 1995 ISO/IEC 132114 : 1995(E)
.................................. 65
8.1.4 Examples
65
8.1.5 Bootstrapped built-in predicates . , , , , , . , , , . , , , , , , ,
................................... 65
8.2 Term unification
.......................... 65
8.2.1 (=)/2 - Prolog unify
66
8.2.2 unify_with_occurs_check/2 - unify . . , , , , , , , , , , , , , . .
8.2.3 (\=)/2 - . 67
not Prolog unifiable
..................................... 67
8.3 Type testing
..................................... 67
8.3.1 var/l
.................................... 68
8.3.2 atom/l
................................... 68
8.3.3 integer/1
.................................... 68
8.3.4 floatll
................................... 68
8.3.5 atomic/l
................................ 69
8.3.6 compound/l
................................... 69
8.3.7 nonvar/l
.................................. 69
8.3.8 number/l
.................................. 70
8.4 Term comparison
8.4.1 (@=<)/2 - term less than or equal, (==)/2 - term identical,
(\==)/2 - term not identical, (@<)/2 - term less than,
(@>)/2 - term greater than, (@>=)/2 - term greater than
or equal . 70
....................... 71
8.5 Term creation and decomposition
.................................. 71
8.5.1 functor/3
8.5.2 arg/3 . 72
8.5.3 (=.)/ 2 - univ . 72
8.5.4 copy-term/2 . 73
............................... 74
8.6 Arithmetic evaluation
8.6.1 (is)/2 - evaluate expression . 74
8.7 Arithmetic comparison . 74
8.7.1 (=:=)/2 - arithmetic equal, (=\=)/2 - arithmetic not equal,
(<)/2 - arithmetic less than, (=<)/2 - arithmetic less
than or equal, (>)/2 - arithmetic greater than, (>=)/2 -
arithmetic greater than or equal . 76
8.8 Clause retrieval and information . 77
................................... 77
8.8.1 clause/2
............................ 78
8.8.2 current-predicate/l
........................ 78
8.9 Clause creation and destruction
................................... 78
8.9.1 asserta/l
................................... 79
8.9.2 assertz/l
................................... 80
8.9.3 retract/l
.................................. 81
8.9.4 abolishll
..................................... 82
8.10 All solutions
................................... 82
8.10.1 findall/
................................... 83
8.10.2 bagof/3
.................................... 84
8.10.3 setof/3
.......................... 86
8.11 Stream selection and control
8.11.1 currenL.input/l . 86
8.11.2 current-output/1 . 86
................................. 87
8.11.3 set-input/l
................................ 87
8.11.4 set-output/1
8.11.5 open/4, open/3 . 87
8.11.6 close/2, close/l . 88
.................... 89
8.11.7 flush-output/l, flush-output/0
8.11.8 stream_property/2, at-end-ofstream/O, at-end-of-strearn/l . 89
.......................... 90
8.11.9 set_stream_position/2
.............................. 9 1
8.12 Character input/output
8.12.1 get_char/2, getcharll , get-Code/1 , get-Code/2 . 9 1
8.12.2 peek_char/2, peek-charll , peek-codell , peekcodel2 . 92
V

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@ ISO/IEC 1995
ISO/IEC 13211-1 : 1995(E)
94
8.12.3 put-charl2, put-charll, put-codell, putcodel2, nl/O, nlll ,
.................................. 95
8.13 Byte inputloutput
.......................... 95
8.13.1 get-bytel2, get-bytell
....................... 96
8.13.2 peek-bytel2, peek-bytell
......................... 97
8.13.3 put-bytel2, put-bytell
.................................. 98
8.14 Term inputloutput
............. 98
8.14.1 read_term/3, read_term/2, readll, readl2
8.14.2 write_term/3, write_term/2, writell, writel2, writeqll,
......... 99
writeql2, writecanonicalll, write-canonicall2
...................................... 101
8.14.3 op/3
................................ 102
8.14.4 current-op/3
............................ 103
8.14.5 char-conversionl2
....................... 103
8.14.6 currentcharconversionl2
................................. 104
8.15 Logic and control
......................... 104
8.15.1 (\+)/l - not provable
.................................... 105
8.15.2 oncell
................................... 105
8.15.3 repeat/O
............................. 105
8.16 Atomic term processing
............................... 106
8.16.1 atomJengthl2
............................... 106
8.16.2 atomconcatl3
................................. 107
8.16.3 sub-atom/5
................................
108
8.16.4 atomcharsl2
............................... 109
8.16.5 atomcodes
................................. 109
8.16.6 char-codel2
.............................. 110
8.16.7 numbercharsl2
.............................. 111
8.16.8 numbercodesl2
......................... 112
8.17 Implementation defined hooks
............................. 112
8.17.1 set-prologflagl2
.......................... 112
8.17.2 current-prologflagl2
..................................... 113
8.17.3 halt/0
..................................... 113
8.17.4 halt/1
.................................... 114
9 Evaluable functors
........................ 114
9.1 The simple arithmetic functors
................. 114
9.1.1 Evaluable functors and operations
............................ 114
9.1.2 Exceptional values
................... 114
9.1.3 Integer operations and axioms
115
..............
9.1.4 Floating Point operations and axioms
............... 116
9.1.5 Mixed mode operations and axioms
.....................
117
9.1.6 Type conversion operations
.................................. 117
9.1.7 Examples
........... 119
9.2 The format of other evaluable functor definitions
.................................
119
9.2.1 Description
..........................
119
9.2.2 Template and modes
....................................
119
9.2.3 Errors
..................................
119
9.2.4 Examples
............................ 119
9.3 Other arithmetic functors
.............................. 119
9.3.1 (**)/2 - power
.....................................
120
9.3.2 sin/1
..................................... 120
9.3.3 cos/1
....................................
120
9.3.4 atanll
..................................... 121
9.3.5 expll
..................................... 121
9.3.6 log/1
.....................................
122
9.3.7 sqrtll
9.4 Bitwise functors . , , , , , , , , , , , , , , . , , . , , , , , , , , , , , , , , . . 122
......................
122
9.4.1 (>>)/2 - bitwise right shift
......................
122
9.4.2 (<<)/2 - bitwise left shift
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@ ISO/IEC 1995 ISO/IEC 13211-1 : 1995(E)
9.4.3 (/\)/2 - bitwise and . 123
9.4.4 (\/)/2 - bitwise or . 123
9.4.5 (\)/l - bitwise complement . 124
Annex
A Formal semantics . 125
A.l Introduction . 125
A.l.l Specification language: Syntax . 125
A.1.2 Specification language: semantics . 126
A.1.3 Comments in the formal specification . 126
A.1.4 About the style of the Formal Specification . 127
A.l.5 References . 127
A.2 An informal description . 127
A.2.1 Search-tree for “pure” Prolog . 128
A.2.2 Search tree for “pure” Prolog with tut . 13 1
A.2.3 Search-tree for kerne1 Prolog . 132
A.2.4 Database and database update view . 134
A.2.5 Exception handling . 135
A.2.6 Environments . 135
A.2.7 The semantics of a Standard program . 136
A.2.8 Getting acquainted with the formal specification . 136
A.2.9 Built-in predicates . 137
A.2.10 Relationships with the informal semantics of 7.7 and 7.8 138
A.3 Data structures . 138
A.3.1 Abstract databases and terms . 138
A.3.2 Predicate indicator . 143
A.3.3 Forest . 143
A.3.4 Abstract lists, atoms, characters and lists . 146
A.3.5 Substitutions and unification . 148
A.3.6 Arithmetic . 149
A.3.7 Differente lists and environments . 149
A.3.8 Built-in predicates and packets . 150
A.3.9 Input and output . 154
A.4 The Formal Semantics . 155
.................................
A.4.1 The kerne1 155
A.5 Control constructs and built-in predicates . 170
............................ 170
A.5.1 Control constructs
............................. 171
A.5.2 Term unification
................................ 172
A.5.3 Type testing
A.5.4 Term comparison . 172
................. 173
A.5.5 Term creation and decomposition
.................... 174
A.5.6 Arithmetic evaluation - (is)/2
......................... 174
A.5.7 Arithmetic comparison
.................. 174
A.5.8 Clause retrieval and information
A.5.9 Clause creation and destruction . 175
A.5.10 All solutions . 178
A.5.11 Stream selection and control . 180
A.5.12 Character inputloutput . 183
A.5.13 Byte inputloutput . 189
A.5.14 Term inputloutput . 192
A.5.15 Logic and control . 194
........................
A.5.16 Atomic term processing 195
A.5.17 Implementation defined hooks . 198
vii

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ISO/IEC 132114 : 1995(E) @ ISO/IEC 1995
Foreword
ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized System for worldwide
standardization. National bodies that are members of ISO or IEC participate
in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in fields
of mutual interest. Other international organizations, governmental and non-
governmental, in liaison with ISO and IEC, also take part in the work.
In the field of information technology, ISO and IEC have established a joint
technical committee ISOIIEC JTC 1. Draft International Standards adopted
by the joint technical committee are circulated to national bodies for voting.
Publication as an International Standard requires approval by at least 75% of
the national bodies casting a vote.
International Standard ISOIIEC 13211 was prepared by Joint Technical Committee
ISOIIEC JTC 1, Information technology, Subcommittee SC 22, Programming
languages, their environments and System software interfaces.
Annex A of this part of ISOIIEC 13211 is for information only.
. . .
VI11

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@ ISO/IEC 1995
ISO/IEC 132114 : 1995(E)
Introduction
This is the first International Standard for Prolog, Part 1 (General Core). It was
produced on 20 April 1995.
There is no other International Standard for Prolog.
Prolog (Programming in Logic) combines the concepts of logical and algorithmic
programming, and is recognized not just as an important tool in AI (Artificial
Intelligente) and expert Systems, but as a general purpose high-level programming
language with some unique properties.
The language originates from work in the early 1970s by Robert A. Kowalski
while at Edinburgh University (and ever since at Imperial College, London) and
Alain Colmerauer at the University of Aix-Marseilles in France. Their efforts
led in 1972 to the use of formal logic as the basis for a programming language.
Kowalski’s research provided the theoretical framework, while Colmerauer’s
gave rise to the programming language Prolog. Colmerauer and his team then
built the first interpreter, and David Warren at the AI Department, University of
Edinburgh, produced the first Compiler.
The crucial features of Prolog are unification and backtracking. Unification
Shows how two arbitrary structures tan be made equal, and Prolog processors
employ a search strategy which tries to find a Solution to a Problem by
backtracking to other paths if any one particular search Comes to a dead end.
Prolog is good for windowing and multimedia because of the ease of building
complex data structures dynamically, and also because the concept of backing
out of an Operation is built into the language.
Prolog is taught in more UK university computing degrees than any other
programming language,
This part of ISOIIEC 13211 defines the general core features of Prolog, and
part 2 will define modules.
ix

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INTERNATIONAL STANDARD 0 ISO/IEC ISO/IEC 13211=1:1995(E)
- Programming languages
Information technology - Prolog -
Part 1:
General core
1 Scope 1.1 Notes
Notes in this part of ISOIIEC 13211 have no effect on the
ISOIIEC 13211 is designed to promote the applicability
language, Prolog text or Prolog processors that are defined
and portability of Prolog text and data among a variety of
as conforming to this part of ISOIIEC 13211. Reasons for
data processing Systems.
including a note include:
This part of ISOIIEC 13211 specifies:
a) Cross references to other clauses and subclauses of
this part of ISOIIEC 13211 in Order to help readers find
a) The representation of Prolog text,
their way around,
b) Warnings when a built-in predicate as defined in
b) The Syntax and constraints of the Prolog language,
this part of ISOIIEC 13211 has a different meaning in
some existing implernentations.
c) The semantic rules for interpreting Prolog text,
d) The representation of input data to be processed by
2 Normative references
Prolog,
The following Standards contain provisions which, through
e) The representation of output produced by Prolog,
reference in this text, constitute provisions of this part of
and
ISOIIEC 13211. At the time of publication, the editions
indicated were valid. All Standards are subject to revision,
f) The restrictions and limits imposed on a conforming
and Parties to agreements based on this part of ISOIIEC
Prolog processor.
13211 are encouraged to investigate the possibility of
applying the most recent editions of the Standards listed
below. Members of IEC and ISO maintain registers of
NOTE - This part of ISOAEC 13211 does not specify:
currently valid International Standards.
a) the size or complexity of Prolog text that will exceed the
ISOIIEC 646 : 1991, Information technology - ISO 7-bit
capacity of any specific data processing System or language
processor, or the actions to be taken when the corresponding
coded Character set for information interchange.
limits are exceeded;
ISO 2382-15 : 1985, Data processing - Vocabulary -
b) the minimal requirements of a data processing System
Part 15: Programming languages.
that is capable of supporting an implementation of a Prolog
processor;
ISO 8859-1 : 1987, Information technology - a-bit
Single-byte coded graphic Character sets - Part I: Latin
c) the methods of activating the Prolog processor or the
alphabet No. 1.
set of commands used to control the environment in which
Prolog text is prepared for execution and executed;
ISOIIEC 9899 : 1990, Programming languages - C.
d) the mechanisms by which Prolog text is prepared for
ISOIIEC TR 10034 : 1990, Guidelines for the prepara-
use by a data processing System;
tion of conformity clauses in programming language
Standards.
e) the typographical representation of Prolog text published
for human reading;
ISOIIEC 10967-1 : 1994, Information technology - Lan-
guage independent arithmetic - Part I: Integer and
1) the user environment (top level loop, debugger, library
Joating Point arithmetic.
System, editor, Compiler etc.) of a Prolog processor.
BS 6154 : 198 1, Method of defining - Syntactic meta-
This part of ISOAEC 13211 is intended for use by implernentors
language.
and knowledgeable programmers, and is not a tutorial.
1

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@ ISO/IEC 1995
ISO/IEC 13211-1 : 1995(E)
arity: The number of arguments of a compound
3 Definitions 39
k-m. Syntactically, a non-negative integer associated with
a functor or predicate.
This terminology for Prolog has a format modelled on that
of ISO 2382.
3.10 assert, to: To assert a clause is to add it to the
An entry consists of a Phrase (in bold type) being defined,
user-defined procedure in the database defined by the
followed by its definition. Words and phrases defined in
predicate of that clause.
the glossary are printed in italics when they are used in
other entries. When a definition contains two words or
It is unnecessary for the user-dejrzed procedure to
NOTE -
phrases defined in separate entries directly following each
already exist.
other (or separated only by a punctuation sign), * (an
asterisk) separates them.
3.11 associativity (of an Operator): Property of being
non-associative, right-associative, or left-associative (see
Words and phrases not defined in this glossary are assumed
63.4, table 4).
to have the meaning given in ISO 2382-15; if they do not
appear in ISO 2382- 15, then they are assumed to have
their usual meaning.
3.12 atom: A basic Object, denoted by an identifier
(see 6.1.2 b, 7.1.4).
For the purposes of ISO/IEC 13211, the following defini-
tions apply:
3.13 atom, null: See 3.117 - null atom.
3.1 A: The set of atoms (see 61.2 b, 7.1.4).
3.14 atom, one-char: See 3.119 - one-char atom.
32 . activation: The process of executing an activator.
3.15 atomic term: An atom or a number.
33 activator: The result of preparing a goal for exe-
3.16 axiom: A rule satisfied an operati on and all
bY
cution (see 7.7.3).
belongs.
values of the data type to which the Operation
3.4 algorithm, Herbrand: See 3.85 - Herbrand al-
backtrack, to: To return to the choicepoint of the
3.17
gorithm.
current goal in Order to attempt to re-execute it (see 7.7.8).
3.5 alias: An atom associated with an open stream (see
3.18 bias, exponent: See 3.68 - exponent bias.
7.10.2.2).
3.19 body: A goal, distinguished by its context as part
The Standard input stream has the alias user-input, and
of a rule (see 3.154).
the Standard output stream has the alias User-output (see
7.10.2.3).
3.20 bootstrapped (built-in predicate): Defined as a
NOTE - A s tream tan have many aliases, but an atom tan be
special case of a more general buiZt-in predicate (see
at most one stream.
the alias of
8.15).
36 . anonymous variable: A variable (represented in a
A procedure whose execution
3.21 built-in predicate:
term or Prolog text by -) which differs from every other
is implemented by the processor (see 8).
variable (and anonymous variable) (see 6.1.2, 6.4.3).
3.22 byte: An integer in the range [0.255] (see 7.1.2.1).
37 argument: A term which is associated with a
piedication or compound term.
3.23 C: The set of characters (see 7.1.4.1).
38 arithmetic data type: A data type whose values
3.24 callable term: An atonz or a compound term.
a;e members of 2 or R.
2

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@ ISO/IEC 1995
ISO/IEC 132114 : 1995(E)
3.25 CC: The set of Character Codes (see 7.1.2.2). 3.37 compound term:
A functor of arity N, N positive,
together with a sequence of N arguments (see 6.1.2 e,
7.1.5).
3.26 Character: A member of C - an implementation
defined Character set (see 6.5, 7.1.4.1).
3.38 configuration: Host and target Computers, any op-
erating System(s) and Software used to operate a processor.
3.27 Character,
...