2.1 Data objects 33 data objects ,/\ simple objects structures /\ of Prolog constants variables /\ atoms numbers Figure 2.1 Data objects in Prolog. Atoms and numbers 2.1.1 44 In chapter 1 we have seen some examples of atoms and variables. In general, however, atoms can take more complicated forms - that is, strings of the follow- ing characters: . upper-case letters A,B, . . . ,2 . lower-case letters a, b, . . . , z . digitsO,'J.,2,...,9 syntax and semantics of basic con- . special characters such as * -* I < >: : . & ects. The topics included are: Atoms can be constructed iri three ways: (1) Strings of ietters, digits and the underscore character, ,_,, starting with a lower- ts case letter: am anna nil meanings of a program x25 x_25 lauses and goals. x_2548 r Chapter 1. Here the treatment will .x_ xv alpha_beta procedure missJones (2) Strings of special characters: in Prolog. The Prolog system lm by its syntactic form. This is + different forms for each type of br distinguishing between atoms upper-case letters whereas atoms rmation (such as data-type decla- When using atoms of this form, some care is necessary because some strings ¡rder to recognize the t¡re of an of special characters already have a predefined meaning. An example is ,:-,.
Terms 2.1 Data objects 33 data objects ,/\ simple objects structures /\ of Prolog constants variables /\ atoms numbers Figure 2.1 Data objects in Prolog. Atoms, numbers, variables and structures are all terms. Atoms and numbers 2.1.1 44 In chapter 1 we have seen some examples of atoms and variables. In general, however, atoms can take more complicated forms - that is, strings of the follow- ing characters: . upper-case letters A,B, . . . ,2 . lower-case letters a, b, . . . , z . digitsO,'J.,2,...,9 syntax and semantics of basic con- . special characters such as * -* I < >: : . & ects. The topics included are: Atoms can be constructed iri three ways: (1) Strings of ietters, digits and the underscore character, ,_,, starting with a lower- ts case letter: am anna nil meanings of a program x25 x_25 lauses and goals. x_2548 r Chapter 1. Here the treatment will .x_ xv alpha_beta procedure missJones (2) Strings of special characters: in Prolog. The Prolog system lm by its syntactic form. This is + different forms for each type of br distinguishing between atoms upper-case letters whereas atoms rmation (such as data-type decla- When using atoms of this form, some care is necessary because some strings ¡rder to recognize the t¡re of an of special characters already have a predefined meaning. An example is ,:-,.
Summary 175 facilities. Therefore an interpreter is typically used in the program development phase, and a compiler is used with the final program. It should be noted, again, that the details of consulting and compiling files : in the current input file that depend on the implementation of Prolog. usually a prolog implementation also Lits original form, represented as allows the user to enter and edit the program interactively. getsentence of this section can be 5ummary A Prolog implementation normally provides a set of built-in procedures to accomplish several useful operations that are not possible in pure prolog. In this cg system by means of built-in chapter, such a set of predicates available in many Prolog implementations was ams. The details of 'consulting, introduced. :ion of Prolog. Here we look at The type of a term can be tested by the following predicates: clogs. X is a (non-instantiated) variable var( X) ile F with a goal of the forr¡ X is not a variable nonvar( X) atom( X) X is an atom X is an integer integer( X) X is a real number float( X) program3 will typically have an X is either an atom or a number atomic( X) rgram file. The effect of this goal compound( X) X is a structure and loaded into the memory, so Terms can be constructed or decomposed: rrther questions from the user. Term :.. I Functor I Argumentlist] time during the same session. this new flle are added into the functor( Term, Functor, Arity) lementation and other circum- arg( N, Term, Argument) Lt a procedure defined in the name( Atom, CharacterCodes) ry be simply added at the end of Terms can be compared: ition of this procedure may be X:Y XandYmatch X::Y XandYareidentical sult goal, for example: X \:: Y X and Y are not identical X::: Y X and Y are arithmetically equal X:\: Y X and Y are not arithmetically equal X < Y IS: adthmetic value of X is less than Y (related: :<, ), ):) X @< Y term X precedes term Y (related: @:<, @>, @>:) A Prolog program can be viewed as a relational database that can be updated by . If a Prolog implementation also the following procedures: n a compiled form. This enables r factor of 5 or 10 between the assert( Clause) add Clause to the program ed into memory in the compiled asserta( Clause) add at the beginning e: assertz( Clause) add at the end retract( Clause) remove a clause that matches Clause All the obiects that satisfy a given condition can be collected into a list by the predicates: bagof( X, P, L) L is the list of all X that satisfy condition P setof( X, P, L) d, but interpreted programs are L is the sorted list of all X that satisfy condition P findall( X, P, L) similar to bagof rd traced by Prolog's debugging
Summary 175 facilities. Therefore an interpreter is typically used in the program development phase, and a compiler is used with the final program. It should be noted, again, that the details of consulting and compiling files : in the current input file that depend on the implementation of Prolog. usually a prolog implementation also Lits original form, represented as allows the user to enter and edit the program interactively. getsentence of this section can be 5ummary A Prolog implementation normally provides a set of built-in procedures to accomplish several useful operations that are not possible in pure prolog. In this cg system by means of built-in chapter, such a set of predicates available in many Prolog implementations was ams. The details of 'consulting, introduced. :ion of Prolog. Here we look at The type of a term can be tested by the following predicates: clogs. X is a (non-instantiated) variable var( X) ile F with a goal of the forr¡ X is not a variable nonvar( X) atom( X) X is an atom X is an integer integer( X) X is a real number float( X) program3 will typically have an X is either an atom or a number atomic( X) rgram file. The effect of this goal X is a structure compound( X) and loaded into the memory, so Terms can be constructed or decomposed: rrther questions from the user. Term :.. I Functor I Argumentlist] time during the same session. this new flle are added into the functor( Term, Functor, Arity) lementation and other circum- arg( N, Term, Argument) Lt a procedure defined in the name( Atom, CharacterCodes) ry be simply added at the end of Terms can be compared: ition of this procedure may be X:Y XandYmatch X::Y XandYareidentical sult goal, for example: X \:: Y X and Y are not identical X::: Y X and Y are arithmetically equal X:\: Y X and Y are not arithmetically equal X < Y IS: adthmetic value of X is less than Y (related: :<, ), ):) X @< Y term X precedes term Y (related: @:<, @>, @>:) A Prolog program can be viewed as a relational database that can be updated by . If a Prolog implementation also the following procedures: n a compiled form. This enables r factor of 5 or 10 between the assert( Clause) add Clause to the program ed into memory in the compiled asserta( Clause) add at the beginning e: assertz( Clause) add at the end retract( Clause) remove a clause that matches Clause All the obiects that satisfy a given condition can be collected into a list by the predicates: bagof( X, P, L) L is the list of all X that satisfy condition P setof( X, P, L) d, but interpreted programs are L is the sorted list of all X that satisfy condition P findall( X, P, L) similar to bagof rd traced by Prolog's debugging
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