|Did you know ...||Search Documentation:|
|library(record): Access named fields in a term|
library(record) provides named access to
fields in a record represented as a compound term such as
Y). The Prolog world knows various approaches to solve this
problem, unfortunately with no consensus. The approach taken by this
library is proposed by Richard O'Keefe on the SWI-Prolog mailinglist.
The approach automates a technique commonly described in Prolog text-books, where access and modification predicates are defined for the record type. Such predicates are subject to normal import/export as well as analysis by cross-referencers. Given the simple nature of the access predicates, an optimizing compiler can easily inline them for optimal preformance.
A record is defined using the directive record/1. We introduce the library with a short example:
:- record point(x:integer=0, y:integer=0). ..., default_point(Point), point_x(Point, X), set_x_of_point(10, Point, Point1), make_point([y(20)], YPoint),
The principal functor and arity of the term used defines the name and arity of the compound used as records. Each argument is described using a term of the format below.
In this definition, <name> is an atom defining the
name of the argument,
<type> is an optional type specification as defined by must_be/2
library(error), and <default>
is the default initial value. The
<type> defaults to
any. If no default
value is specified the default is an unbound variable.
A record declaration creates a set of predicates through term-expansion. We describe these predicates below. In this description, <constructor> refers to the name of the record (`point' in the example above) and <name> to the name of an argument (field).