## A.49 library(record): Access named fields in a term

The library `library(record)` provides named access to fields in a record represented as a compound term such as `point(X, 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 performance.

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.

> <name>[:<type>][=<default>]

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 from library `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).

- default_<constructor>(-Record)
  Create a new record where all fields have their default values. This is the same as make_<constructor>([], Record) .

- make_<constructor>(+Fields, -Record)
  Create a new record where specified fields have the specified values and remaining fields have their default value. Each field is specified as a term <name>(<value>). See example in the introduction.

- make_<constructor>(+Fields, -Record, -RestFields)
  Same as make_<constructor>/2, but named fields that do not appear in Record are returned in RestFields. This predicate is motivated by option-list processing. See library `library(option)`.

- <constructor>_<name>(Record, Value)
  Unify Value with argument in Record named <name>.251Note this is not called‘get_’as it performs unification and can perfectly well instantiate the argument.

- <constructor>_data(?Name, +Record, ?Value)
  True when Value is the value for the field named Name in Record. This predicate does not perform type-checking.

- set_<name>_of_<constructor>(+Value, +OldRecord, -NewRecord)
  Replace the value for <name> in OldRecord by Value and unify the result with NewRecord.

- set_<name>_of_<constructor>(+Value, !Record)
  Destructively replace the argument <name> in Record by Value based on setarg/3. Use with care.

- nb_set_<name>_of_<constructor>(+Value, !Record)
  As above, but using non-backtrackable assignment based on nb_setarg/3. Use with extreme care.

- set_<constructor>_fields(+Fields, +Record0, -Record)
  Set multiple fields using the same syntax as make_<constructor>/2, but starting with Record0 rather than the default record.

- set_<constructor>_fields(+Fields, +Record0, -Record, -RestFields)
  Similar to set_<constructor>_fields/4, but fields not defined by <constructor> are returned in RestFields.

- set_<constructor>_field(+Field, +Record0, -Record)
  Set a single field specified as a term <name>(<value>).

record(+Spec)
  The construct `:- record Spec, ...` is used to define access to named fields in a compound. It is subject to term-expansion (see expand_term/2) and cannot be called as a predicate. See section A.49 for details.