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>.^{252Note 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.