local +SpecList

+SpecList

One or a comma-separated sequence of valid local specifications

Description

Name/Arity

procedure specification

domain(Spec)

domain declaration

struct(Prototype)

structure declaration

variable(Name)

non-logical variable declaration

reference(Name)

reference declaration

reference(Name,InitialValue)

reference declaration with initial value (which must be a simple value like atom, small integer or the empty list)

array(Name)

untyped non-logical array declaration

array(Name,Type)

typed non-logical array declaration

record(Name)

record key declaration

store(Name)

store name declaration

op(Prec,Assoc,Name)

operator declaration

chtab(Char,Class)

character class declaration

syntax_option(Option)

syntax option setting

macro(Functor,Transformation,Options)

macro (input transformation) declaration

portray(Functor,Transformation,Options)

portray (output transformation) declaration

initialization(Goal)

initialization goal specification

Local Procedures

For procedures, the local-declaration is normally redundant because local visibility is the default. However, it might be necessary to explicitly declare a procedure as local to resolve a name conflict when an imported module exports a procedure of the same name.

Local declarations should be placed at the beginning of a module text. They must occur before the first reference to the declared prodecure:

A procedure can have four kinds of visibility in a given module: local, exported, imported or reexported. A local-declaration is silently ignored if the procedure has already been exported before. If a procedure of the given name has already been imported or reexported, the local-declaration raises an error 94. If there is one or more imported modules which export a procedure of the same name, these all get hidden silently by the local declaration.

A local procedure can only be called from within the module where it is defined, even when explicit module qualification via :/2 is used.

Local Initialization

The local initialization declaration is used to specify an initialization goal. All initialization goals which occur within a compilation unit (usually a file), will be executed just after this compilation unit has been loaded by the system.

Other Local Items

All other local declarations also have an effect only in the module where they occur. Some of them have corresponding export-variants.

Further Hints

The local/1 primitive can not only occur as a directive but can also be called at runtime.

Duplicate local declarations are accepted silently.

Fail Conditions

Resatisfiable

Exceptions

(4) instantiation fault

SpecList is not instantiated.

(5) type error

SpecList is instantiated, but not to a sequence of valid local specifications.

(94) trying to redefine an existing imported procedure

SpecList is already imported.

Examples

% Normally, local declarations for predicates are redundant:
  :- module(m).

  :- local p/1.         % can be omitted since the default is local
  p(99).


% Redefining a built-in predicate:

    :- module(m)
    :- local writeln/1.   % stop writeln/1 from being imported

    main :-
       writeln(hello).    % local-declaration must be before this use!

    writeln(X) :-         % the local version
       printf("I don't like the normal writeln/1 predicate: %w%n",[X]).


% Redefining an imported predicate:

    :- module(m)
    :- lib(lists).        % module 'lists' defines a predicate subtract/3
    :- local subtract/3.  % stop subtract/3 being imported from 'lists'

    decr(N, N1) :-
       subtract(N,1,N1).  % local-declaration must be before this use!

    subtract(X,Y,Z) :-    % the local version of subtract/3
       Z is X-Y.


% Other local declarations:

   :- local
   	op(500, xfx, before),
	struct(book(author,title,publisher)).

   :- local initialization(writeln("I am being initialized!")).


% Error cases:

  :- local P.                           (Error 4).
  :- local p/a.                         (Error 5).
  :- (import p/0 from m), local(p/0)    (Error 94).

See Also