directive

# `use_module/2`

## Description

    use_module(Module, [Name/Arity, ...])
    use_module(Module, [Name/Arity as Alias/Arity, ...])

    use_module(Module, [Predicate as Alias, ...])

    use_module(Module, [Name//Arity, ...])
    use_module(Module, [Name//Arity as Alias//Arity, ...])

    use_module(Module, [op(Precedence,Associativity,Operator), ...])

This directive declares that all calls (made from predicates defined in the category or object containing the directive) to the specified predicates (or non-terminals) are to be interpreted as calls to explicitly-qualified module predicates (or non-terminals). Thus, this directive may be used to simplify the writing of predicate definitions by allowing the programmer to omit the `Module:` prefix when using the predicates listed in the directive (as long as the predicate calls do not occur as arguments for non-standard Prolog meta-predicates not declared on the adapter files). It is also possible to include operator declarations in the second argument.

This directive is also taken into account when compiling calls to the database and reflection built-in methods by looking into these methods predicate arguments if bound at compile-time.

It is possible to specify a predicate alias using the notation `Name/Arity as Alias/Arity` or, in alternative, the notation `Name/Arity:Alias/Arity`. Aliases may be used either for avoiding conflicts between predicates specified in `use_module/2` and uses/2 directives or for giving more meaningful names considering the calling context of the predicates. For predicates, it is also possible to define alias shorthands using the notation `Predicate as Alias` or, in alternative, the notation `Predicate::Alias`, where `Predicate` and `Alias` are callable terms where some or all arguments may be instantiated.

Note that this directive differs from the directive with the same name found on some Prolog implementations by requiring the first argument to be a module name (an atom) instead of a file specification. In Logtalk, there’s no mixing between loading a resource and (declaring the) using (of) a resource. As a consequence, this directive doesn’t automatically load the module. Loading the module file is dependent on the used backend Prolog compiler and must be done separately (usually, using a source file directive such as `use_module/1` or `use_module/2` in the entity file or preferably in the application loader file file). Also, note that the name of the module may differ from the name of the module file.

Warning

The modules must be loaded prior to the compilation of entities that call the module predicates. This is required in general to allow the compiler to check if the called module predicate is a meta-predicate and retrieve its meta-predicate template to ensure proper call compilation.

The module identifier argument can also be a parameter variable when using the directive in a parametric object or a parametric category defined in a source file (the common case). In this case, dynamic binding will be used for all listed predicates (and non-terminals). The parameter variable must be instantiated at runtime when the calls are made.

## Template and modes

    use_module(+module_identifier, +predicate_indicator_list)
    use_module(+module_identifier, +module_predicate_indicator_alias_list)

    use_module(+module_identifier, +predicate_template_alias_list)

    use_module(+module_identifier, +non_terminal_indicator_list)
    use_module(+module_identifier, +module_non_terminal_indicator_alias_list)

    use_module(+module_identifier, +operator_list)

## Examples

    :- use_module(lists, [append/3, member/2]).
    :- use_module(store, [data/2]).
    :- use_module(user,  [foo/1 as bar/1]).

    foo :-
        ...,
        % same as findall(X, lists:member(X, L), A)
        findall(X, member(X, L), A),
        % same as lists:append(A, B, C)
        append(A, B, C),
        % same as assertz(store:data(X, C))
        assertz(data(X, C)),
        % same as retractall(user:foo(_))
        retractall(bar(_)),
        ...

Another example, using the extended notation that allows us to define predicate aliases:

    :- use_module(ugraphs, [transpose_ugraph/2 as transpose/2]).

    convert_graph :-
        ...,
        % the same as ugraphs:transpose_ugraph(Graph0, Graph)
        transpose(Graph0, Graph),
        ...

An example of defining a predicate alias that is also a shorthand:

    :- use_module(pairs, [
        map_list_to_pairs(length, Lists, Pairs) as length_pairs(Lists, Pairs)
    ]).

An example of using a parameter variable in place of the module identifier to delay to runtime the actual module to use:

    :- object(bar(_OptionsModule_)).

        :- use_module(_OptionsModule_, [
            set/2, get/2, reset/0
        ]).

See also

use_module/1, uses/2, uses/1, alias/2