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14.7 Adding foreign declarations

Foreign language declarations (such as type declarations, header file inclusions or macro definitions) can be included in the Mercury source file as part of a ‘foreign_decl’ declaration of the form

:- pragma foreign_decl("Lang", DeclCode).

This declaration will have effects equivalent to including the specified DeclCode in an automatically generated source file of the specified programming language, in a place appropriate for declarations, and linking that source file with the Mercury program (after having compiled it with a compiler for the specified programming language, if appropriate).

Entities declared in ‘pragma foreign_decl’ declarations are visible in ‘pragma foreign_code’, ‘pragma foreign_type’, ‘pragma foreign_proc’, and ‘pragma foreign_enum’ declarations that specify the same foreign language and occur in the same Mercury module.

By default, the contents of ‘pragma foreign_decl’ declarations are also visible in the same kinds of declarations in other modules that import the module containing the ‘pragma foreign_decl’ declaration. This is because they may be required to make sense of types defined using ‘pragma foreign_type’ and/or predicates defined using ‘pragma foreign_proc’ in the containing module, and these may be visible in other modules, especially in the presence of intermodule optimization,

If you do not want the contents of a ‘pragma foreign_decl’ declaration to be visible in foreign language code in other modules, you can use the following variant of the declaration:

:- pragma foreign_decl("Lang", local, DeclCode).

Note: currently only the C and Erlang backends support this variant of the ‘pragma foreign_decl’ declaration.

The Melbourne Mercury implementation additionally supports the forms

:- pragma foreign_decl("Lang", include_file("Path")).
:- pragma foreign_decl("Lang", local, include_file("Path")).

These have the same effects as the standard forms except that the contents of the file referenced by Path are included in place of the string literal in the last argument, without further interpretation. Path may be an absolute path to a file, or a path to a file relative to the directory that contains the source file of the module containing the declaration. The interpretation of the path is platform-dependent. If the filesystem uses a different character set or encoding from the Mercury source file (which must be UTF-8), the file may not be found.

mmc --make’ and ‘mmake’ treat included files as dependencies of the module.


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