Next: , Previous: Adding foreign declarations, Up: Foreign language interface   [Contents]


14.8 Declaring Mercury exports to other modules

The declarations for Mercury predicates or functions exported to a foreign language using a ‘pragma foreign_export’ declaration are visible to foreign code in a ‘pragma foreign_code’ or ‘pragma foreign_proc’ declaration of the same module, and also in those of any submodules. By default, they are not visible to the foreign code in ‘pragma foreign_code’ or ‘pragma foreign_proc’ declarations in any other module, but this default can be overridden (giving access to all other modules) using a declaration of the form:

:- pragma foreign_import_module("Lang", ImportedModule).

where ImportedModule is the name of the module containing the ‘pragma foreign_export’ declarations.

If Lang is "C", this is equivalent to

:- pragma foreign_decl("C", "#include ""ImportedModule.mh""").

where ImportedModule.mh is the automatically generated header file containing the C declarations for the predicates and functions exported to C.

pragma foreign_import_module’ should be used instead of the explicit #include because ‘pragma foreign_import_module’ tells the implementation that ImportedModule.mh must be built before the object file for the module containing the ‘pragma foreign_import_module’ declaration.

Note that the Melbourne Mercury implementation often behaves as if ‘pragma foreign_import_module’ declarations were implicitly added to modules. However, programmers should not should not depend on this behaviour; they should always write explicit ‘pragma foreign_import_module’ declarations wherever they are needed.