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Limbo has three kinds of objects:
Data objects exist in the storage associated with a module; they can be manipulated by arithmetic operations, assignment, selection of component entities, and other concrete operations. Each data object has a type that determines what can be stored in it and what operations are applicable.
The second kind of object is the function. Functions are characterized by the types of the arguments they accept and the values they return, and are associated with the modules in which they are defined. Their names can be made visible in their module's declaration, or they can be encapsulated within the adt (abstract data types) of their modules, or they can exist privately within their module.
Finally, Limbo programs are organized into modules: a named collection of data objects, constants, abstract data types, and functions made available by that module. A module declaration displays the members visible to other modules; the module's implementation defines both the publicly visible members and its private parts, including the data objects it uses. A module that wishes to use the facilities of another includes its declaration in order to understand what it exports, but before using them it explicitly loads the new module.