# Modules and interfaces

> Art, it seems to me, should simplify. That, indeed, is very nearly the whole of the higher artistic process; finding what conventions of form and what detail one can do without and yet preserve the spirit of the whole. ---Willa Cather

A module is like a block, except for the visibility of names. An entity is visible in a block if it is declared in the block or in some enclosing block; an entity is visible in a module if it is declared in the module or in an interface that is imported or exported by the module.

An interface is a group of declarations. Declarations in interfaces are the same as in blocks, except that any variable initializations must be constant, and procedure declarations must specify only the signature, not the body.

A module `X` exports an interface `I` to supply bodies for one or more of the procedures declared in the interface. A module or interface `X` imports an interface `I` to make the entities declared in `I` visible in `X`.

A program is a collection of modules and interfaces that contains every interface imported or exported by any of its modules or interfaces, and in which no procedure, module, or interface is multiply defined. The effect of executing a program is to execute the bodies of each of its modules. The order of execution of the modules is constrained by the initialization rule.

The module whose body is executed last is called the main module. Implementations are expected to provide a way to specify the main module, in case the initialization rule does not determine it uniquely. The recommended rule is that the main module be the one that exports the interface `Main`, whose contents are implementation-dependent.

Program execution terminates when the body of the main module terminates, even if concurrent threads of control are still executing.

The names of the modules and interfaces of a program are called global names. The method for looking up global names---for example, by file system search paths---is implementation-dependent.

The following topics are discussed below

- Import statements
- Interfaces
- Modules
- An example interface and module
- Generics
- Initialization
- Safety

    Last modified on Tue Oct  3 17:29:56 PDT 1995 by heydon
         modified on Mon Apr 18 13:44:15 PDT 1994 by kalsow