13.2.5 Simple example of libraries and modules

To illustrate these concepts, we repeat the definition of the library.dylan file, first shown in Chapter 2, Quick Start. Here, we have used a more verbose, but also more precise, format.

module: dylan-user

define library hello
  use dylan, import: { dylan }; 
  use format-out, import: { format-out };
end library hello;
define module hello
  use dylan, import: all; 
  use format-out, import: all;
end module hello; 

The first line of library.dylan states that the expressions and definitions in this file are in the dylan-user module. In this predefined module, you define the modules and library that make up your component or program. Every library has a unique dylan-user module. In the file library.dylan, we define a library named hello and a module named hello.

The module definition names the other modules whose names the hello module will use. In this case, the hello module uses the dylan and format-out modules. Here, we have explicitly stated that we are importing all the names from the modules that we use — using the import: all clause is not strictly necessary, because it is the default that is used if we do not specify what to import. By using another module, we import the names exported from that module, making them available in our namespace. For example, format-out is exported from the format-out module, so the use format-out clause enables our program to call the format-out function. The use dylan clause in the module definition makes available all the built-in Dylan language elements exported from the dylan module. When we define a module, it must use all the modules that export the definitions used by the definitions in our module.

The library definition tells the compiler which other libraries our program uses. Here, we have explicitly stated that we are interested in only the dylan and format-out modules from these other libraries. This clause is not strictly necessary, since the module definition tells the compiler which modules it uses; but it is good practice to document our intent. For example, the format-out module is in the format-out library. Therefore, our hello library must use the format-out library, and must import the format-out module for the hello module to use the format-out module. Similarly, the dylan module is in the dylan library, and therefore our hello library must use the dylan library and import the dylan module in order for the hello module to use the dylan module. When we define a library, it must use all the libraries that export the modules used by the modules in our library.

The module definition also specifies which variables and constants are exported from the module for use by other modules. The library definition specifies which modules are exported from the library for use by other libraries. In our simple example, the hello module exports no variables or constants, and the hello library exports no modules.

Figure 13.1 illustrates the relationships between libraries and modules in our example program. In Figure 13.1, and in the other figures in this chapter, we draw libraries as heavy bold boxes and modules as light boxes. We have tried to illustrate how libraries and modules build on one another through their "use" relationships. A library that uses another library is shown above the library that it uses, so we show the hello library above the format-out and dylan libraries. An exported module is illustrated as being on top of (overlapping) the library that

exports it (we have also shaded them, to illustrate this overlap). And a module that uses another module is illustrated as being on top of (overlapping) the used module. Try to envision the modules as semitransparent overlays, layered up from the surface of the paper. Thus, the hello module overlays the format-out and dylan modules that it uses.

Note that we intentionally do not show all the modules in the format-out and dylan libraries in Figure 13.1. The format-out and dylan libraries might well have other modules, but either those modules are not exported or our program does not use them.