Modules & Libraries¶
Modules and libraries provide the structure of a Dylan program. Modules represent namespaces and control access to objects and functions. Libraries contain modules, and act as units of compilation in a finished Dylan program.
Modules import the symbols of other modules and export their own. The dependencies between modules must form a directed, acyclic graph. Two modules may not use each other, and no circular dependencies may exist.
Modules only export variables. Since the names of classes and generic functions are actually stored in variables, this represents no hardship. A sample module containing the vehicle classes from earlier chapters might resemble:
define module vehicles use dylan; export <vehicle>, serial-number, owner, owner-setter, tax, <car>, <truck>, capacity; end module;
Like all normal modules, this one uses the
dylan module, which
contains all of the standard built-in functions and classes. In turn,
vehicles module exports all three of the vehicle classes, the
tax, several getter functions and a single
To control access to a slot, export some combination of its
getter and setter functions. To make a slot public, export both. To
make it read-only, export just the getter function. To make it
private, export neither. In the above example, the slot
serial-number is read-only, while the slot
owner is public.
Note that when some module adds a method to a generic function, the change affects all modules using that function. The new method actually gets added into the variable representing the generic function. Since the variable has been previously exported, all clients can access the new value.
Dylan allows very precise control over how symbols are imported from other modules. For example, individual symbols may be imported by name. They may be renamed, either one at a time, or by adding a prefix to all of a module’s symbols at once. Some or all of them may be re-exported immediately. See the DRM for specific examples.
Dylan’s import system has a number of advantages. Name conflicts occur rarely. Programmers don’t need to define or maintain function prototypes. There’s no need for header files. Modules may also provide different interfaces to the same objects – one module exports a complete interface, which another module imports, redefines and re-exports.
Libraries contain modules. For example, the
library contains the
described earlier, the
extensions module, and
possibly several other implementation-dependent modules. Note that
a library and a module may share the same name. Modules with the
same name may also appear in more than one library.
By default, a Dylan environment provides a library called
dylan-user for the convenience of the programmer.
This is typically used for short, single library programs which
depend only on modules found in the Dylan library.
Additionally, every library contains an implicit module, also
dylan-user, which imports all of the
modules found in the
dylan library. This may be
used for single module programs. Many Dylan environments, however,
use it to bootstrap new library definitions. The vehicle library,
for example, might be defined as follows in a
define library vehicles use dylan; // This is the library! export // These are modules. vehicles, // (Defined above.) traffic-simulation, crash-testing, inspection; // (Hypothetical.) end library vehicles;
This library could in turn be imported by another library:
define library vehicle-application use dylan; use my-gui-classes; use vehicles; end;
Libraries import other libraries and export modules, whereas
modules import other modules and export variables. In general, a
module may import any module found in its own library or exported
from a library imported by its own library. The following module, for
example, could belong to the
define module sample-module // module name source library use dylan; // dylan use extensions; // dylan use menus; // my-gui-classes use vehicles; // vehicles use inspection; // vehicles end module;
Classes and generic functions may be sealed using a number of Dylan forms. This prevents code in other libraries from subclassing objects or adding methods to generic functions, and lets the compiler optimize more effectively. Both classes and generic functions are sealed by default.
To allow code in other libraries to subclass a given class,
declare it as
define open class <sample> (<object>) end;
To allow other libraries to add methods to a generic function, use a similar syntax:
define open generic sample-function (o :: <object>) => ();
A third form,
define sealed domain, partially
seals a generic function, disallowing only some additions from outside
For more information on sealing, see the chapter “Controlling Dynamism” in the DRM.