(define-simple-class foo ...)

The definitions that a module exports are accessible to other modules. These are the "public" definitions, to use Java terminology. By default, all the identifiers declared at the top-level of a module are exported, except those defined using define-private. However, a major purpose of using modules is to control the set of names exported. One reason is to reduce the chance of accidental name conflicts between separately developed modules. An even more important reason is to enforce an interface: Client modules should only use the names that are part of a documented interface, and should not use internal implementation procedures (since those may change).

If there is a module-export declaration in the module, then only those names listed in a module-export are exported. There can be more than one module-export, and they can be anywhere in the Scheme file. As a matter of good style, I recommend a single module-export near the beginning of the file.

Syntax: module-export name ...

In this module, fact is public and worker is private:

(module-export fact)
(define (worker x) ...)
(define (fact x) ...)

Alternatively, you can write:

(define-private (worker x) ...)
(define (fact x) ...)

If you want to just use a Scheme module as a module (i.e. load or require it), you don’t care how it gets translated into a module class. However, Kawa gives you some control over how this is done, and you can use a Scheme module to define a class which you can use with other Java classes. This style of class definition is an alternative to define-class, which lets you define classes and instances fairly conveniently.

The default name of the module class is the main part of the filename of the Scheme source file (with directories and extensions sripped off). That can be overridden by the -T Kawa command-line flag. The package-prefix specified by the -P flag is prepended to give the fully-qualified class name.

Syntax: module-name name

Syntax: module-name <name>

By default, the base class of the generated module class is unspecified; you cannot count on it being more specific than Object. However, you can override it with module-extends.

Syntax: module-extends class

Syntax: module-implements interface ...

Note that the compiler does not currently check that all the abstract methods requires by the base class or implemented interfaces are actually provided, and have the correct signatures. This will hopefully be fixed, but for now, if you are forgot a method, you will probably get a verifier error

For each top-level exported definition the compiler creates a corresponding public field with a similar (mangled) name. By default, there is some indirection: The value of the Scheme variable is not that of the field itself. Instead, the field is a gnu.mapping.Symbol object, and the value Scheme variable is defined to be the value stored in the Symbol. Howewer, if you specify an explicit type, then the field will have the specified type, instead of being a Symbol. The indirection using Symbol is also avoided if you use define-constant.

If the Scheme definition defines a procedure (which is not re-assigned in the module), then the compiler assumes the variable as bound as a constant procedure. The compiler generates one or more methods corresponding to the body of the Scheme procedure. It also generates a public field with the same name; the value of the field is an instance of a subclass of <gnu.mapping.Procedure> which when applied will execute the correct method (depending on the actual arguments). The field is used when the procedure used as a value (such as being passed as an argument to map), but when the compiler is able to do so, it will generate code to call the correct method directly.

You can control the signature of the generated method by declaring the parameter types and the return type of the method. See the applet (see Applet compilation) example for how this can be done. If the procedures has optional parameters, then the compiler will generate multiple methods, one for each argument list length. (In rare cases the default expression may be such that this is not possible, in which case an "variable argument list" method is generated instead. This only happens when there is a nested scope inside the default expression, which is very contrived.) If there are #!keyword or #!rest arguments, the compiler generate a "variable argument list" method. This is a method whose last parameter is either an array or a <list>, and whose name has $V appended to indicate the last parameter is a list.

Top-leval macros (defined using either define-syntax or defmacro) create a field whose type is currently a sub-class of kawa.lang.Syntax; this allows importing modules to detect that the field is a macro and apply the macro at compile time.

Unfortunately, the Java class verifier does not allow fields to have arbitrary names. Therefore, the name of a field that represents a Scheme variable is "mangled" (see Mangling) into an acceptable Java name. The implementation can recover the original name of a field X as ((gnu.mapping.Named) X).getName() because all the standard compiler-generate field types implemented the Named interface.

There are two kinds of module class: A static module is a class (or gets compiled to a class) all of whose public fields a static, and that does not have a public constructor. A JVM can only have a single global instance of a static module. An instance module has a public default constructor, and usually has at least one non-static public field. There can be multiple instances of an instance module; each instance is called a module instance. However, only a single instance of a module can be registered in an environment, so in most cases there is only a single instance of instance modules. Registering an instance in an environment means creating a binding mapping a magic name (derived from the class name) to the instance.

In fact, any Java class class that has the properties of either an instance module or a static module, is a module, and can be loaded or imported as such; the class need not have written using Scheme.

You can control whether a module is compiled to a static or a non-static class using either a command-line flag to the compiler, or using the module-static special form.

Syntax: module-static name ...

Syntax: module-static #t

Syntax: module-static #f

Syntax: module-static 'init-run

The top-level actions of a module will get compiled to a run method. If there is an explicit method-extends, then the module class will also automatically implement java.lang.Runnable. (Otherwise, the class does not implement Runnable, since in that case the run method return an Object rather than void. This will likely change.)

Certain compilation options can be be specified either on the command-line when compiling, or in the module itself. The can be any of the --wan- options, or the --[full-]tailcalls options.

Syntax: module-compile-options [key: value] ...

Syntax: with-compile-options [key: value] ... body

You can import a module into the current namespace with require.

Syntax: require ’featureName

Syntax: require classname ["sourcepath"]

Syntax: require "sourcepath"

Syntax: import import-set ...

Syntax: provide ’featurename

Using require and provide with featurenames is similar to the same-named macros in SLib, Emacs, and Common Lisp. However, in Kawa these are not functions, but instead they are syntax forms that are processed at compile time. That is why only quoted featurenames are supported. This is consistent with Kawa emphasis on compilation and static binding.

For some examples, you may want to look in the gnu/kawa/slib directory.