Kawa

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.

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) ...)

In addition to define (which can take an optional type specifier), Kawa has some extra definition forms.

For define-namespace and define-private-namespace see Namespaces and compound symbols.

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.

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.

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 Compiling to an applet) 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.

Unfortuntely, 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 Mapping Scheme names to Java names) 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.

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.)

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

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.