Ideas and tasks for contributing to Kawa

Kawa (like other Free Software projects) has no lack of tasks and projects to work on. Here are some ideas.

The ones marked (GSoC) are probably most suitable for a Google Summer of Code project, in being a reasonable size, self-contained, and not depending on other tasks.

Recusively initialized data structures


Kawa has convenient syntax to allocate and initialize objects, but it gets messier it you want to initialize multiple objects that reference each other. Likewise for a single object “tree” which contains links to the root. In this example, we will looks at two vectors, but the feature is more useful for tree structures. Assume:

(define-constant list1 [1 2 list2])
(define-constant list2 ['a 'b list1])

The compiler translates this to:

(define-constant list1
   (let ((t (object[] length: 3))) ;; allocate native Java array
      (set! (t 0) 1)
      (set! (t 1) 2)
      (set! (t 2) list2)
      (FVector:makeConstant t)))
(define-constant list2
   (let ((t (object[] length: 3))) ;; allocate native Java array
      (set! (t 0) 'a)
      (set! (t 1) 'b)
      (set! (t 2) list1)
      (FVector:makeConstant t)))

The problem is that list2 has not been created when we evaluate the initializing expression for list.

We can solve the problem by re-writing:

(define-private tmp1 (object[] length: 3))
(define-constant list1 (FVector:makeConstant tmp1)
(define-private tmp2 (object[] length: 3))
(define-constant list2 (FVector:makeConstant tmp2)
(set! (tmp1 0) 1)
(set! (tmp1 1) 2)
(set! (tmp1 2) list2)
(set! (tmp2 0) 1)
(set! (tmp2 1) 2)
(set! (tmp2 2) list1)

The complication is that the code for re-writing vector and object constructors is spread out (depending on the result type), and not where we deal with initializing the variables. One solution is to introduce an inlineable helper function $build$ defined as:

(define ($build$ raw-value create init)
  (let ((result (create raw-value))
    (init raw-value result)

Then we can re-write the above code to:

(define-constant list1
    (object[] length: 3)
    (lambda (raw) (FVector:makeConstant raw))
    (lambda (raw result)
      ($init-raw-array$ raw 1 2 list2))))
(define-constant list2
    (object[] length: 3)
    (lambda (raw) (FVector:makeConstant raw))
    (lambda (raw result)
      ($init-raw-array$ raw 'a 'b list1))))

Note that the call to $build$, as well as the generated lambda expressions, are all easily inlineable.

Now assume if at the top-level body if there is a sequence of define-constant definitions initialized with calls to $build$. Now it is relatively easy to move all the init calls after all alloc and create expressions. The $init-raw-array$ calls are expanded after the code has been re-ordered.

The project includes both implementing the above framework, as well as updating type-specific (and default) object creation to use the framework. It would also be good to have compiler warnings if accessing an uninitialized object.

Enhance texinfo-js documentation browser for Kawa documentation


Run interactive process in separate Java Virtual Machine:


When developing and testing it is useful for the REPL to support hot-swapping (replacing functions on-the-fly) and debugging. The main goal being able to smoothly reload changed modules (files or functions), and have other modules not break. Debugging (such as setting breakpoints) would not be a priority for this project, but could be a follow-on project. Skills: Should be experienced with Java, and interested in learning about JVM TI and similar low-level parts of the platform. Difficulty: Challenging, but you can study how Java-9’s new jshell uses the JVM TI.

Better dynamic reload

(GSoC - this is related to the previous item)

Kawa does a lot of optimizations and inlining. This conflicts with being able to “reload” a module into an already-running interactive environment.

We could add an option to load a module in “reloadable” mode. Kawa already patches an old function object (a ModuleMethod) so existing references to the function get automatically updated. However, there are problems if the “signature” of the function changes - for example if the return type (declared or inferred) becomes more general. In those cases the best thing is to re-compile any code that depends on the modified function.

Reloading a module that defines a class is even trickier, at least if there are existing instances that should work as the updated class. We can handle the special case where only method bodies change: In reloadable mode, each method body is compiled to a separate function, the actual body indirects to the function. We must also recognize when compiling a new version of the same class, which requires a textual comparison between the old and new versions, or a structural comparison between the old class and the new code.

When it comes to top-level variables, an issue is when to re-evaluate the initializing expression. It is reasonable to do so if and only if the expression is modified, which again requires a textual comparison.

Easier Access to Native Libraries using JNA/JNR


The traditional way to access native (C/C++) functions is using JNI, but it’s very awkward. JNA and JNR are much easier to use. This project would design and implement an easy-to-use Kawa wrapper for for JNR. You should study existing JNR wrappers, such as that for JRuby. Difficulty: Medium. Need to study existing wrappers and "foreign function interfaces" (in multiple languages) and design one suitable for Kawa. Some Scheme (Kawa) experience would be helpful.

Types for units


Kawa supports units (such as cm^2 for square centimeters) and quantities (such as 4cm^2). We would like to integrate these into the type system, both for performance and compile-time type checking.

For syntax we can use a pseudo-parameterized type quantity. For example:

(define a1 ::quantity[cm^2] 4cm^2)
(* 2.0 a1) ;; ⇒ 8cm^2
(+ 2.0 a1) ;; compile-time error

The run-time type of the variable a1 should be a primitive double, without object allocation. Of course when a1 is converted to an object, we create a Quantity, not a Double. We can build on Kawa’s existing framework for non-standard primitive types such as character and ulong. Skills: Need good Java experience, and somewhat familiar with the Java Virtual Machine. You will need to become comfortable reading javap output. Difficulty: Modest.

Compiler should use class-file reading instead of reflection

The Kawa compiler currently uses reflection to determine properties (such as exported function definitions) from referenced classes. It would be better to read class files. This should not be too difficult, since the gnu.bytecode library abstracts over class information read by reflection or class reading.

Mutually dependent Java and Scheme modules

(GSoC - maybe)

We’d like a command for compiling a list of Java and Scheme source files that may have mutual dependencies. A good way to do this is to hook into javac, which is quite extensible and pluggable.

One could do something like:

  1. Read the “header" of each Kawa source file, to determine the name of the generated main class.

  2. Enter these class names into the javac tables as “uncompleted” classes.

  3. Start compiling the Java files. When this requires the members of the Kawa classes, switch to the Kawa files. From javac, treat these as pre-compiled .class files. I.e. we treat the Kawa compiler as a black box that produces Symbols in the same way as reading class files. At this point we should only need the initial “scan” phase on Kawa.

  4. If necessary, finish compiling remaining Kawa files.

This approach may not immediately provide as robust mixed-language support as is ideal, but it is more amenable to incremental improvement than a standalone stub-generator.

This project is good if you know or want to learn how javac works.

Use Java-7 MethodHandles and invokedynamic

Java 7 supports MethodHandles which are meant to provide better performance (ultimately) for dynamic languages. See JSR 292 and the Da Vinci Machine Project. Kawa makes limited use of MethodHandles, and no use of invokedynamic. There is more to be done. For example, we can start by optimizing arithmetic when the types are unknown at compile-time. They could make implementing generic functions (multimethods) more efficient. At some point we want to compile lambdas in the same way as Java 8 does. This can potentially be more efficient than Kawa’s current mechanism.

Remi Forax’s vmboiler is a small library on top of ASM that generates optimistically typed bytecodes. It could be useful for ideas.

Parameterized types


Kawa has some limited support for parameterized types, but it’s not used much. Improve type inferencing. Support definition of parameterized classes. Better use of parameterized types for sequence class. Support wildcards. (It might be better to have wild-carding be associated with declarations, as in Scala or proposed for Java, rather than uses.) See also

Optimized function types and values using MethodHandles


Kawa doesn’t have true function types: Parameter and result types are only handled for “known” functions. The general case with optional and keyword parameter is complicated, but simple fixed-arity procedure types would be very useful.

The following syntax is suggested:

procedure[(T1 .. Tn) Tr]

T1 through T1 are types of the parameters, and Tr is the type of the result. For example: procedure[(vector int) string]. We call this a typed-procedure type (in contrast to plain procedure).

If a value has a typed-procedure type then its run-time representation is a just a MethodHandle. If such a procedure is called, the generated bytecode is to just call its invokeExact method. The argument expressions are converted (and type-checked) the same way as if we were calling a statically-known procedure.

Note that passing an int argument of to procedure[(vector int) string] value does not require allocating an object to “box” the int; we can pass a plain int as-is. Thus using typed-procedure types can lead to major speed-up. For example the lib-test.scm should become much faster.

Converting a known procedure to a typed-procedure type is usually just a matter of creating a MethodHandle that references the method implementing the procedure. Some glue code may be needed if the types aren’t identical, or if the procedure is a closure.

Converting a type-procedure value p to generic value (such as untyped procedure or object) can be though of as wrapping it in a lambda:

((lambda (arg1::vector arg2::int)::string (p arg1 arg2))

Coercing a generic value or an untyped procedure to a typed-procedure would need to generate a method whose signature matches the typed-procedure type, and in the body of the method use a generic apply.

Coercing from one typed-procedure type to a different typed-procedure type is a combination of the above techniques (as if converting first to object and then to the target type), though some optimizations are worth doing.

Adding varargs support can be done later.

We need a fall-back mechanism for platforms (such as Android) that don’t support MethodHandles. The easiest is to just treat a typed-procedure type as plain procedure at run-time, though we still want the compile-time type-checking,

Full continuations

Currently being worked on.

Add support for full continuations, which is the major feature missing for Kawa to qualify as a “true Scheme”. One way to implement continuations is to add a add that converts the abstract syntax tree to continuation-passing-style, and then expand the existing full-tail-call support to manage a stack. There are other ways to solve the problem. This may benefit from Faster tailcalls.

Faster tailcalls

Make --full-tailcalls run faster. This may depend on (or incorporate) TreeList-optimization.


The TreeList class is a data structure for “flattened” trees. It is used for XML-style nodes, for multiple values, and for the full-tail-call API. The basic concept is fine, but it could do with some re-thinking to make make random-access indexing fast. Also, support for updating is insufficient. (This needs someone into designing and hacking on low-level data-structures, along with lots of profiling and testing.)

Asynchronous evaluation

C# recently added asynch and await keywords for asynchronous programming. Kawa’s recently improved support for lazy programming seems like a good framework for equivalent functionality: Instead of an asynch method that returns a Task<T>, the Kawa programmer would write a function that returns a lazy[T]. This involves some design work, and modifying the compiler to rewrite the function body as needed.

This is related to full continuations, as the re-writing is similar.

REPL console and other REPL improvement

Currently being worked on.

Improvements to the read-eval-print console. In addition to a traditional Swing console, it would be useful to support using a web browser as a remote terminal, possibly using web-sockets. (This allows “printing” HTML-expressions, which can be a useful way to learn and experiment with web technologies.) See here for an article on the existing Swing REPL, along with some to-do items. Being able to hide and show different parts of the output might be nice. Being able to link from error messages to source might be nice. Better handling of redefinitions is discussed here in the context of JavaXF Script; this is a general REPL issue, mostly independent of the GUI for it.

An interesting possibility is to use the IPython framework. There are existing ports for Scala: either IScala or Scala Notebook.

XQuery-3.0 functionality

(GSoC, for some subset)

It would be nice to update the XQuery (Qexo) support to some subset of XQuery 3.0.


It would be nice to support XQuery updates. This depends on TreeList-optimization.

Common Lisp support

Kawa supports a small subset of the Common Lisp language, but it supports a much larger subset of core Common Lisp concepts and data structures, some designed with Common Lisp functionality in mind. Examples include packages, arrays, expanded function declarations, type specifications, and format. A lot could be done to improve the Common Lisp support with modest effort. Some Common Lisp features could also be useful for Scheme: Documentation strings (or markup) as Java annotations, better MOP-like introspection, and generic methods a la defmethod (i.e. with multiple definition statements, possibly in separate files, as opposed to the current make-procedure) all come to mind. Being able to run some existing Common Lisp code bases with at most modest changes should be the goal. One such package to start with might be an existing test framework, perhaps FivaAM. Full Common Lisp compatibility is nice, but let’s walk before we can run.

JEmacs improvements

(GSoC, for some subset)

A lot of work is needed to make JEmacs useful. One could try to import a useful package and see what works and what fails. Or one may look at basic editing primitives. Enhancements may be needed to core Emacs Lisp language primitives (enhancing Common Lisp support may help), or to the display engine.

Emacs now supports lexical bindings - we should do the same.

Improved IDE integration

There is some Kawa support for Eclipse (Schemeway), and possibly other IDEs (NetBeans, IntelliJ). But many improvements are desirable. REPL improvements may be a component of this.

Plugin for NetBeans IDE

Kawa-Scheme support for the NetBeans IDE would be useful. One could perhaps build on the Clojure plugin.

Plugin for Eclipse IDE

Kawa-Scheme support for the Eclipse IDE would be useful. Probably makes sense to enhance SchemeWay. It may also make sense to build on the Dynamic Languages Toolkit, possibly making use of Schemeide, though DLTk seems more oriented towards interpreted non-JVM-based languages.

Improve Emacs integration

SLIME is an Emacs mode that provides IDE-like functionality. It supports Kawa.

JDEE is a Java development environment, so might have better hooks to the JVM and Java debugging architecture.

CEDET is a more general framework of development tools.

Hop-style web programming

Hop is an interesting design for integrating server-side and client-side programming using a Scheme dialect. These ideas seem like they would port quite well to Kawa.

String localization


Support localization by extending the SRFI_109 syntax, in the manner of (and compatible with) GNU gettext. I.e. optionally specify a localization key (to use as an index in the translation database); if there is no key specified, default to using the literal parts of the string.

Data binding

Implement a “bind” mechanism similar to that of JavaFX Script. The idea is that when you initialize a variable or field, instead of initializing it to a fixed value, you bind it to an expression depending on other variables. We install “listeners” on those variables, so when those variables change, we update the bound variable. This feature is useful in many applications, but the initial focus could be GUI programming and perhaps web programming.

Decimal arithmetic and repeated decimals

(GSoC. Possibly a bit light for a full Summer project, but can be extended or combined with other projects.)

Exact decimal arithmetic is a variation of exact rational arithmetic, but may be more user-friendly. In particular, printing using decimals is generally nicer than fractions. It is also sometimes useful to specify an explicit scale, so we can distinguish 0.2 from 0.20. We can use the Java BigDecimal class, but run into problems with division - for example (/ 1.0 3.0). We should implement a subclass of RatNum that generalizes BigDecimal to also handle repeating decimals. We need a lexical syntax for repeating decimals. Possible ideas: 0._81_ or 0.#81. If a Scheme number literal is specified as exact and has either a decimal point or an exponent (for example #e1.25), then it should read as an exact decimal, not a fraction.

Optional strict typing along with an explicit dynamic type


Kawa currently implements “optimistic” typing: The compiler only complains if an expression has no values in common with the target type - for example, if assigning a string expression to an integer variable. It would be interesting to experiment with a --strict-typing option (which would never be the default): Strict typing would only allow “widening” conversions - i.e. that the expression type be a subtype of the target type. For example it would complain if assigning a number to an integer unless you used an explicit cast.

To make this easier to work with we’d make use of the dynamic type, similar to what C# does: Any expression can be converted to or from dynamic without the compiler complaining. Similarly, if x is dynamic then x:name is allowed by the compiler regardless of name, with all checking being deferred to run-time. If a variable is declared without a type, it should default to dynamic. The dynamic type is represented in the VM as object but with an annotation (like we do with character).

The type-checker might need some changes to better distinguish implicit conversions from explicit casts.