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kawa-tour
*********

Kawa:  Compiling Scheme to Java by Per Bothner <bothner@cygnus.com>,
Cygnus Solutions

   Abstract:

   Kawa is an implementation of the Scheme programming language written
entirely in Java.  It compiles Scheme procedures into Java bytecode
classes, either on-the-fly, or in batch mode for later use.  Kawa also
includes a full Scheme run-time environment written in an
object-oriented style, with class hierarchies for collections,
procedures, and the full numeric "tower."

   This paper gives an overview of the features of Kawa.  It
concentrates on the compiler, and how Scheme values are represented by
Java objects.  The paper is structured as a "tour" of the Java classes
used in the implementation.

* Menu:

* Introduction::
* Background::
* Objects and Values::
* Symbols::
* Numbers::
* Mixed-type arithmetic::
* Quantities::
* Collections::
* Procedures::
* Overview of compilation::
* Environments:: Top-level environments
* Expressions::
* Translation::
* Syntax and Macros::
* Code generation::
* Codegen:: The `codegen' package
* Literals::
* Low-level procedures::
* Continuations::
* Tail-calls::
* Types:: Classes, types, and declarations::
* Future:: Current and Future Work
* Conclusion::
* Biographical Information::


File: kawa-tour.info,  Node: Introduction,  Next: Background,  Prev: Top,  Up: Top

Introduction
============

While Java is a decent programming language, its success is largely due
to using portable bytecodes, and its integration into secure web
browsers.  Many prefer other programming languages.  If they are
implemented on top of Java, programmers can gain many of the
extra-linguistic benefits of Java, including libraries, portable
bytecodes, web applets, and the existing efforts to improve Java
implementations and tools.

   Scheme is a simple yet powerful language.  It is a non-pure
functional language (i.e. it has first-class functions, lexical
scoping, non-lazy evaluation, and side effects).  It has dynamic
typing, and usually has an interactive read-evaluate-print interface.
The dynamic nature of Scheme (run-time typing, immediate expression
evaluation) may be a better match for the dynamic Java environment
(interpreted bytecodes, dynamic loading) than Java is!

   _Note:_  Kawa is also the trademarked name of a commercial Java
development environment.  To avoid confusion, we may soon change the
name of the system described here, but in any case, information and
source code will be available from
`http://www.cygnus.com/~bothner/kawa.html'.


File: kawa-tour.info,  Node: Background,  Next: Objects and Values,  Prev: Introduction,  Up: Top

Background
==========

Starting in 1995 Cygnus (on behalf of the Free Software Foundation)
developed Guile, an implementation of Scheme suitable as a general
embedding and extension language.  Guile was based on Aubrey Jaffar's
SCM interpreter;  the various Guile enhancements were initially done by
Tom Lord.  In 1995 we got a major contract to enhance Guile, and with
our client we added more features, including threads (primarily done by
Anthony Green), and internationalization.

   The contract called for a byte-code compiler for Guile, and it
looked like doing a good job on this would be a major project.  One
option we considered was compiling Scheme into Java bytecodes and
executing them by a Java engine.  The disadvantage would be that such a
Scheme system would not co-exist with Guile (on the other hand, we had
run into various technical and  non-technical problems with Guile that
led us to conclude that Guile would after all not be strategic to
Cygnus).  The advantage of a Java solution was leveraging off the tools
and development being done in the Java "space", plus that Java was more
likely to be strategic long-term.

   The customer agreed to using Java, and I started active development
June 1996.  Kawa 1.0 was released to our customer and "the Net"
September 1996.  Development has continued since then, at a less
intense pace!  The long-term goal is an object-oriented environment
that harmoniously integrates Scheme, Java, and other languages.

   As a base, I used the Kawa Scheme interpreter written by R.
Alexander Milowski.  He needed an object-oriented Scheme interpreter to
implement DSSSL `International Standard ISO/IEC 10179:1996(E),
Document Style Semantics and Specification Language. 1996', a
Scheme-like environment for expressing style, formatting, and other
processing of SGML documents `International Standards Organization:
Standard Generalized Markup Language, ISO 8879'.  DSSSL is an subset of
"pure" Scheme with some extensions.  Kawa 0.2 was a simple interpreter
which was far from complete.  It provided a useful starting point, but
almost all of the original code has by now been re-written.

   There are three basic ways of implementing Scheme in Java.
  1. One could write an interpreter in Java, but this has poor
     performance.

  2. One could translate into Java source code.  However, invoking an
     external Java source-to-bytecode compiler is too slow except for
     batch compilation.  Also, some features of Java bytecodes (such as
     goto) are not available in the Java language.

  3. One could translate into Java bytecodes, either ahead of time (a
     batch compiler), or for immediate execution.

   We have implemented the last option for Kawa, partly because that
was what our contract called for, but also because it is the best
solution.  To generate bytecodes, we use a new "`codegen'" package,
which is an intermediate-level toolkit to generate Java bytecodes and
"`.class'" files.

   We will discuss the compiler later, but first we will give an
overview of the run-time environment of Kawa, and the classes used to
implement Scheme values.


File: kawa-tour.info,  Node: Objects and Values,  Next: Symbols,  Prev: Background,  Up: Top

Objects and Values
==================

Java `J. Gosling, B. Joy, G.~Steele:   The Java Language Specification,
Addison-Wesley, 1996' has primitive types (such as 32-bit `int') as well
reference types.  If a variable has a reference type, it means that it
can contains references (essentially pointers) to objects of a class,
or it can contain references to objects of classes that "extend"
(inherit from) the named class.  The inheritance graph is "rooted"
(like Smalltalk and unlike C++); this means that all classes inherit
from a distinguished class `java.lang.Object' (or just `Object' for
short).

   Standard Scheme `W. Clinger and J. Rees (editors):  Revised 4 report
on the algorithmic language Scheme.   LISP Pointers, 4(3):1-55, 1991.'
has a fixed set of types, with no way of creating new types.  It has
run-time typing, which means that types are not declared, and a
variable can contain values of different types at different times.  The
most natural type of a Java variable that can contain any Scheme value
is therefore `Object', and all Scheme values must be implemented using
some class that inherits from `Object'.

   The task then is to map each Scheme type into a Java class.  Whether
to use a standard Java class, or to write our own is a tradeoff.  Using
standard Java classes simplifies the passing of values between Scheme
functions and existing Java methods.  On the other hand, even when Java
has suitable built-in classes, they usually lack functionality needed
for Scheme, or are not organized in any kind of class hierarchy as in
Smalltalk or Dylan.  Since Java lacks standard classes corresponding to
pairs, symbols, or procedures, we _have_ to write some new classes, so
we might as well write new classes whenever the existing classes lack
functionality.  One extreme would be to define a new `SchemeObject'
class, and derive from it classes for all Scheme values.  While this
might make implementing Scheme easier, Kawa does not go that far,
partly because we want to allow Scheme variables to contain arbitrary
Java objects.

   The Scheme boolean type is one where we use a standard Java type, in
this case `Boolean' (strictly speaking `java.lang.Boolean').  The
Scheme constants `#f' and `#t' are mapped into static fields (i.e.
constants) `Scheme.falseObject' and `Scheme.trueObject'.  These are
initialized to `new Boolean(false)' and `new Boolean(true)',
respectively, when Kawa starts up.

   On the other hand, numbers and collections are reasonably organized
into class hierarchies, which Java does not do well.  So Kawa has its
own classes for those, discussed in the following sections.


File: kawa-tour.info,  Node: Symbols,  Next: Numbers,  Prev: Objects and Values,  Up: Top

Symbols
=======

Symbols represent names, and do not need much functionality.  Scheme
needs to be able to convert them to and from strings, and they need to
be "interned" (which means that there is a global table to ensure that
there is a unique symbol for a given string).  Symbols are immutable
and have no accessible internal structure.  Currently, Scheme symbols
are implemented using a Kawa-specific `Symbol' class.

   I am planning to re-implement symbols to use the the standard Java
`String' class, which supports an `intern' operator.  Note that the
Java `String' class implements immutable strings, and is therefore
cannot be used to implement Scheme strings.  However, it makes sense to
use it to implement symbols, since the way Scheme symbols are used is
very similar to how Java `String's are used.  While Kawa currently
assumes a Java implementation corresponding to version 1.0 of JDK
(Sun's Java Development Kit), a future version will depend on JDK 1.1
features.  One such new 1.1 feature is that literal Java `String's will
be required to be automatically interned.  This will make it even more
appealing to use `String' to implement Scheme symbols.


File: kawa-tour.info,  Node: Numbers,  Next: Mixed-type arithmetic,  Prev: Symbols,  Up: Top

Numbers
=======

Scheme defines a "numerical tower" of numerical types: number, complex,
real, rational, and integer.  Kawa implements the full "tower" of
Scheme number types, which are all sub-classes of the abstract class
`Quantity' discussed in *Note Quantities::.

     class Complex extends Quantity {
       public abstract RealNum re ();
       public abstract RealNum im ();
       ...
     }

   `Complex' is the class of abstract complex numbers.  It has three
subclasses:  the abstract class `RealNum' of real numbers;  the general
class `CComplex' where the components are arbitrary `RealNum' fields;
and the optimized `DComplex' where the components are represented by
`double' fields.

     class RealNum extends Complex {
       public final RealNum re ()
       { return this; }
       public final RealNum im ()
       { return IntNum.zero(); }
       public abstract boolean isNegative ();
       ...
     }

     class DFloNum extends RealNum {
       double value;
       ...
     }

   Concrete class for double-precision (64-bit) floating-point real
numbers.

     class RatNum extends RealNum {
       public abstract IntNum numerator();
       public abstract IntNum denominator();
       ...
     }

   `RatNum', the abstract class for exact rational numbers, has two
sub-classes:  `IntFraction' and `IntNum'.

     class IntFraction extends RatNum {
       IntNum num;
       IntNum den;
       ...
     }

   The `IntFraction' class implements fractions in the obvious way.
Exact real infinities are identified with the fractions `1/0' and
`-1/0'.

     class IntNum extends RatNum {
       int ival;
       int[] words;
       ...
     }

   The `IntNum' concrete class implements infinite-precision integers.
The value is stored in the first `ival' elements of `words', in 2's
complement form (with the low-order bits in `word[0]').

   There are already many bignum packages, including a couple written
in Java.  What are the advantages of this one?
   * A complete set of operations, including gcd and lcm;  logical, bit,
     and shift operations; power by repeated squaring; all of the
     division modes from Common Lisp (floor, ceiling, truncate, and
     round); and exact conversion to `double'.

   * Consistency and integration with a complete "numerical tower."
     Specifically, consistency and integration with "fixnum" (see
     below).

   * Most bignum packages use a signed-magnitude representation, while
     Kawa uses 2's complement.  This makes for easier integration with
     fixnums, and also makes it cheap to implement logical and
     bit-fiddling operations.

   * Use of all 32 bits of each "big-digit" word, which is the
     "expected" space-efficient representation.  More importantly, it
     is compatible with the `mpn' routines from the Gnu Multi-Precision
     library (`gmp' by T. Granlund).  The `mpn' routines are low-level
     algorithms that work on unsigned pre-allocated bignums;  they have
     been transcribed into Java in the `MPN' class.  If better
     efficiency is desired, it is straight-forward to replace the `MPN'
     methods with native ones that call the highly-optimized `mpn'
     functions.

   If the integer value fits within a signed 32-bit `int', then it is
stored in `ival' and `words' is null.  This avoids the need for extra
memory allocation for the `words' array, and also allows us to
special-case the common case.

   As a further optimization, the integers in the range -100 to 1024
are pre-allocated.


File: kawa-tour.info,  Node: Mixed-type arithmetic,  Next: Quantities,  Prev: Numbers,  Up: Top

Mixed-type arithmetic
=====================

Many operations are overloaded to have different definitions depending
on the argument types.  The classic examples are the functions of
arithmetic such as `+', which needs to use different algorithms
depending on the argument types.  If there is a fixed and reasonably
small set of number types (as is the case with standard Scheme), then
we can just enumerate each possibility.  However, the Kawa system is
meant to be more extensible and support adding new number types.

   The solution is straight-forward in the case of a one-operand
function such as "negate", since we can use method overriding and
virtual method calls to dynamically select the correct method.
However, it is more difficult in the case of a binary method like `+',
since classic object-oriented languages (including Java) only support
dynamic method selection using the type of the first argument
("`this'").  Common Lisp and some Scheme dialects support dynamic method
selection using all the arguments, and in fact the problem of binary
arithmetic operations is probably the most obvious example where
"multi-dispatch" is useful.

   Since Java does not have multi-dispatch, we have to solve the
problem in other ways.  Smalltalk has the same problems, and solved it
using "coercive generality":  Each number class has a generality
number, and operands of lower generality are converted to the class
with the higher generality.  This is inefficient because of all the
conversions and temporary objects (see `T. A. Budd:   Gneralized
arithmetic in C++.    Journal of Object-Oriented Programming,
3(6):11-22, Feb. 1991', and it is limited to what extent you can add
new kinds of number types.

   In "double dispatch" `D. Ingalls:   A simple technique for handling
multiple polymorphism.    ACM SIGPLAN Notices, 21(11):347-349, Nov.
1986.', the expression `x-y' is implemented as `x.sub(y)'.  Assuming
the (run-time) class of `x' is `Tx' and that of `y' is `Ty', this
causes the `sub' method defined in `Tx' to be invoked, which just does
`y.subTx(x)'.  That invokes the `subTx' method defined in `Ty' which
can without further testing do the subtraction for types `Tx' and `Ty'.

   The problem with this approach is that it is difficult to add a new
`Tz' class, since you have to also add `subTz' methods in all the
existing number classes, not to mention `addTz' and all the other
operations.

   In Kawa, `x-y' is also implemented by `x.sub(y)'.  The `sub' method
of `Tx' checks if `Ty' is one of the types it knows how to handle.  If
so, it does the subtraction and returns the result itself.  Otherwise,
`Tx.sub' does `y.sub_reversed(x)'.  This invokes `Ty.sub_reversed' (or
`sub_reversed' as defined in a super-class of `Ty').  Now `Ty' (or one
of its super-classes) gets a chance to see if it knows how to subtract
itself from a `Tx' object.

   The advantage of this scheme is flexibility.  The knowledge of how
to handle a binary operation for types `Tx' and `Ty' can be in either
of `Tx' or `Ty' or either of their super-classes.  This makes is easier
to add new classes without having to modify existing ones.


File: kawa-tour.info,  Node: Quantities,  Next: Collections,  Prev: Mixed-type arithmetic,  Up: Top

Quantities
==========

The DSSSL language is a dialect of Scheme used to process SGML
documents.  DSSSL has "quantities" in addition to real and integer
numbers.  Since DSSSL is used to format documents, it provides length
values that are a multiple of a meter (e.g. `0.2m'), as well as derived
units like `cm' and `pt' (point).  A DSSSL quantity is a product of a
dimension-less number with an integral power of a length unit (the
meter).  A (pure) number is a quantity where the length power is zero.

   For Kawa, I wanted to merge the Scheme number types with the DSSSL
number types, and also generalize the DSSSL quantities to support other
dimensions (such as mass and time) and units (such as `kg' and
seconds).  Quantities are implemented by the abstract class `Quantity'.
A "quantity" is a product of a `Unit' and a pure number.  The number
can be an arbitrary complex number.

     class Quantity extends Number {
       public Unit unit ()
       { return Unit.Empty; }
       public abstract Complex number ();
       ...
     }

     class CQuantity extends Quantity {
       Complex num;
       Unit unt;
       public Complex number ()
       { return num; }
       public Unit unit ()
       { return unt; }
       ...
     }

   A `CQuantity' is a concrete class that implements general
`Quantities'.  But usually we don't need that much generality, and
instead use `DQuanity'.

     class DQuantity extends Quantity {
       double factor;
       Unit unt;
       public final Unit unit ()
       { return unt; }
       public final Complex number ()
       { return new DFloNum(factor); }
       ...
     }

     class Unit extends Quantity {
       String name; // Optional.
       Dimensions dims;
       double factor;
       ...
     }

   A `Unit' is a product of a floating-point `factor' and one or more
primitive units, combined into a `Dimensions' object.  The `Unit' name
have a name (such as "`kg'"), which is used for printing, and when
parsing literals.

     class BaseUnit extends Unit {
       int index;
       ...
     }

   A `BaseUnit' is a primitive unit that is not defined in terms of any
other `Unit', for example the meter.  Each `BaseUnit' has a different
`index', which is used for identification and comparison purposes.  Two
`BaseUnit's have the same `index' if and only if they are the same
`BaseUnit'.

     class Dimensions {
       BaseUnit[] bases;
       short[] powers;
       ...
     }

   A `Dimensions' object is a product and/or ratio of `BaseUnit's.  You
can think of it as a data structure that maps every `BaseUnit' to an
integer power.  The `bases' array is a list of the `BaseUnit's that
have a non-zero power, in order of the `index' of the `BaseUnit'.  The
`powers' array gives the power (exponent) of the `BaseUnit' that has
the same index in the `bases' array.

   Two `Dimensions' objects are equal if they have the same list of
`bases' and `powers'.  `Dimensions' objects are "interned" (using a
global hash table) so that they are equal only if they are the same
object.  This makes it easy to implement addition and subtraction:

       public static DQuantity add (DQuantity x, DQuantity y)
       {
         if (x.unit().dims != y.unit().dims)
           throw new ArithmeticException ("units mis-match");
         double r = y.unit().factor / x.unit().factor;
         double s = x.factor + r * y.factor;
         return new DQuantity (s, x.unit());
       }

   The `Unit' of the result of an addition or subtraction is the `Unit'
of the first operand.  This makes it easy to convert units:
     kawa> (+ 0cm 2.5m)
     250cm

   Because Kawa represents quantities relative to user-specified units,
instead of representing them relative to primitive base units, it can
automatically print quantities using the user's preferred units.
However, this does make multiplication and division more difficult.
The actual calculation (finding the right `Dimensions' and multiplying
the constant factors) is straight-forward.  The problem is generating
the new compound unit, and later printing out the result in a
human-friendly format.  There is no obvious right way to do this.  Kawa
creates a `MulUnit' to represent a compound unit, but it is not obvious
which simplifications should be done when.  Kawa uses a few heuristics
to simplify compound units, but this is an area that could be improved.


File: kawa-tour.info,  Node: Collections,  Next: Procedures,  Prev: Quantities,  Up: Top

Collections
===========

Kawa has a rudimentary hierarchy of collection classes.

     class Sequence {
       abstract public int length();
       abstract public Object elementAt (int index);
       ...
     }

   A `Sequence' is the abstract class that includes lists, vectors, and
strings.

     class FString extends Sequence {
       char[] value;
       ...
     }

   Used to implement fixed-length mutable strings (array of Unicode
character).  This is used to represent Scheme strings.

     class FVector extends Sequence {
       Object[] value;
       ...
     }
   Used to implement fixed-length mutable general one-dimensional array
of `Object'.  This is used to represent Scheme vectors.

     class List extends Sequence {
       protected List () { }
       static public List Empty = new List ();
       ...
     }

   Used to represent Scheme (linked) lists.  The empty list `'()' is
the static (global) value `List.Empty'.  Non-empty-lists are
implemented using `Pair' objects.

     class Pair extends Sequence {
       public Object car;
       public Object cdr;
       ...
     }

   Used for Scheme pairs.

     class PairWithPosition extends Pair {
       ...
     }

   Like `Pair', but includes the filename and linenumber in the file
from which the pair was read.

   Future plans include more interesting collection classes, such a
sequences implemented as a seekable disk file; lazily evaluated
sequences; hash tables;  APL-style multi-dimensional arrays; stretchy
buffers.(1)

   ---------- Footnotes ----------

   (1) Many of 	these ideas were implemented in my earlier
experimental 	language `Q' - see `P. Bothner:   Efficiently
Combining Logical Constraints with Functions.    PhD thesis, Department
of Computer Science, Stanford University, 1988' and
`ftp://ftp.cygnus.com/pub/bothner/Q/'.


File: kawa-tour.info,  Node: Procedures,  Next: Overview of compilation,  Prev: Collections,  Up: Top

Procedures
==========

Scheme has procedures as first-class values.  Java does not.  However,
we can simulate procedure values, by overriding of virtual methods.

     class Procedure {
       public abstract Object applyN (Object[] args);
       public abstract Object apply0 ();
       public abstract Object apply1 (Object arg1);
       ...;
       public abstract Object apply4 (Object arg1, ..., Object arg4);
       ...
     }

   We represent Scheme procedures using sub-classes of the abstract
class `Procedure'.  To call (apply) a procedure with no arguments, you
invoke its `apply0' method;  to invoke a procedure, passing it a single
argument, you use its `apply1' method; and so on using `apply4' if you
have 4 arguments.  Alternatively, you can bundle up all the arguments
into an array, and use the `applyN' method.  If you have more than 4
arguments, you have to use `applyN'.

   Notice that all `Procedure' sub-classes have to implement all 6
methods, at least to the extent of throwing an exception if it is
passed the wrong number of arguments.  However, there are utility
classes `Procedure0' to `Procedure4' and `ProcedureN'.

     class Procedure1 extends Procedure {
       public abstract Object applyN (Object[] args)
       {
         if (args.length != 1)
           throw new WrongArguments();
         return apply1(args[0]);
       }
       public Object apply0 ()
        { throw new WrongArguments(); }
       public abstract Object apply1 (Object arg1);
       public Object apply2 (Object arg1, Object arg2)
       { throw new WrongArguments();}
       ...
     }

   Primitive procedures are generally written in Java as sub-classes of
these helper classes. For example:

     class cdr extends Procedure1 {
       public Object apply1 (Object arg1)
        { return ((Pair) arg1).cdr; }
     }

   A user-defined Scheme procedure is compiled to a class that is
descended from `Procedure'.  For example, a variable-argument procedure
is implemented as a sub-class of `ProcedureN', with an `applyN' method
comprising the bytecode compiled from the Scheme procedure body.  Thus
primitive and user-defined procedure have the same calling convention.

   If a nested procedure references a lexical variable in an outer
procedure, the inner procedure is implemented by a "closure".  Kawa
implements a closure as a `Procedure' object with a "static link" field
that points to the inherited environment.  In that case the lexical
variable must be heap allocated, but otherwise lexical variables use
local Java variable slots.  (This is conceptually similar to the "Inner
classes" proposed for JDK 1.1.)

     class ModuleBody extends Procedure0 {
       public Object apply0 ()
       {return run(Environment.current()); }
       public abstract Object run (Environment env);
       ...
     }

   Top-level forms (including top-level definitions) are treated as if
they were nested inside a dummy procedure.  A `ModuleBody' is such a
dummy procedure.  When a file is `load'ed, the result is a `ModuleBody';
invoking `run' causes the top-level actions to be executed.


File: kawa-tour.info,  Node: Overview of compilation,  Next: Environments,  Prev: Procedures,  Up: Top

Overview of compilation
=======================

These are the stages of compilation:
*Reading*
     The first compilation stage reads the input from a file, from a
     string, or from the interactive command interpreter.  The result
     is one or more Scheme forms (S-expressions), usually lists.  If
     reading commands interactively, only a single form is read; if
     reading from a file or string, all the forms are read until
     end-of-file or end-of-string;  in either case, the result is
     treated as the body of a dummy function (i.e. a `ModuleBody').

*Translation*
     The source form body is rewritten into an `Expression' object,
     specifically a `ModuleExp'.  This stage handles macro expansion
     and lexical name binding.  Many (future) optimizations can be done
     in this phase by annotating and re-arranging `Expression' trees.

*Code generation*
     The resulting `ModuleExp' is compiled into one or more byte-coded
     classes.  This is done by invoking the virtual `compile' method
     recursively on the `Expression's, which generates instructions
     (using the `codegen' package) to evaluate the expression and leave
     the result on the Java operand stack.  At the end we ask the
     `codegen' package to write out the resulting classes and methods.
     They can be written to a file (for future use), or into byte
     arrays in memory.

*Loading*
     The compiled bytecodes are loaded into the Kawa interpreter.  In
     the case of code that is compiled and then immediately executed,
     the compiled code can be immediately turned into Java classes
     using the Java `ClassLoader' feature.  (That is how the
     read-eval-print loop works.)  An instance of the compiled
     sub-class of `ModuleBody' is created and `run', which normally
     produces various side-effects.


File: kawa-tour.info,  Node: Environments,  Next: Expressions,  Prev: Overview of compilation,  Up: Top

Top-level environments
======================

     class Environment {
       ...
     }

   An `Environment' is a mapping from symbols to bindings.  It contains
the bindings of the user top-level.  There can be multiple top-level
`Environments', and an `Environment' can be defined as an extension of
an existing `Environment'.  The latter feature is used to implement the
various standard environment arguments that can be passed to `eval', as
adopted for the next Scheme standard revision ("R5RS").  Nested
environments were also implemented to support threads, and fluid
bindings (even in the presence of threads).


File: kawa-tour.info,  Node: Expressions,  Next: Translation,  Prev: Environments,  Up: Top

Expressions
===========

     class Expression {
       public abstract Object eval (Environment env);
       public abstract void compile (Compilation comp, int flags);
       ...
     }

   The abstract `Expression' class represents partially processed
expressions.  These are in principle independent of the source language,
but in practice there are some Scheme assumptions.

   The `eval' method evaluates the `Expression' in the given
`Environment'.  The interactive command interface uses `eval' to
evaluate expressions typed by the user.  However, `eval' only support
"simple" expressions, such as literals, identifiers, and applications.
Expressions that define new local bindings (such lambda expressions and
`let' forms) do not implement `eval'.  If the user types in such an
expression, it is wrapped inside a dummy function, compiled, and
immediately executed.  This is to avoid dealing with lexical binding in
the evaluator.  (We could compile _all_ user expressions, but that
entails a certain amount of overhead.  Code generation creates new
classes, and JDK 1.0 does not garbage-collect unused classes.)

   The `compile' method is called when we are compiling the body of a
procedure.  It is responsible for generating bytecodes that evaluate
the expression, and leave the result on the Java evaluation stack.

     class QuoteExp extends Expression {
     Object value;
       public QuoteExp (Object val)
       { value = val; }
       public Object eval (Environment env)
       { return value; }
       public void compile (Compilation comp, int flags)
       { comp.compileConstant (value); }
       ...
     }

   A `QuoteExp' represents a literal (self-evaluating form), or a
quoted form.

     class ReferenceExp extends Expression {
       Symbol symbol;
       Declaration binding;
       ...
     }

   A `ReferenceExp' is a reference to a named variable.  The `symbol'
is the source form identifier.  If `binding' is non-null, it is the
lexical binding of the identifier.

     class ApplyExp extends Expression {
       Expression func;
       Expression[] args;
       ...
     }

   An `ApplyExp' is an application of a procedure `func' to an argument
list `args'.

     class ScopeExp extends Expression {
       ScopeExp outer; // Surrounding scope.
       public Declaration add_decl (Symbol name)
       { Create new local variable. }
     ...
     }

   A `ScopeExp' is a abstract class that represents a lexical scoping
construct.  Concrete sub-classes are `LetExp' (used for a `let' binding
form) and `LambdaExp'.

     class LambdaExp extends ScopeExp {
       Symbol name; // Optional.
       Expression body;
       int min_args;
       int max_args;
       ...
     }

   The Scheme primitive syntax `lambda' is translated into a
`LambdaExp', which represents anonymous procedures.  Each `LambdaExp'
is compiled into a different bytecoded class.  Invoking `eval' causes
the `LambdaExp' to be compiled into a class, the class to be loaded,
and instance of the class to be created, and the result coerced to a
`Procedure'.

   Other sub-classes of `Expression' are `IfExp' (used for conditional
expressions); `BeginExp' (used for compound expressions); `SetExp'
(used for assignments);  and `ErrorExp' (used to mark code that has a
syntax error);


File: kawa-tour.info,  Node: Translation,  Next: Syntax and Macros,  Prev: Expressions,  Up: Top

Translation
===========

The translation phase takes a top-level form (or body), and generates a
`ModuleExp', which is a top-level expression.  This is done using a
`Translator', which keeps track of lexical bindings and other
translation state.

     class Translator {
       public Expression rewrite (Object exp)
       { ... }
       public Expression syntaxError (String message)
       { ... }
       ...
     }

   The `rewrite' method converts a Scheme source form to an
`Expression'.  The `syntaxError' method is called when a syntax error
is seen.  It prints out the current source filename and line number
with the given `message'.

   A `ModuleExp' represents a top-level form.

     class ModuleExp extends LambdaExp {
       public Object eval_module (Environment env)
       {
         if (body_is_simple) // Optimization
           return body.eval (env);
         Object v = eval (env);
         return ((ModuleBody) v).run (env);
       }
       ...
     }

   `ModuleExp' is a sub-class of `LambdaExp', since it is actually a
dummy function created by wrapping the top-level forms in an implicit
`lambda'.  The `eval_module' method evaluates the top-level forms.  It
invokes the `eval' in `LambdaExp' (which invokes the compiler).  The
result of `eval' is a `ModuleBody' (*note Procedures::), which we can
`run'.  If the `body' is "simple" we don't bother actually evaluating
the `ModuleExp', since that entails compiling it to a bytecoded class;
we just `eval' the `body' instead.


File: kawa-tour.info,  Node: Syntax and Macros,  Next: Code generation,  Prev: Translation,  Up: Top

Syntax and Macros
=================

     class Syntax {
       public abstract Expression rewrite (Object obj, Translator tr);
       ...
     }

   The `rewrite' method in `Translator' checks for syntactic keywords
and macros.  If the `car' of a "call" is a `Syntax' or if it is a
`Symbol' that is bound to a `Syntax', then its `rewrite' method is
called.

   As an example, this trivial class implements `quote'.

     class quote extends Syntax {
       public Expression rewrite (Object obj, Translator tr)
       {
         // Error-checking is left out.
         Object value = ((Pair)obj).car;
         return new QuoteExp(value);
       }
       ...
     }

   Much more complicated is the `Syntax' that implements
`define-syntax'.

     class define_syntax extends Syntax {
       public Expression rewrite (Object obj, Translator tr)
       {
         enter (new SyntaxRules (...));
       }
       ...
     }

   The result is a `SyntaxRules' object, which contains an encoded
representation of the patterns and templates in the `syntax-rules'.
This is in its own right a `Syntax' object.

     class SyntaxRules extends Syntax {
     SyntaxRule[] rules;
       public Expression rewrite (Object obj, Translator tr)
       {
         Object[] v = new Object[maxVars];
         for (int i = 0;  i < rules.length;)
           {
             SyntaxRule r = rules[i++];
             if (r.match (obj, v))
               return r.execute_template(v, tr);
           }
         return tr.syntaxError ("no matching syntax-rule");
       }
       ...
     }

   Contrast evaluating a procedure definition (`lambda'), which causes
a new _sub-class_ of `Procedure' to be created and compiled, while
evaluating a `define-syntax' only causes a new _instance_ of
`SyntaxRules' to be created.  This is because the `syntax-rules' can be
represented using relatively simple and compact data structures.  A
traditional low-level macro facility specifies the transformations using
executable code, and that probably would need a new `Procedure'
sub-class.


File: kawa-tour.info,  Node: Code generation,  Next: Codegen,  Prev: Syntax and Macros,  Up: Top

Code generation
===============

A `Compilation' object keeps track of the classes, methods, and
temporary state generated as a result of compiling a single top-level
`ModuleExp'.

     class Compilation {
       ClassType[] classes;
       boolean immediate;
       public ClassType addClass (LambdaExp lexp, String name)
       { ... }
       public ClassType (ModuleExp exp, ...)
       { ...; addClass (exp, ...); }
       ...
     }

   Each `Compilation' may create one or more `ClassType' objects, each
of which generates the bytecodes for one class.  Each `ClassType' is
generated from a `LambdaExp', including the top `ModuleExp'.  The
boolean `immediate' is true if we are compiling for immediate loading,
and is false if the target is one or more `.class' files.

   The `addClass' method does all the work to compile a given
`LambdaExp'.  It creates a `ClassType', adds it to `Compilation''s
`classes' array, and generates `Method' objects for the constructor and
the main `applyX' method.  Once the `applyX' `Method' has been created,
`addClass' emits some bytecodes to set up the incoming parameters, and
then invokes the virtual `compile' method on the body of the
`LambdaExp', which generates the code that does the actual work of the
procedure.

   The `Compilation' constructor gets a `ModuleExp', which it passes to
`addClass'.  The `compile' method of `LambdaExp' (which gets called for
all `lambda's except the dummy top-level) also calls `addClass' to
generate the class corresponding to the `lambda', and then it emits
instructions to create a new instance of the generated `Procedure'
class, and pushes it on the Java stack.


File: kawa-tour.info,  Node: Codegen,  Next: Literals,  Prev: Code generation,  Up: Top

The `codegen' package
=====================

The `ClassType' and `Method' classes are in a separate `codegen'
package, which is an intermediate-level interface to code generation
and Java `.class' files.  It is essentially independent of Scheme or
the rest of Kawa, and could be used for generating code for other
languages.

     class ClassType extends Type {
       CpoolEntry[] constant_pool;
       Method methods; // List of methods.
       Field fields; // List of fields.
       public Field new_field (String name, Type type, int flags)
       { Create new field. }
       public method new_method (String name, ...)
       { Create new method. }
       public void emit_to_stream (OutputStream stream)
       { ... }
       public void emit_to_file (String filename)
       { ... }
       public byte[] emit_to_array ()
       { ... }
       ...
     }

   The `ClassType' class is the main class of the `codegen' package.
It  manages a list `Field's , a list of `Method's, and the constant
pool.  There are utility methods for adding new fields, methods, and
constant pool entries.

   When the `ClassType' has been fully built, the `emit_to_file' method
can be used to write out the contents into a file.  The result has the
format of a `.class' file `T. Lindholm and F.~Yellin:   The Java
Virtual Machine Specification, Addison-Wesley, 1996'.  Alternatively,
the class can be written to an internal byte array (that has the same
layout as a `.class' file) using the `emit_to_array' method.  The
resulting byte array may be used by a `ClassLoader' to define a new
class for immediate execution.  Both of the these methods are
implemented on top of the more general `emit_to_stream'.

   The largest class in the `codegen' package is `Method'.

     class Method {
       Variable new_local (Type type, String name)
       { ... }
       public void compile_push_value (Variable var)
       { ... }
       public void compile_push_int (int i)
       { ... }
       public void compile_linenumber (int linenumber)
       { ... }
       ...
     }

   As an example of the level of functionality, `compile_push_int'
compiles code to push an integer I on stack.  It selects the right
instruction, and if I is too big for one of the instructions that take
an inline value, it will create a constant pool entry for `i', and push
that.

   The method `new_local' creates a new local variable (and makes sure
debugging information is emitted for it), while `compile_push_value'
pushes the value of the variable on the stack.

   Kawa calls `compile_linenumber' to indicate that the current location
corresponds to a given line number.  These are emitted in the `.class
file', and most Java interpreters will use them when printing a stack
trace.


File: kawa-tour.info,  Node: Literals,  Next: Low-level procedures,  Prev: Codegen,  Up: Top

Literals
========

A Scheme quoted form or self-evaluating form expands to a `QuoteExp'.
Compiling a `QuoteExp' would seem a trivial exercise, but it is not.
There is no way to embed (say) a list literal in Java code.  Instead we
create a static field in the top-level class for a each (different)
`QuoteExp' in the body we are compiling.  The code compiled for a
`QuoteExp' then just needs to load the value from the corresponding
static field.  The tricky part is making sure that the static field gets
initialized (when the top-level class is loaded) to the value of the
quoted form.

   The basic idea is that for:

     (define (foo) '(3 . 4))

   we compile:

     class foo extends Procedure0 {
       Object static lit1;
       public foo ()
       { // Initializer
         lit1 = new Pair(IntNum.make(3), IntNum.make(4));
       }
       public Object apply0 ()
       { return lit1; }
     }

   When the compiled class `foo' is loaded, we do:

     Class fooCl = Class.forName("foo");
     Procedure fooPr = (Procedure) fooCl.newInstance ();

     // Using foo:
     Object result = fooPr.apply0 ();

   How does the Kawa compiler generate the appropriate `new Pair'
expression as shown above?  A class whose instances may appear in a
quoted form implements the `Compilable' interface:

     interface Compilable {
       Literal makeLiteral (Compilation comp);
       void emit (Literal literal, Compilation comp);

   The `makeLiteral' creates a `Literal' object that represents the
value of `this' object.  That `Literal' is later passed to `emit',
which emits bytecode instructions that (when evaluated) cause a value
equal to `this' to be pushed on the Java evaluation stack.

   This two-part protocol may be overkill, but it makes it possible to
combine duplicate constants and it also supports circularly defined
constants.  (Standard Scheme does not support self-referential
constants, but Common Lisp does.  See section 25.1.4 `Similarity of
Constants' in    `G. L. Steele Jr.:    Common Lisp - The Language,
Digital Press and Prentice-Hall, second edition, 1990'.

   It is likely that the `Compilable' interface will be replaced in the
future with the serialization features of JDK 1.1.

   If we are compiling for immediate execution, we do not need to
generate code to regenerate the literal.  In fact, we want to re-use
the literal from the original source form.  The problem is passing the
source literal to the byte-compiled class.  To do that, we use the
`CompiledProc' interface.

     interface CompiledProc {
       public abstract void setLiterals (Object[] values);
     }

   An immediate class compiled from a top-level form implements the
`CompiledProc' form.  After an instance of the `ModuleBody' has been
created, it is coerced to a `CompiledProc', and `setLiterals' is
called.  The argument to `setLiterals' is an array of the necessary
literal values, and the method that implements it in the compiled code
causes the array of literal values to be saved in the `ModuleBody'
instance, so it can be accessed by the compiled code.


File: kawa-tour.info,  Node: Low-level procedures,  Next: Continuations,  Prev: Literals,  Up: Top

Low-level procedures
====================

Using a named global `Procedure' object has a bit of overhead.  We
refer to it by name, and finding the right `Procedure' is done by a run
time name lookup.  Once we have the procedure, we do a virtual method
call to apply the procedure, which is relatively fast, but we would
like to be able to inline known functions.  Also, in the future, we
want to support type declarations and the use of machine types such as
`int'.  Having to convert an `int' to an `IntNum' just because that is
what the apply interface requires is expensive.

   Another problem is that being able to call a Java method from Scheme
requires writing by hand a new `Procedure' subclass.  This is not a
friendly interface.

   The plan is to provide new syntax for defining Scheme procedures in
terms of Java methods.  The syntax will look something like:

     (define-virtual exec-process "java.lang.RunTime" "exec"
          "java.lang.Process" ("java.lang.String"))

     (set! ls (exec-process "ls -l"))

   The primitive syntax `define-syntax' defines a `PrimitiveProcedure'
object (which inherits from `PrcedureN'), which refers to a named
virtual method in a named class with specified parameter and return
types.  The primitives `define-static' and `define-interface' are
similar, but for static and interface methods.

   When the `compile' method of `ApplyExp' sees that the function is a
`PrimitiveProcedure', it will emit code to call the method.  It will
also emit code to coerce the arguments to the required types.  It will
not do any checking that the method actually exists and has declared
type, since the method may not be available yet (and it is also a pain
to do, though the JDK 1.1 reflection facility will make it easier).

   The name declared by `define-virtual' has the same scope as that of
a macro.  It must be seen at compile-time.  A `PrimitiveProcedure'
cannot be applied or called at top-level interactively.

   We can lift these restrictions when Kawa switches to JDK 1.1 and its
reflective features.  In that case, using a `PrimitiveProcedure' may be
the preferred way to define many builtin functions.  The advantage is
simpler declarations, fewer standard classes, much faster invocation
when the compiler knows which method is being called, but at the cost
of somewhat slower calls when we have to use the general `applyN'
interface, which would use the probably slower reflection facilities.


File: kawa-tour.info,  Node: Continuations,  Next: Tail-calls,  Prev: Low-level procedures,  Up: Top

Continuations
=============

Scheme continuations "capture" the current execution state.  They can
be implemented by copying the stack, but this requires non-portable
native code.  Kawa continuations are implemented using Java exceptions,
and can be used to prematurely exit (throw), but not to implement
co-routines (which should use threads anyway).

     class callcc extends Procedure1 {
       public Object apply1 (Object arg1)
       {
         Procedure proc = (Procedure) arg1;
         Continuation cont = new Continuation ();
         try { return proc.apply1(cont); }
         catch (CalledContinuation ex)
           {
             if (ex.continuation != cont)
                  throw ex;  // Re-throw.
             return ex.value;
           }
       }
       ...
     }

   This is the `Procedure' that implements
`call-with-current-continuation'.  It creates `cont', which is the
"current continuation", and passes it to the incoming `proc'.  If
`callcc' catches a `CalledContinuation' exception it means that `proc'
invoked some `Continuation'.  If it is "our" continuation, return the
value passed to the continuation;  otherwise re-throw it up the stack
until we get a matching handler.  (I have left out code to detect
unsupported invocation of `cont' after `callcc' returns.)

     class Continuation extends Procedure1 {
       public Object apply1 (Object arg1)
       {
         throw new CalledContinuation (arg1, this);
       }
       ...
     }

   A `Continuation' is the actual continuation object that is passed to
`callcc''s argument; when it is invoked, it throws a
`CalledContinuation' that contains the continuation and the value
returned.

     class CalledContinuation extends RuntimeException {
       Object value;
       Continuation continuation;
       CalledContinuation (Object value, Continuation continuation)
       {
         super ("call/cc called");
         this.value = value;
         this.continuation = continuation;
       }
       ...
     }

   `CalledContinuation' is the exception that is thrown when the
continuation is invoked.


File: kawa-tour.info,  Node: Tail-calls,  Next: Types,  Prev: Continuations,  Up: Top

Tail-calls
==========

Scheme requires that tail-calls be implemented without causing stack
growth.  This is difficult to do portably in Java (or for that matter
in C), since implementing it efficiently requires low-level access to
the hardware stack.  There are tricks one can use (a function returns a
pointer to the next function to be called, rather than calling it
directly), but these are rather expensive, especially in Java (which
does not have function pointers).

   Compiler optimizations can re-write many tail-calls into `goto's.
The most important case is self-tail-calls or tail recursion.  Kawa
rewrites these when it can prove that is safe.  It is rather
conservative:  it must be able to prove that the procedure binding
cannot be re-assigned to some other value.  Thus only `letrec'
tail-recursion is supported, not tail-recursion of global procedures.
This restriction may be excessively paranoid, but it is required by the
language, and it is sufficient to optimize the standard `do' and
named-`let' forms.  A future version will provide a way to specify that
Kawa can be less conservative (using either a module system or compiler
switches).

   While full tail-call elimination will probably never be supported by
Kawa (except perhaps on modified Java interpreters), some forms of
mutual tail-recursion can be eliminated with more sophisticated
compiler analysis, though there are no concrete plans for that yet.


File: kawa-tour.info,  Node: Types,  Next: Future,  Prev: Tail-calls,  Up: Top

Classes, types, and declarations
================================

An extensive type system that supports user-defined classes is
important for extensibility and for integration with other Java
packages.  This section describes current ideas for future extensions.

   The record extension proposed (and rejected) for R5RS will probably
be added.  It has been partially written; however a new implementation
based on the JDK 1.1 reflective features seems preferable, since that
means the record accessor features can be used on arbitrary Java
objects.  After than, the next step is to add methods, and we have a
way to define and access Java classes from Scheme.

   Type declarations and some form of first-class types are also
planned.  This will improve documentation, error-checking, and code
efficiency.  Among the types to be supported are "unboxed" number
types, so the compiler could use raw `double' instead of having to
allocate a `DFloNum' object.  In addition to allowing better code, this
feature will also make it it easier to integrate with primitive Java
types.  (The `define-virtual' syntax in *Note Low-level procedures::
will be extended to support type values.)

   Some kind of module system is desirable.  One reason is to control
names and names clashes;  another reason is so that the compiler can
map global variables references to their definitions. This makes it
easier to do better optimizations.  It is tempting to define a module
as a kind of class: A simple module is a packaging of data and function
definitions, and it is easy to map these into static fields and methods
of a class.  Controlling which components are exported is similar to
specifying visibility (`public' or `private').  What is more
challenging is how to model import lists, or module
signature/interfaces.  Perhaps we can use Java interface types.

   Given all this new infrastructure (types and modules), then the
compiler will need some re-writing.  At the very least it needs to know
that expressions may have other types than just plain `Object'.


File: kawa-tour.info,  Node: Future,  Next: Conclusion,  Prev: Types,  Up: Top

Current and Future Work
=======================

There is very  preliminary threads support in Kawa.  It provides an
interface to Java threads that looks somewhat like `delay', except that
the delayed expression is evaluated in a new thread.  (The model is
similar to to the "futures" concept of MultiScheme `J. S. Miller:
MultiScheme: A Parallel Processing System based on MIT Scheme,   PhD
thesis, Department of Electrical Engineering and Computer Science,
MIT, Aug. 1987.  MIT-LCS//MIT/LCS/TR-402'.)

   but there is no implicit force, at least yet.)  Recent
re-implementation of core classes (such `Environment' and `Translator')
has been done to support threads with optionally separate top-level
environments.

   An interface to graphics primitives is needed, but no work has been
done.  Unfortunately, Java's Abstract Windowing Toolkit is commonly
considered among the weaker parts of Java, and lacks important
features, such as a decent multi-line text editor.

   More sophisticated Scheme code rewriting, optimizations, and
inlining should also be investigated after Kawa has been taught to know
about types as discussed in *Note Types::.

   One important goal of Kawa is that it should support multiple
languages.  This includes an interface to plug in new parsers,
pre-defined functions, data types, and output formatting.  Of special
interest is re-implementing some of the ideas and syntax from my
earlier Q language. These include a line-oriented syntax with fewer
parentheses, and high-level sequence and array operations (as in APL).

   Also of interest is support for Emacs Lisp.  This would require an
extensive library to implement the Emacs data types (such as buffers
and windows), in addition to the challenges of the Emacs Lisp language
itself (it has different data types and name binding rules than
Scheme), but may be a good way to build a next-generation Emacs.


File: kawa-tour.info,  Node: Conclusion,  Next: Biographical Information,  Prev: Future,  Up: Top

Conclusion
==========

Kawa is a solid implementation of Scheme with many features.  It is
portable to any environment that can run Java applications.  It has
active use and development.  I have not bothered running benchmarks,
because the state of the art in Java implementation is in such flux,
because many optimizations are planned but have not been implemented,
and because the different feature sets of the various Scheme
implementations makes them difficult to compare fairly.  But Kawa has
the potential of being a reasonably fast Scheme implementation (though
probably never among the very fastest), and will reap benefits from
current efforts in Java compilation (see `P.~Bothner:   A Gcc-based
Java implementation.    IEEE Compcon 1997 Proceedings (to appear),
1997').


File: kawa-tour.info,  Node: Biographical Information,  Prev: Conclusion,  Up: Top

Biographical Information
========================

Per Magnus Alfred Bothner studied at the University of Oslo, Norway,
and received his Ph.D. in Computer Science from Stanford University in
1989.  He then worked for Digital Equipment; University of Wisconsin;
and has worked for Cygnus since 1991.  He can be reached at
`<bothner@cygnus.com>' or `<bothner@gnu.bai.mit.edu>'.

   His highly unimpressive home page is
`http://www.cygnus.com/~bothner', but at least it tells you where to
get Kawa.



Tag Table:
Node: Top79
Node: Introduction1423
Node: Background2694
Node: Objects and Values5925
Node: Symbols8654
Node: Numbers9918
Node: Mixed-type arithmetic13523
Node: Quantities16765
Node: Collections21238
Ref: Collections-Footnote-122876
Node: Procedures23159
Node: Overview of compilation26361
Node: Environments28304
Node: Expressions29032
Node: Translation32417
Node: Syntax and Macros34020
Node: Code generation36175
Node: Codegen37925
Node: Literals40767
Node: Low-level procedures43935
Node: Continuations46487
Node: Tail-calls48683
Node: Types50209
Node: Future52346
Node: Conclusion54322
Node: Biographical Information55204

End Tag Table
