GitHub - atgreen/cl-kawa: Scheme on Java on Common Lisp

cl-kawa

Common Lisp / Kawa Scheme interop via OpenLDK.

Kawa Scheme, created by Per Bothner, compiles Scheme code to Java bytecode and runs on the JVM. OpenLDK is a JVM implemented in Common Lisp; it transpiles Java bytecode to Common Lisp code. In SBCL, that Common Lisp code is then compiled to native assembly. Since both share the same SBCL process and heap, cl-kawa enables deep interoperability between Common Lisp and Scheme with no serialization or process boundaries.

Project status: technology demonstration. Not a performant or production ready implementation.

What you can do

• Evaluate Scheme from Common Lisp (strings or s-expressions).
• Call Scheme procedures from Common Lisp.
• Register Common Lisp functions and call them from Scheme.
• Exchange basic values (numbers, strings, booleans, lists) across the boundary.

Prerequisites

• SBCL
• OpenLDK
• Java 8 JDK (JAVA_HOME pointing to a JRE with lib/rt.jar)
• Kawa 3.1.1 JAR (downloaded automatically via Maven, or manually placed in libs/)

Install

1. Ensure prerequisites are installed and JAVA_HOME is set.
2. Download Kawa (or let Maven fetch it) into libs/ as kawa-3.1.1.jar.
3. Load the ASDF system from your Common Lisp image.

Quick start

(asdf:load-system :cl-kawa)

;; Initialize the runtime (point to the Kawa JAR)
(kawa:startup :classpath "libs/kawa-3.1.1.jar")

;; Evaluate Scheme expressions -- as s-expressions or strings
(kawa:eval '(+ 1 2))              ; => 3
(kawa:eval '(string-length "hello")) ; => 5
(kawa:eval '(list 1 2 3))         ; => (1 2 3)
(kawa:eval "(* 6 7)")             ; => 42  (strings still work)

;; Look up a Scheme procedure and call it from CL
(let ((add (kawa:lookup "+")))
  (kawa:funcall add 10 20))        ; => 30

;; Register a CL function so Scheme can call it
(kawa:register "cl-square" (lambda (x) (* x x)))
(kawa:eval '(cl-square 7))        ; => 49

Hello World: three languages in one process

hello.lisp demonstrates the full Common Lisp → Scheme → Java interop chain in a single SBCL process:

(format t "~A~%"
  (kawa:eval '(let ((s (|java.lang.String| (string-append "Hello" ", " "World!"))))
               (|s:toUpperCase|))))
;; prints: HELLO, WORLD!

This single expression crosses three language boundaries:

1. Common Lisp calls kawa:eval with a quoted s-expression.
2. Kawa Scheme assembles the greeting with string-append and wraps it in a java.lang.String.
3. Java's String.toUpperCase() runs -- its bytecode was transpiled by OpenLDK into Common Lisp, then compiled by SBCL to native x86-64 assembly.

The result flows back through Scheme's value system into Common Lisp and is printed by format. No FFI, no sockets, no serialization -- just nested function calls inside a single Lisp image.

LDK_CLASSPATH=libs/kawa-3.1.1.jar \
  JAVA_HOME=/path/to/java8/jre \
  sbcl --load hello.lisp

Environment variables

• JAVA_HOME: path to a Java 8 JRE that contains lib/rt.jar.
• LDK_CLASSPATH: optional classpath override for OpenLDK.

API

(kawa:startup &key classpath)

Initialize the Kawa Scheme runtime. classpath is a colon-separated string of JAR paths. Safe to call multiple times (subsequent calls are no-ops).

(kawa:eval expr &optional env) => value

Evaluate a Scheme expression. expr can be a string of Scheme source or a Common Lisp s-expression (symbols are automatically downcased, quote becomes ', t/nil become #t/'()). Returns the result converted to a Common Lisp value.

(kawa:lookup name &optional env) => object

Look up a Scheme binding by name. Returns the raw Java/Kawa object (typically a Procedure).

(kawa:funcall proc &rest args) => value

Call a Scheme procedure with Common Lisp arguments. Arguments are automatically converted to Kawa values; the result is converted back.

(kawa:register name function &optional env)

Register a Common Lisp function as a Scheme procedure. The function becomes callable from Scheme code via kawa:eval.

(kawa:scheme->cl obj) => value

Convert a Java/Kawa object to a Common Lisp value. Handles integers, floats, strings, booleans, and lists. Returns the object unchanged if no conversion applies.

(kawa:cl->scheme val) => object

Convert a Common Lisp value to a Java/Kawa object. Handles integers, floats, strings, and cons cells.

(kawa:make-environment &optional parent) => environment

Create a new Scheme environment, optionally inheriting from a parent.

Value conversions

Running the tests

Or manually:

LDK_CLASSPATH=libs/kawa-3.1.1.jar \
  JAVA_HOME=/path/to/java8/jre \
  sbcl --load test.lisp

Limitations and notes

• Designed as a proof-of-concept; performance and completeness are not goals.
• The conversion layer only handles basic scalar and list types.
• Requires Java 8 because OpenLDK depends on rt.jar.

Author and License

cl-kawa was written by Anthony Green and is distributed under the terms of the MIT license.