Overview

toytown-lispy-java is a Java re(re-re-re)implementation of Peter Norvig's famous hack demonstrating how to implement Lisp in less than 100 lines of Python.

Unlike Python, Java's language features do not give a lot away for free, so the author had a lot more work to do, and the resulting code weighed in at slightly more than 100 lines of Java.

This isn't really a problem, because the code is still quite compact and readable, and the built JAR weighs in at a mere 18 kilobytes in size.

Norvig actually wrote two versions of Lispy: the first, which was his first attempt to write a Lisp interpreter in less than 100 lines of code; this lacks error checking and a lot of important features, but is surprisingly powerful, given its size. His second attempt is a more complete version, which has better error checking, a more complete set of builtins, hygienic macros, tail call optimisation, additional data types, call/cc and a few other refinements.

Toytown implements a modest superset of the former, which includes Boolean types and (with big limitations) Java interoperability features lifted from Clojure. It also comes with Peter Norvig's test suite, which it passes.

Support

Toytown is supplied as-is. If it breaks, you get to keep both pieces.

Example Usage

A few examples are below, but you my also find example usage in the /src/test directory.

Hello World

(Whoops. Toytown doesn't yet support string literals, so this will have to wait. Sorry!!)

Fibonacci Sequences

(begin 

  (define fact (lambda (n) 
    (if (<= n 1) 
      1 
      (* n (fact (- n 1))))))
      
  (fact 13))

Closures

(begin
              
  (define filter (lambda (predicate argument)
    (if (null? argument)
      (quote ())
      (if (predicate (car argument))
        (cons (car argument) (filter predicate (cdr argument)))
        (filter predicate (cdr argument))))))

  (define notDivisibleBy (lambda (n)
    (lambda (x)
      (> (% x n) 0))))

  (filter (notDivisibleBy 2) (list 1 2 3 4 5 6 7 8)))

Java Interop

Some examples. Toytown's support for Java interoperability was only added to easily allow implementing all the language builtins in Java, and has a long way to go before it's truly useful.

(.nanoTime java.util.System)

(.toHexString 65534)

(begin
    (define fred 65534)
    (.isInfinite fred))