lava

lava is an interpreter implemented in Java. It supports parsing a subset of the Lisp language. The internal implementation of lava is very concise, and it has some basic operators built in for the user to implement various abstractions.

Builtin values and operators

• constants: #t #f

• arithmetic: + - * /

• comparator: < <= > >= =

• list: cons car cdr list

• procedure: begin lambda

• macro: defmacro

• other: quote cond if atom?

For lava all values of non-empty lists are treated as #t, and arithmetic operators only support integer.

Usage

The minimum running version of lava is Java 17. Because of the preview feature used in the source code, you need to run lava in the following way

java --enable-preview -jar lava.jar

Examples

lava> (+ 1 2)
=> 3

lava> (> 1 2)
=> #t

lava> (car '(a b c))
=> a

lava> (cdr '(a b c))
=> (b c)

lava> (cons 'a '(b c))
=> (a b c)

lava> (list 'a 1 'b 2)
=> (a 1 b 2)

lava> (begin (define fib (lambda (n) (if (<= n 2) 1 (+ (fib (- n 1)) (fib (- n 2)))))) (fib 10))
=> 55

lava> (((lambda (b) ((lambda (f) (b (lambda (x) ((f f) x)))) (lambda (f) (b (lambda (x) ((f f) x))))))
                  (lambda (fact) (lambda (n) (if (= 0 n) 1 (* n (fact (- n 1)))))))
                  5)
=> 120

Acknowledgments

• lisp.re
• (How to Write a (Lisp) Interpreter (in Python))