/little-scheme-in-kotlin

A Scheme interpreter in Kotlin with first-class continuations

Primary LanguageKotlinMIT LicenseMIT

A Little Scheme in Kotlin

This is a small interpreter of a subset of Scheme written in Kotlin 1.3. It implements almost the same language as

and their meta-circular interpreter, little-scheme.

As a Scheme implementation, it optimizes tail calls and handles first-class continuations properly.

How to run

$ make
kotlinc -include-runtime -d scm.jar scm.kt scm_j.kt
java -jar scm.jar
> (+ 5 6)
11
> (cons 'a (cons 'b 'c))
(a b . c)
> (list
| 1
| 2
| 3
| )
(1 2 3)
>

Press EOF (e.g. Control-D) to exit the session.

> Goodbye
$

You can run it with a Scheme script. Examples are found in little-scheme; download it at .. and you can try the following:

$ cat ../little-scheme/examples/yin-yang-puzzle.scm 
;; The yin-yang puzzle 
;; cf. https://en.wikipedia.org/wiki/Call-with-current-continuation

((lambda (yin)
   ((lambda (yang)
      (yin yang))
    ((lambda (cc)
       (display '*)
       cc)
     (call/cc (lambda (c) c)))))
 ((lambda (cc)
    (newline)
    cc)
  (call/cc (lambda (c) c))))

;; => \n*\n**\n***\n****\n*****\n******\n...
$ java -jar scm.jar ../little-scheme/examples/yin-yang-puzzle.scm | head

*
**
***
****
*****
******
*******
********
*********
^C$
$ java -jar scm.jar ../little-scheme/examples/amb.scm
((1 A) (1 B) (1 C) (2 A) (2 B) (2 C) (3 A) (3 B) (3 C))
$ java -jar scm.jar ../little-scheme/examples/dynamic-wind-example.scm
(connect talk1 disconnect connect talk2 disconnect)
$ java -jar scm.jar ../little-scheme/examples/nqueens.scm
((5 3 1 6 4 2) (4 1 5 2 6 3) (3 6 2 5 1 4) (2 4 6 1 3 5))
$ java -jar scm.jar ../little-scheme/scm.scm < \
> ../little-scheme/examples/nqueens.scm
((5 3 1 6 4 2) (4 1 5 2 6 3) (3 6 2 5 1 4) (2 4 6 1 3 5))
$

Press INTR (e.g. Control-C) to terminate the yin-yang-puzzle.

Put a "-" after the script in the command line to begin a session after running the script.

$ cat ../little-scheme/examples/fib90.scm 
;; Fibonacci numbers: F(n) = F(n-1) + F(n-2) with F(0) = 0 and F(1) = 1. 
;; cf. https://oeis.org/A000045
(define fibonacci
  (lambda (n)
    (define _fib
      (lambda (i F_i F_i+1)
        (if (= i n)
            F_i
          (_fib (+ i 1) F_i+1 (+ F_i F_i+1)))))
    (_fib 0 0 1)))                      ; i=0, F(0)=0, F(1)=1

(display (fibonacci 90))
(newline)
;; => 2880067194370816120
$ java -jar scm.jar ../little-scheme/examples/fib90.scm -
2880067194370816120
> (globals)
(globals error number? = < * - + apply call/cc symbol? eof-object? read newline
display list not null? pair? eq? cons cdr car fibonacci)
> (fibonacci 16)
987
> (fibonacci 1000)
43466557686937456435688527675040625802564660517371780402481729089536555417949051
89040387984007925516929592259308032263477520968962323987332247116164299644090653
3187938298969649928516003704476137795166849228875
>

The implemented language

Scheme Expression Internal Representation
numbers 1, 2.3 Int, Double or BigInteger
#t true
#f false
strings "hello, world" String
symbols a, + class Sym
() null
pairs (1 . 2), (x y z) class Cell
closures (lambda (x) (+ x 1)) class Closure
built-in procedures car, cdr class Intrinsic
continuations class Continuation

Expression types

  • v [variable reference]

  • (e0 e1...) [procedure call]

  • (quote e)
    'e [transformed into (quote e) when read]

  • (if e1 e2 e3)
    (if e1 e2)

  • (begin e...)

  • (lambda (v...) e...)

  • (set! v e)

  • (define v e)

For simplicity, this Scheme treats (define v e) as an expression type.

Built-in procedures
(car lst) (display x) (+ n1 n2)
(cdr lst) (newline) (- n1 n2)
(cons x y) (read) (* n1 n2)
(eq? x y) (eof-object? x) (< n1 n2)
(pair? x) (symbol? x) (= n1 n2)
(null? x) (call/cc fun) (number? x)
(not x) (apply fun arg) (globals)
(list x ...) (error reason arg)

  • (error reason arg) raises an exception with the message "Error: reason: arg". It is based on SRFI-23.

  • (globals) returns a list of keys of the global environment. It is not in the standards.

See GLOBAL_ENV in scm.kt for the implementation of the procedures except call/cc and apply.
call/cc and apply are implemented particularly at applyFunction in scm.kt.