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A Meta-circular Little Scheme

This is a meta-circular interpreter of a subset of Scheme, inspired by Zick Standard Lisp.

It implements the same language as

and runs on them. It also runs on other Schemes such as guile or any R5RS Schemes.

How to use

Run scm.scm on another Scheme. The following example uses little-scheme-in-go.

$ little-scheme-in-go scm.scm
(+ 5 6)
=> 11
(cons 'a (cons 'b 'c))
=> (a b . c)
(list
1
2
3
)
=> (1 2 3)
+
=> ($Intrinsic . #<(x):((+ (fst x) (snd x))):#<GlobalEnv>>)
(globals)
=> (globals error number? = < * - + apply call/cc symbol? eof-object? read newline display list not 
null? pair? eq? cons cdr car)

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

The implemented language

Expression types

  • v [variable reference]

  • (e0 e1...) [procedure call]

  • (quote e) ['e will be 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) displays Error: reason: arg and goes back to the top level. It is based on SRFI-23.

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

See Global-Env in scm.scm for the implementation of the procedures except call/cc and apply.
call/cc and apply are implemented particularly at apply-fun in scm.scm.

Examples

There are five files under the examples folder:

$ guile scm.scm < examples/fib90.scm 
2880067194370816120
$ guile scm.scm < 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))
$ guile scm.scm < examples/dynamic-wind-example.scm 
(connect talk1 disconnect connect talk2 disconnect)
$ guile scm.scm < examples/amb.scm
((1 A) (1 B) (1 C) (2 A) (2 B) (2 C) (3 A) (3 B) (3 C))
$ guile scm.scm < examples/yin-yang-puzzle.scm | head

*
**
***
****
*****
******
*******
********
*********
$

Performance

The following table shows the times to run scm.scm < examples/nqueens.scm on each Schemes. I used MacBook Pro (15-inch, 2016), 2.6GHz Core i7, 16GB 2133MHz LPDDR3, macOS Mojave 10.14.6.

Scheme Compiled/Executed on Time [sec] Rel. Speed
GNU Guile 2.2.7 guile 0.13 14.4
little-scheme-in-cs 1.1.0 .NET Core 3.1.2: dotnet build -c Release 1.87 1.00
little-scheme-in-go 1.2.0 Go 1.14.2: go build 2.00 0.94
little-scheme-in-java 1.1.0 AdoptOpenJDK jdk-11.0.6+10 2.02 0.93
little-scheme-in-crystal 0.2.0 Crystal 0.34.0: crystal build --release scm.cr 2.15 0.87
little-scheme-in-lisp 0.4.0 SBCL 2.0.2: sbcl --script scm.l 2.38 0.79
little-scheme-in-kotlin 0.2.0 Kotlin 1.3.71/AdoptOpenJDK jdk-11.0.6+10 2.38 0.79
little-scheme-in-cs 1.1.0 Mono 6.8.0: csc -o -r:System.Numerics.dll *.cs 2.77 0.68
little-scheme-in-dart 0.4.0 Dart 2.7.2: dart scm.dart 3.71 0.50
little-scheme-in-dart 0.4.0 Dart 2.7.2: dart2native scm.dart; ./scm.exe 3.72 0.50
little-scheme-in-python 3.2.0 PyPy 7.3.0 (Python 2.7.13): pypy scm.py 4.73 0.40
little-scheme-in-python 3.2.0 PyPy 7.3.0 (Python 3.6.9): pypy3 scm.py 5.19 0.36
little-scheme-in-typescript 1.2.1 TypeScript 3.8.3/Node.js 13.12.0: tsc -t ESNext ... 7.17 0.26
little-scheme-in-crystal 0.2.0 Crystal 0.34.0: crystal scm.cr 9.88 0.19
little-scheme-in-php 0.3.0 PHP 7.1.33: php scm.php 44.84 0.04
little-scheme-in-python 3.2.0 Python 3.8.2: python3 scm.py 81.72 0.02
little-scheme-in-ruby 0.3.0 Ruby 2.3.7: ruby scm.rb 84.80 0.02
little-scheme-in-python 3.2.0 Python 2.7.16: python scm.py 88.78 0.02

Tower of meta-circular interpreters

Being meta-circular, this interpreter is able to run itself recursively.

  1. Copy the interpeter file scm.scm to scm-scm.scm.

  2. Comment out the last line (read-eval-print-loop) of scm-scm.scm.

;; (read-eval-print-loop)
  1. Append two new lines (global-eval '(begin and )) to scm-scm.scm.
;; (read-eval-print-loop)
(global-eval '(begin
))
  1. Insert the whole contents of scm.scm between the new lines.
;; (read-eval-print-loop)
(global-eval '(begin
;; A meta-circular little Scheme v1.3 R02.04.12 by SUZUKI Hisao
...
(read-eval-print-loop)
))
  1. Run scm-scm.scm on another Scheme.

For your convenience, I have built it as tower/scm-scm.scm.

$ little-scheme-in-go tower/scm-scm.scm
(+ 5 6)
=> 11
+
=> ($Intrinsic $Closure (x) ((+ (fst x) (snd x))) #<(op):((if (eq? op (quote car)) CAR_ (if (eq? op 
(quote cdr)) CDR_ (if (pair? op) (set! CDR_ (car op)) (_error "unknown op" op))))):#<| CAR_ CDR_ Glo
balEnv>>)

Note that the intrinsic function + is now implemented by a closure of scm.scm, the underlying Scheme here.

You can repeat the above process any times. Try tower/scm-scm-scm.scm and you will find it runs prohibitively slowly as might be expected.

$ time ./little-scheme-in-go examples/yin-yang-puzzle.scm | head -4

*
**
***

real	0m0.007s
user	0m0.004s
sys	0m0.005s
$ time ./little-scheme-in-go scm.scm < examples/yin-yang-puzzle.scm | head -4

*
**
***

real	0m0.010s
user	0m0.006s
sys	0m0.005s
$ time ./little-scheme-in-go tower/scm-scm.scm < examples/yin-yang-puzzle.scm | head -4

*
**
***

real	0m0.386s
user	0m0.434s
sys	0m0.026s
$ time ./little-scheme-in-go tower/scm-scm-scm.scm < examples/yin-yang-puzzle.scm | head -4

*
**
***

real	1m46.486s
user	2m33.903s
sys	0m5.011s
$

Performance on the tower

The following table shows the times to run tower/scm-scm.scm < examples/nqueens.scm on each Schemes. I used the same MacBook Pro as above.

Scheme Compiled/Executed on Time [sec] Rel. Speed
GNU Guile 2.2.7 guile 27.32 18.5
little-scheme-in-cs 1.1.0 .NET Core 3.1.2: dotnet build -c Release 506.15 1.00
little-scheme-in-java 1.1.0 AdoptOpenJDK jdk-11.0.6+10 506.79 1.00
little-scheme-in-kotlin 0.2.0 Kotlin 1.3.71/AdoptOpenJDK jdk-11.0.6+10 598.57 0.85
little-scheme-in-go 1.2.0 Go 1.14.2: go build 604.27 0.84
little-scheme-in-crystal 0.2.0 Crystal 0.34.0: crystal build --release scm.cr 624.52 0.81
little-scheme-in-lisp 0.4.0 SBCL 2.0.2: sbcl --script scm.l 676.82 0.75