little-scheme-in-dart

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little-scheme-in-dart

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A Scheme interpreter with first-class continuations

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# A Little Scheme in Dart

This is a small interpreter of a subset of Scheme
in circa 800 lines of Dart 2.7.
It implements almost the same language as

- [little-scheme-in-crystal](https://github.com/nukata/little-scheme-in-crystal)
- [little-scheme-in-cs](https://github.com/nukata/little-scheme-in-cs)
- [little-scheme-in-go](https://github.com/nukata/little-scheme-in-go)
- [little-scheme-in-java](https://github.com/nukata/little-scheme-in-java)
- [little-scheme-in-kotlin](https://github.com/nukata/little-scheme-in-kotlin)
- [little-scheme-in-lisp](https://github.com/nukata/little-scheme-in-lisp)
- [little-scheme-in-php](https://github.com/nukata/little-scheme-in-php)
- [little-scheme-in-python](https://github.com/nukata/little-scheme-in-python)
- [little-scheme-in-ruby](https://github.com/nukata/little-scheme-in-ruby)
- [little-scheme-in-typescript](https://github.com/nukata/little-scheme-in-typescript)

and their meta-circular interpreter, 
[little-scheme](https://github.com/nukata/little-scheme).

As a Scheme implementation, 
it optimizes _tail calls_ and handles _first-class continuations_ properly.

## How to run

```
$ ./scm.dart
> (+ 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](https://github.com/nukata/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...
$ ./scm.dart ../little-scheme/examples/yin-yang-puzzle.scm | head

*
**
***
****
*****
******
*******
********
*********
^C
$ ./scm.dart ../little-scheme/examples/amb.scm
((1 A) (1 B) (1 C) (2 A) (2 B) (2 C) (3 A) (3 B) (3 C))
$ ./scm.dart ../little-scheme/examples/dynamic-wind-example.scm
(connect talk1 disconnect connect talk2 disconnect)
$ ./scm.dart ../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))
$ ./scm.dart ../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
$ ./scm.dart ../little-scheme/examples/fib90.scm -
2880067194370816120
> (globals)
(fibonacci globals error number? = < * - + apply call/cc symbol? eof-object? rea
d newline display list not null? pair? eq? cons cdr car)
> (fibonacci 16)
987
> (fibonacci 1000)
43466557686937456435688527675040625802564660517371780402481729089536555417949051
89040387984007925516929592259308032263477520968962323987332247116164299644090653
3187938298969649928516003704476137795166849228875
> 
```

Note the result of `(fibonacci 1000)` above.
Since Dart 2.0.0, `int` does not have inifinite-precision
and arithmetic operations wrap around on overflow
(cf. [Dart - Fixed-Size Integers](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/int64.md)).
By using [BigInt class](https://api.dart.dev/stable/2.7.2/dart-core/BigInt-class.html),
this Scheme calculates `(fibonacci 1000)` correctly.

## The implemented language

| Scheme Expression                   | Internal Representation             |
|:------------------------------------|:------------------------------------|
| numbers `1`, `2.3`                  | `num` (`int`, `double`) or `BigInt` |
| `#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](https://srfi.schemers.org/srfi-23/srfi-23.html).

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

See [`globalEnv`](scm.dart#L425-L469)
in `scm.dart` for the implementation of the procedures
except `call/cc` and `apply`.  
`call/cc` and `apply` are implemented particularly at 
[`applyFunction`](scm.dart#L603-L636) in `scm.dart`.

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