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A set of Lisp-1 implementations that share the VM instruction set, built-in functions, and the bootstrapping code

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# Rosetta Lisp

Rosetta Lisp is a set of Lisp-1 implementations that share the VM instruction
set, built-in functions, and the bootstrapping code.

- [Introduction article (Japanese)](https://zenn.dev/yubrot/articles/0ea5405ea53de5)

## Implementations

Currently there are 6 implementations available. Each of them is implemented in
different languages.

- [ocalisp](https://github.com/yubrot/ocalisp) in [OCaml](https://ocaml.org/) (Reference implementation)
- [scalisp](https://github.com/yubrot/scalisp) in [Scala 3](https://www.scala-lang.org/)
- [golisp](https://github.com/yubrot/golisp) in [Go](https://go.dev/)
- [fslisp](https://github.com/yubrot/fslisp) in [F#](https://fsharp.org/)
- [kokalisp](https://github.com/yubrot/kokalisp) in [Koka 3](https://koka-lang.github.io/)
- [idrlisp](https://github.com/yubrot/idrlisp) in [Idris 1](https://www.idris-lang.org/) (Out of date)
- [wonderlisp](https://github.com/yubrot/wonderlisp) in Rosetta Lisp itself

## VM Design

The design of the Rosetta Lisp implementation is heavily inspired by
[SECD machine](https://en.wikipedia.org/wiki/SECD_machine). Every S-expression
is macro-expanded and then compiled into a code, a sequence of instructions.
Rosetta Lisp implementations share an extremely simple, small instruction set
that targeting an SECD-like virtual machine.

### VM Instruction Set

Rosetta Lisp VM consists of four states:

- `[S]tack` - a list of values
- `[E]nvironment` - an abstract representation of a collection of key-value
  pairs
- `[C]ode` - a list of instructions
- `[D]ump` - a list of pairs of Environment and Code

#### ldc value

Push a `value` on top of `S`.

```lisp
(define (ldc value)
  (lambda (S E C D k)
    (k (cons value S) E C D)))
```

#### ldv name

Find a binding for `name` from `E` and push the value on top of `S`.

```lisp
(define (ldv name)
  (lambda (S E C D k)
    (k (cons (find-env name E) S) E C D)))
```

#### ldf pattern code

Make a function capturing `E` and push it on top of `S`.

```lisp
(define (ldf pattern code)
  (lambda (S E C D k)
    (k (cons (make-function-closure pattern code E) S) E C D)))
```

#### ldm pattern code

Make a macro capturing `E` and push it on top of `S`.

```lisp
(define (ldm pattern code)
  (lambda (S E C D k)
    (k (cons (make-macro-closure pattern code E) S) E C D)))
```

#### ldb name

Find a built-in function named `name` and push it on top of `S`.

```lisp
(define (ldb name)
  (lambda (S E C D k)
    (k (cons (find-builtin-function name) S) E C D)))
```

#### sel then-code else-code

Pop a value from `S`, set `C` to `then-code` if the value is `#f`, set `C` to
`else-code` otherwise. Set `E` to a new environment derived from `E`. Push the
previous `E` and `C` on top of `D`.

```lisp
(define (sel then-code else-code)
  (lambda ((s . S) E C D k)
    (k S
       (new-env E)
       (if s then-code else-code)
       (cons (cons E C) D))))
```

#### leave

Pop a pair of Environment and Code from `D` and set `E` and `C` to it.

```lisp
(define (leave)
  (lambda (S E C ((e . c) . D) k)
    (k S e c D)))
```

#### app argc

Pop `argc` values as function arguments from `S`, pop a function from `S`, call
the function with the arguments.

```lisp
(define (app argc)
  (lambda ((argN argN-1 ... arg1 f . S) E C D k)
    (apply-function f (list arg1 .. argN) S E C D k)))
```

`apply-function` is defined for built-in functions and function closures
obtained by `ldf`. `apply-function` on function closures is defined as follows:

```lisp
(define (apply-function (FUNCTION-CLOSURE pattern code env)
                        args
                        S E C D k)
  (k S
     (bind-args-with-pattern args pattern (new-env env))
     code
     (cons (cons E C) D)))
```

#### pop

Pop a value from `S`.

```lisp
(define (pop)
  (lambda ((s . S) E C D k)
    (k S E C D)))
```

#### def name

Pop a value, create a binding from `name` to the value on `E`.

```lisp
(define (def name)
  (lambda ((s . S) E C D k)
    (k S (env-define name s E) C D)))
```

#### set name

Pop a value, update a binding from `name` to the value on `E`.

```lisp
(define (set name)
  (lambda ((s . S) E C D k)
    (k S (env-set name s E) C D)))
```

### Compilation to VM instructions

All literals and quoted expressions are compiled into `ldc`:

```
; compiling 123
[0 entry]
  ldc 123
```

All unquoted symbols are compiled into `ldv`:

```
; compiling foo
[0 entry]
  ldv foo
```

Lists are compiled into a sequence of element evaluations and an `app`.

```
; compiling (compare a b)
[0 entry]
  ldv compare
  ldv a
  ldv b
  app 2

; compiling (+ foo (* bar baz) 111)
[0 entry]
  ldv +
  ldv foo
  ldv *
  ldv bar
  ldv baz
  app 2
  ldc 111
  app 3
```

All other instructions are produced by **Syntax**. Applications of Syntax are
not compiled into an usual `app`.

`(builtin cons)` produces a `ldb`.

```
[0 entry]
  ldb hello
```

`(if a b c)` produces a `sel` and two branch codes terminated by a `leave`.

```
[0 entry]
  ldv a
  sel [1 then] [2 else]
[1 then]
  ldv b
  leave
[2 else]
  ldv c
  leave
```

`(begin a b c)` produces a sequence of evaluations and `pop`s.

```
[0 entry]
  ldv a
  pop
  ldv b
  pop
  ldv c
```

`(fun (x y) y)` produces a `ldf`, `(macro (x y) x)` produces a `ldm`. Body of
functions are terminated by a `leave` but macros are not.

```
[0 entry]
  ldf [1 fun (x y)]
[1 fun (x y)]
  ldv y
  leave
```

```
[0 entry]
  ldm [1 macro (x y)]
[1 macro (x y)]
  ldv x
```

`(def x 123)` produces a `def`, `(set! x 123)` produces a `set`. Both of them
also produce a `ldc ()` to adjust stack size.

```
[0 entry]
  ldc 123
  def x
  ldc ()
```

```
[0 entry]
  ldc 123
  set x
  ldc ()
```

`(quote x)` is also one of Syntax that produces a `ldc`.

## Built-in functions

All other functionalities are injected as built-in functions. Every
implementation provides the same built-in function set. By doing this, Every
implementation can use [the same bootstrapping code](./boot.lisp) to get
frequently used functions and macros.

### Built-in functions required by the bootstrapping code

To reduce the size of required built-in functions, there are a lot of functions
and macros that are defined in the bootstrapping code instead of in host
languages. It's not optimal in terms of performance, but it makes easy to add
another Rosetta Lisp implementation.

In the following documentation, `result<a>` means a result cons cell.

- If `car` is `#t`, the result `a` is set to `cdr`.
- If `car` is `#f`, the failure error information `str` is set to `cdr`.

#### Cons cell operators

| name   | overloads        |
| ------ | ---------------- |
| `cons` | `(_, _) -> cons` |
| `car`  | `(cons) -> _`    |
| `cdr`  | `(cons) -> _`    |

#### Terminators

| name    | overloads     |
| ------- | ------------- |
| `exit`  | `(?num) -> !` |
| `error` | `(?str) -> !` |

#### Macro supports

NOTE: `quote` is a syntax, `quasiquote` and `unquote` are implemented on
`boot.lisp`.

| name     | overloads   | note                                  |
| -------- | ----------- | ------------------------------------- |
| `gensym` | `() -> sym` | Must not overlap with symbol literals |

#### Control operators

| name      | overloads           |
| --------- | ------------------- |
| `apply`   | `(proc, list) -> _` |
| `call/cc` | `(proc) -> _`       |
| `never`   | `(proc, ..._) -> !` |

#### Test operators

| name    | overloads     | note                                  |
| ------- | ------------- | ------------------------------------- |
| `num?`  | `(_) -> bool` |                                       |
| `sym?`  | `(_) -> bool` |                                       |
| `str?`  | `(_) -> bool` |                                       |
| `cons?` | `(_) -> bool` |                                       |
| `nil?`  | `(_) -> bool` |                                       |
| `bool?` | `(_) -> bool` |                                       |
| `proc?` | `(_) -> bool` | Built-in functions and user functions |
| `meta?` | `(_) -> bool` | Language syntax and user macros       |
| `vec?`  | `(_) -> bool` |                                       |

#### Arithmetic operators

| name | overloads              |
| ---- | ---------------------- |
| `+`  | `(...num) -> num`      |
| `-`  | `(num, ...num) -> num` |
| `*`  | `(...num) -> num`      |
| `/`  | `(num, ...num) -> num` |
| `%`  | `(num, ...num) -> num` |

#### Relational operators

| name | overloads                              |
| ---- | -------------------------------------- |
| `=`  | `(..._) -> bool`                       |
| `<`  | `(...num) -> bool`, `(...str) -> bool` |
| `>`  | `(...num) -> bool`, `(...str) -> bool` |
| `<=` | `(...num) -> bool`, `(...str) -> bool` |
| `>=` | `(...num) -> bool`, `(...str) -> bool` |

#### `str` operators

String encoding is not specified, but is expected to be ASCII compatible through
these operators. This requirement is usually satisfied by using a Unicode scalar
sequence or byte sequence.

| name          | overloads                     | note                                                                         |
| ------------- | ----------------------------- | ---------------------------------------------------------------------------- |
| `str`         | `(...num) -> str \| !`        | Each `num` corresponds to a character. Raises an error on invalid characters |
| `str-char-at` | `(str, num) -> num \| nil`    | Takes an index. Returns a character                                          |
| `str-length`  | `(str) -> num`                | Returns character count                                                      |
| `str-concat`  | `(...str) -> str`             |                                                                              |
| `substr`      | `(str, num, num) -> str \| !` | Takes a str, index, and length. Raises an error on out of range              |
| `sym->str`    | `(sym) -> str`                |                                                                              |
| `num->str`    | `(num) -> str`                |                                                                              |
| `str->num`    | `(str) -> num \| nil`         |                                                                              |

#### `vec` operators

`vec` is a type for minimal support of mutable/fixed-length sequential data.

| name         | overloads                               | note                                                             |
| ------------ | --------------------------------------- | ---------------------------------------------------------------- |
| `vec`        | `(..._) -> vec`                         |                                                                  |
| `vec-make`   | `(num, _) -> vec`                       | Takes a length and initial value                                 |
| `vec-length` | `(vec) -> num`                          |                                                                  |
| `vec-get`    | `(vec, num) -> _ \| nil`                | Get by index                                                     |
| `vec-set!`   | `(vec, num, _) -> nil \| !`             | Set by index. Raises an error on out of range                    |
| `vec-copy!`  | `(vec, num, vec, num, num) -> nil \| !` | Takes a dest, dest-start-index, src, src-start-index, and length |

#### I/O operators

| name                | overloads                   | note                                            |
| ------------------- | --------------------------- | ----------------------------------------------- |
| `read-file-text`    | `(str) -> result<str>`      | Takes a file path. Returns contents of the file |
| `write-file-text`   | `(str, str) -> result<nil>` | Takes a file path and contents                  |
| `read-console-line` | `() -> result<str \| nil>`  | No line feed at end. Returns nil if terminated  |
| `write-console`     | `(str) -> result<nil>`      |                                                 |

#### Misc

| name            | overloads          | note                                          |
| --------------- | ------------------ | --------------------------------------------- |
| `args`          | `() -> list`       | Returns command line arguments as list of str |
| `eval`          | `(_) -> result<_>` | Evaluates a S-expression                      |
| `macroexpand`   | `(_) -> result<_>` | Perform macro expansion                       |
| `macroexpand-1` | `(_) -> result<_>` | Perform one step macro expansion              |

## Tests

There are [test suites](./testsuites) for the functionalities of the parser, the
compiler, and the VM. Rosetta Lisp's bootstrapping code includes unit tests for
each built-in function, as comments.

```lisp
(def cons (builtin cons))
;! > (cons 1)
;! fail
;! > (cons 1 2)
;! (1 . 2)
;! > (cons 1 2 3)
;! fail
```

Since it's troublesome to parse these comments, there is
[a unified, easy-to-parse test file](./test) available. This test file is
generated by [./gentest](./gentest).

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