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1. About

Rust implementation of an interpreter for Monkey Programming Language.

The basic idea comes from the book "Writing An Interpreter In Go".

$ cargo run

>> let v = 3.14**float(2);
null

>> v
9.8596

>> v % 3.
0.8596000000000004

>> let factorial = fn(x) { if (x == 0) { return 1; }; x * factorial(x - 1) };
null

>> factorial(3)
6

>> factorial(5)
120

>> let recursive_inverse_fizzbuzz = fn(x) {
    if (x == 0) { return; }
    if (x % 15 == 0) {
        print("FizzBuzz");
    } else {
        if (x % 3 == 0) {
            print("Fizz");
        } else {
            if (x % 5 == 0) {
                print("Buzz");
            } else {
                print(x);
            }
        }
    }
    recursive_inverse_fizzbuzz(x - 1);
};
null

>> recursive_inverse_fizzbuzz(15)
FizzBuzz
14
13
Fizz
11
Buzz
Fizz
8
7
Fizz
Buzz
4
Fizz
2
1
null

>> let fibonacci = fn(arr, iter, max) {
    if (iter == max) {
        return arr;
    } else {
        if (iter == 0) {
            return fibonacci([0, 1], iter + 1, max);
        } else {
            return fibonacci(arr + [arr[len(arr) - 1] + arr[len(arr) - 2]], iter + 1, max);
        }
    }
};
null

>> fibonacci([], 0, 10)
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

2. Caveats

The language implemented in this repository is very similar to Monkey, but not exactly the same.

Currently, hashmaps is not yet implemented because, well..., I was just not attracted by it, after implementing arrays and function call expressions. They are syntactically different but the implementation details are similar (as far as we borrow the built-in data structures from Go or Rust).

Instead, some new functionalities which are missing in Monkey are implemented. Here's the non-exhaustive list:

  • vi-binding is enabled in REPL (with the help of GNU Readline library).

  • Remainder operator (%) and power operator (**) are implemented.

  • Cast functions (e.g. bool() and char()) are implemented.

  • String can be indexed to retrieved the nth Unicode character.

  • Arrays can be concatenated by +. For example, [1, 2] + [3] results in [1, 2, 3].

  • etc.

3. How It Works

3.1 How It Works

  1. Lexer (lexer.rs) splits the input string into tokens (token.rs).

  2. Parser (parser.rs) parses the generated tokens to build an AST (abstract syntax tree) (ast.rs).

  3. Evaluator (evaluator.rs) evaluates the AST by top-down parsing.

3.2 Example

  1. Input

    a + b * c

  2. Tokens

    [Identifier("a"), Plus, Identifier("b"), Asterisk, Identifier("c"), Eof]

  3. AST

  4. Result

    a + (b * c)

3.3 Other Interesting Files

  • object.rs: Built-in types such as Int, Float, Function, etc. and their base trait Object.

  • operator.rs: Built-in operators. For example, to which types the binary + can be applied and its behavior are defined in this file.

  • environment.rs: Environment. Scope is represented as a nested environment.

  • builtin.rs: Built-in constants (e.g. pi) and built-in functions (e.g. print(), bool(), etc.).