dlox

A full-featured VM written in dart

Live editor

Head over to the live editor to play with dlox online! The editor is based on the code_text_field flutter widget, and has been compiled in javascript to run entirely in the browser. Indeed, both dlox and the editor

Implementation

dlox is a strict superset of the fantastic lox language designed by Bob Nystrom.

Features

Common with lox VM

• Arithmetic operations
• Control flow
• Loops (for & while)
• Stack based local variables manipulation
• Scoping & closures
• Classes & inheritance

Extensions

• Suport for list and maps

var list = [1, 2, 3];
var map = {"a": 1, "b": 2};

• Container indexing

print [1, 2, 3, 4][1:-1]; // [2,3]
print "a cool feature"[2:]; // "cool feature"

• Container traversal

for v in "test" {
    print v; // t, e, s, t
}
for v in [1, 2, 3] {
    print v; // 1, 2, 3
}
// All containers also support index extraction
for v, k in {"a": 1, "b": 2} {
    print k + " " + v; // 0 a, 1 b
}

• Usual mathematical functions

var theta = 3.14 / 4;
var a = sin(theta);
var b = sqrt(1 - cos(theta)^2);
var c = ceil(abs(round(-theta)));

• Controlled execution

Pause on each line, and limit the number of instructions per run for asyncronous execution

Using the CLI

A repl can be spun up this way

# in dlox
dart lib/main.dart
> fun sum(a, b) { return a + b; }
> print sum(2, 3) # 5

Or a file can be executed

# in dlox
dart lib/main.dart editor/assets/snippets/fibonacci.lox

VM primer

The VM utilizes a stack to maintain all its local variables, function arguments, temporary operation results and return values. As its name suggets, a stack is a simple data structure supporting two main operations

• Pushing and element on top of it
• Popping its topmost element

Let's see how that works Head over to the editor and enter the simple program

var a = (1 + 2) * 3;
print a;

The top right panel shows the compiled bytecode

On small screens, the compiler panel is hidden by default. Press the rightmost button on the top toolbar to show it

Here's a breakdown of the compile program

OP_CONSTANT '1'      // Push 1 on top of the stack
OP_CONSTANT '2'      // Push 2 on top of the stack
OP_ADD               // Pop the two last elements off the stack
                     // Add them and push the result on the stack
OP_CONSTANT '3'      // Push 3 on top of the stack
OP_MULTIPLY          // Pop the two last elements off the stack
                     // Multiply them and push the result on the stack
OP_DEFINE_GLOBAL 'a' // Pop the last stack entry
                     // Assign it to the global variable 'a'
OP_GET_GLOBAL 'a'    // Push the global variable 'a' on the stack
OP_PRINT             // Pop the last stack entry
                     // Print it
OP_NIL               // Push NIL on the stack
OP_RETURN            // Pop and return the last stack value

Wow, that seems a lot for such a simple program! Well, breaking it down in simple instructions allows the Virtual machine to execute it in a really streamlined fashion. All it needs to do is rip through the instructions one by one while carefully bookkeeping the global variables and managing the stack

Now run that nifty piece of code. The terminal output should print 9. The bottom right panel shows the VM trace

On small screens, the VM panel is hidden by default. Press the rightmost button on the bottom toolbar to show it

Here's the first few lines

The VM trace shows instruction per instruction what the state of the stack is. We can see that is starts with the <script> object, and then proceeds to add 1 + 2, and to mutiply the result by 3, before popping it and assigning it to the global variable 'a'.

For further in-depth exploration, make sure to play with the examples in the top left drawer and make your own! Of course, Bob's book explains everything in incredible details.

Benchmark

To really unleash the VM thirst for code, disable the trace by swiching the performance button on the bottom toolbar.

That should make the execution significantly faster. The indicator on the left shows the average number of instructions per second

Running the test suite

Should you feel like tweaking things to you liking, a little regression testing here and there never hurts

# in dlox
dart lib/test.dart