Horribly Underqualified

Saber Notes Factorial

I’ve been working on my language, Saber a lot recently. I figured I should keep some notes on implementation in case someone finds them useful, even if that’s only future-me. Saber is a scripting language intended to be a lightweight row-polymorphic psuedo-functional language that compiles to WebAssembly. Think OCaml meets the best parts of ES6. Of course right now I’m just trying to get basic functionality finished. The compiler is written in Rust.

In my previous post I had gotten parameters and local variables working. After that, I started working on getting general statement code generation finished, control flow and function calls finished. This allowed me to write factorial, my first non trivial Saber program. Pretty neat if I say so myself.

Return and Other Statements

Stack based VMs are kind of interesting in that compiling expressions is a lot more simple (at least to me) than compiling statements. Expressions in stacks are fairly simple. You push the necessary parts to the expression on to the stack, whether those are function arguments, terms in an arithmetic expression, etc., then the VM evaluates them into a single element on the stack. Statements are a little weird in that most of the time they should result in no terms on the stack. In a lot of cases that means you might need to throw away a value. For instance, this is a statement: ``` foo(); `` However if foo returns a value, that means we need to discard it. But if foo doesn’t return anything, we need to not discard it.

Another slightly tricky thing is that code generation for return statements is a little different depending on where the return statement is placed. If it’s at the end of a function, say:

let add = \(a, b) => {
 return a + b
}

Then we don’t need to generate a return operation. By virtue of being in a stack based VM, we naturally return whatever’s on the stack at the end of a function1.

Up until now I had generated return statements with no return op, as I didn’t have any other statements, so the only statement that I was emitting was a return statement. But if I’m adding control flow, I need to have the return op so that I can actually tell WASM “hey, I want to return early”.

This isn’t hard as return is just another opcode. However I don’t want to always emit the code, as if the return statement is the last statement in a function, the return op is redundant. It’s a minor optimization but hey, why not. The answer is easy: thread an is_last boolean through generate_stmt. If you’re generating a return and is_last is true, then don’t emit the return.

fn generate_stmt(&mut self, stmt: &TypedStmt, is_last: bool) -> Result<Vec<OpCode>> {
    match stmt {
        TypedStmt::Return(expr) => self.generate_expr(expr),
        TypedStmt::Return(expr) => {
	        let mut opcodes = self.generate_expr(expr)?;
            if !is_last {
                opcodes.push(OpCode::Return);
            }
			Ok(opcodes)
        }
		...
	}
}

Next came if statements. I actually don’t plan on having if statements long-term. Eventually I want to transition to if expressions that can act as statements (but just evaluate to (), or the empty type). This is how Rust implements a lot of the traditional statements:

// Technically an expression of type ()
if cond {
  foo();
} else {
  bar();
}

// If expr evaluates to an integer
let var = if cond {
  12
} else {
  11
};

But for now we’re doing if statements because it’s easier. The initial code gen wasn’t too bad. WebAssembly has if statements built in, which is kinda high level for an “assembly” language, but hey I’m not gonna look a gift horse in the mouth. You push on the condition expression, the if opcode and the expected type (the empty type in this case). Then the opcodes corresponding to the then block (i.e. the block that runs if the condition is true), then an else opcode and the block corresponding to else (if it exists), then an end opcode. Not crazy.

What’s tricky is that WebAssembly requires some sort of return value if the if statement is the last statement in a function. Even if both branches return, WebAssembly gets a little uneasy. Therefore I figured that you need to push on a special opcode unreachable that indicates that either branch returns.

If by some virtue one branch doesn’t return, we get an error.

Function Calls

After that I implemented functions calls. This is getting a little hazy as I’m actually writing these posts a few months later, but I basically generated a function index for each function, then on a call expression, I check if the name corresponds to a function, get the index, and emit a call opcode with said index. This only works for functions defined statically, i.e. without any funny function-as-value stuff. For that I’d need indirect calls.

One slight complication is that the index generation needs to happen before we typecheck the function body, as otherwise recursive calls won’t work.

Factorial

With if statements, return and function calls down, I was able to write a simple factorial program! It looked like the following:

let fact = \n => {
  if n < 2 {
    return 1;
  }
  return n * fact(n - 1);
}

Very simple but satisfying to compile.


  1. Which actually means Rust/Ruby’s idiom of the last element in a block being the block’s value makes total sense for a stack VM ↩︎