Debugging Your Language

Before we move on, let’s establish debugging techniques you’ll use throughout this book. Language implementations have many moving parts - knowing how to inspect each layer saves hours of frustration.

The Golden Rule

Your language is a pipeline. Data flows through stages: Source → Tokens → AST → Output. When something breaks, find which stage produced the wrong output.

Debug by checking each arrow: Is the input to this stage correct? Is the output?

Print the AST

The AST is your program’s structure. When behavior is wrong, print it:

// In your code
let ast = parse(source)?;
println!("{:#?}", ast);  // Pretty-print with {:#?}

Output:

Binary {
    op: Add,
    left: Int(1),
    right: Int(2),
}

If this looks wrong, your parser has a bug. If it looks right, the bug is in execution.

Check Operator Precedence

A common parsing bug: 1 + 2 * 3 parses as (1 + 2) * 3 instead of 1 + (2 * 3).

let ast = parse("1 + 2 * 3")?;
println!("{:#?}", ast);

// Should be: Add(1, Mul(2, 3))
// Bug if:    Mul(Add(1, 2), 3)

Fix: Check your grammar’s precedence rules. In PEG, earlier alternatives have higher precedence.

Test Small, Test Often

Don’t write 100 lines then debug. Test each feature in isolation:

#[test]
fn test_addition() {
    assert_eq!(eval("1 + 2"), 3);
}

#[test]
fn test_subtraction() {
    assert_eq!(eval("5 - 3"), 2);
}

#[test]
fn test_combined() {
    assert_eq!(eval("1 + 2 - 3"), 0);
}

When a test fails, you know exactly what’s broken.

Use the REPL

The REPL is your best friend for quick experiments:

>>> 1 + 2
3
>>> 1 + 2 * 3
7
>>> (1 + 2) * 3
9

If a complex expression fails, simplify until you find the minimal failing case.

When Using LLVM (Later)

When we add LLVM compilation, two more techniques become useful:

Print the IR

// After code generation
codegen.module.print_to_stderr();

You’ll see LLVM IR:

define i64 @add(i64 %a, i64 %b) {
entry:
  %result = add i64 %a, %b
  ret i64 %result
}

If this looks wrong, your codegen has a bug.

Use LLVM’s Verifier

codegen.module.verify().map_err(|e| {
    eprintln!("LLVM verification failed: {}", e);
    e.to_string()
})?;

The verifier catches:

• Missing terminators (every basic block needs ret or br)
• Type mismatches
• Invalid instructions

Always verify before JIT execution!

The Debugging Mindset

Think like a detective. You have a crime (wrong output). You need to find where in the pipeline the crime occurred. Interrogate each stage until you find the culprit.

1. Reproduce - Find the smallest input that triggers the bug
2. Isolate - Which stage is producing wrong output?
3. Inspect - Print the data at that stage
4. Fix - Change the code
5. Verify - Run your test again

This systematic approach works for any compiler bug.

Quick Reference

Now you have the tools. Let’s continue building!