BoundaryML/baml · error

C-style for loop condition must be boolean

Error message

C-style for loop condition must be boolean

What it means

Raised by the BAML interpreter while executing a C-style `for (init; cond; after)` loop: after evaluating the condition, the result must be Bool(true) to continue or Bool(false) to break. Any other value type (Int, String, null, etc.) makes the match fall through to this bail. BAML does not truthiness-coerce loop conditions.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:1064

                    } => {
                        loop {
                            // Check condition (if present)
                            if let Some(cond_expr) = condition {
                                let cond_val = expect_value(
                                    evaluate_expr(
                                        cond_expr,
                                        scopes,
                                        thir,
                                        run_llm_function,
                                        watch_handler,
                                        function_name,
                                    )
                                    .await?,
                                )?;
                                match cond_val {
                                    BamlValueWithMeta::Bool(false, _) => break,
                                    BamlValueWithMeta::Bool(true, _) => {}
                                    _ => bail!("C-style for loop condition must be boolean"),
                                }
                            }

                            // Execute loop body
                            match evaluate_block_with_control_flow(
                                block,
                                scopes,
                                thir,
                                run_llm_function,
                                watch_handler,
                                function_name,
                            )
                            .await?
                            {
                                ControlFlow::Break => break,
                                ControlFlow::Continue => {
                                    // Execute after statement if present
                                    if let Some(after_stmt) = after {

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Make the condition a real boolean expression, e.g. `i > 0` instead of `i`.
2. Fix any helper function used in the condition to return Bool.
3. Handle null-producing conditions explicitly (`cond != null && cond)` style).

Example fix

// before (BAML)
for (let i = 10; i; i -= 1) { ... } // error: condition must be boolean

// after
for (let i = 10; i > 0; i -= 1) { ... }

// ensure a loop condition is boolean before running the C-for
function assertBoolCondition(cond: unknown): void {
  if (typeof cond !== "boolean") {
    throw new Error("C-style for loop condition must be a Bool expression");
  }
}

const isBool = (v: unknown): v is boolean => typeof v === "boolean";

try {
  await runBaml(program);
} catch (e) {
  if (String(e).includes("C-style for loop condition must be boolean")) {
    // rewrite the condition as an explicit comparison (e.g. `i > 0`) and retry
  } else { throw e; }
}

When it happens

Trigger: A C-for loop whose condition expression evaluates to a non-Bool, e.g. `for (let i = 0; i; i += 1)` (Int condition) or a condition returning a string/null from a function call.

Common situations: Porting C/JS/Python loops that rely on numeric truthiness (`while i`); calling a helper in the condition that returns Int/String instead of bool; using a nullable comparison result.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• unsupported types for += in C-for after clause
• unsupported types for %= operator
• bitwise ^= requires integer operands
• bitwise &= requires integer operands
• bitwise |= requires integer operands

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/c024f0c9b79ae743. Report an issue: GitHub.