BoundaryML/baml · error

length() method takes no arguments at {:?}

Error message

length() method takes no arguments at {:?}

What it means

THIR interpreter runtime error: the `length()` method was invoked with one or more arguments. The builtin length method on lists, strings, and maps takes none; extra arguments mean the call site is malformed (perhaps intending a different function), and evaluation aborts with the method-call location.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2809

            Ok(parsed_value)
        }
        _ => bail!("unknown builtin function '{}' at {:?}", fn_name, meta.0),
    }
}

fn evaluate_method_call(
    receiver: &BamlValueWithMeta<ExprMetadata>,
    method_name: &str,
    args: &[BamlValueWithMeta<ExprMetadata>],
    meta: &ExprMetadata,
) -> Result<BamlValueWithMeta<ExprMetadata>> {
    match method_name {
        "length" => {
            // Array/List/String/Map length method
            match receiver {
                BamlValueWithMeta::List(items, _) => {
                    if !args.is_empty() {
                        bail!("length() method takes no arguments at {:?}", meta.0);
                    }
                    Ok(BamlValueWithMeta::Int(items.len() as i64, meta.clone()))
                }
                BamlValueWithMeta::String(s, _) => {
                    if !args.is_empty() {
                        bail!("length() method takes no arguments at {:?}", meta.0);
                    }
                    Ok(BamlValueWithMeta::Int(
                        s.chars().count() as i64,
                        meta.clone(),
                    ))
                }
                BamlValueWithMeta::Map(map, _) => {
                    if !args.is_empty() {
                        bail!("length() method takes no arguments at {:?}", meta.0);
                    }
                    Ok(BamlValueWithMeta::Int(map.len() as i64, meta.clone()))
                }

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Remove all arguments from the length() call
2. If filtering by a predicate is intended, filter the list first, then call length()

Example fix

// before
let n = items.length(0)
// after
let n = items.length()

fn validate_length_call(args: &[BamlValueWithMeta<ExprMetadata>]) -> Result<(), String> {
    if !args.is_empty() {
        return Err("length() takes no arguments".into());
    }
    Ok(())
}

When it happens

Trigger: Evaluating someList.length(x) or someList.length(a, b) where someList is a BamlValueWithMeta::List.

Common situations: Confusing length() with functions in other languages that take a predicate or separator; mechanically converting len(list) style calls from Python.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• length() method not available on type {:?} at {:?}
• toLowerCase() method takes no arguments at {:?}
• env.get expects exactly one argument
• baml.media.image.from_url expects 1 argument, got {} at {:?}
• baml.fetch_as expects 1 argument (url), got {} at {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/ed90036d86500b9a. Report an issue: GitHub.