BoundaryML/baml · error

unsupported types for - operator at

Error message

unsupported types for - operator at {:?}

What it means

THIR interpreter runtime error: the binary `-` operator was applied to operand types with no defined subtraction — only int/int, float/float, and mixed numeric pairs are supported. Any other combination (string, bool, null, list, map on either side) reaches this bail with the expression's source metadata attached.

Solutions

1. Convert operands to numbers explicitly before subtracting
2. Validate/parse LLM or config outputs into Int/Float types first
3. Check for null and handle missing values before arithmetic

Example fix

// before
let diff = input - 10;    // input is "42"
// after
let diff = toInt(input) - 10;

if (typeof a !== 'number' || typeof b !== 'number') { throw new Error('subtraction requires numeric operands'); }

function isNumeric(v) { return typeof v === 'number' && !Number.isNaN(v); }

try { result = a - b; } catch (e) { if (String(e).includes('unsupported types for -')) { /* convert to numbers and retry */ } else { throw e; } }

When it happens

Trigger: Subtracting strings, nulls, booleans, or arrays; a variable expected to be numeric actually holding a string from an LLM response or config.

Common situations: Numeric strings from parsed LLM output used directly in arithmetic; null from a missing optional value; copying JS code that relies on implicit string-to-number coercion.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• unsupported types for + operator at
• unsupported types for * operator at
• for loop requires iterable (list)
• unsupported types for += operator
• division by zero at

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/423fd3dfb71ea205. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2381

            (BamlValueWithMeta::String(a, _), BamlValueWithMeta::String(b, _)) => {
                BamlValueWithMeta::String(format!("{a}{b}"), meta.clone())
            }
            _ => bail!("unsupported types for + operator at {:?}", meta.0),
        },
        BinaryOperator::Sub => match (left_val.clone(), right_val.clone()) {
            (BamlValueWithMeta::Int(a, _), BamlValueWithMeta::Int(b, _)) => {
                BamlValueWithMeta::Int(a - b, meta.clone())
            }
            (BamlValueWithMeta::Float(a, _), BamlValueWithMeta::Float(b, _)) => {
                BamlValueWithMeta::Float(a - b, meta.clone())
            }
            (BamlValueWithMeta::Int(a, _), BamlValueWithMeta::Float(b, _)) => {
                BamlValueWithMeta::Float((a as f64) - b, meta.clone())
            }
            (BamlValueWithMeta::Float(a, _), BamlValueWithMeta::Int(b, _)) => {
                BamlValueWithMeta::Float(a - (b as f64), meta.clone())
            }
            _ => bail!("unsupported types for - operator at {:?}", meta.0),
        },
        BinaryOperator::Mul => match (left_val.clone(), right_val.clone()) {
            (BamlValueWithMeta::Int(a, _), BamlValueWithMeta::Int(b, _)) => {
                BamlValueWithMeta::Int(a * b, meta.clone())
            }
            (BamlValueWithMeta::Float(a, _), BamlValueWithMeta::Float(b, _)) => {
                BamlValueWithMeta::Float(a * b, meta.clone())
            }
            (BamlValueWithMeta::Int(a, _), BamlValueWithMeta::Float(b, _)) => {
                BamlValueWithMeta::Float((a as f64) * b, meta.clone())
            }
            (BamlValueWithMeta::Float(a, _), BamlValueWithMeta::Int(b, _)) => {
                BamlValueWithMeta::Float(a * (b as f64), meta.clone())
            }
            _ => bail!("unsupported types for * operator at {:?}", meta.0),
        },
        BinaryOperator::Div => match (left_val.clone(), right_val.clone()) {
            (BamlValueWithMeta::Int(a, _), BamlValueWithMeta::Int(b, _)) => {

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