BoundaryML/baml · error · VmPanic

baml.panics.MapKeyNotFound

baml.panics.MapKeyNotFound

Error message

key not found in map

What it means

Raised when a map lookup does not find the requested key. Unlike some runtimes it returns a panic rather than an optional value, so absence of a key is treated as an exceptional condition. The error carries no payload since the key is known at the call site.

Solutions

1. Check membership before indexing, or use the map API that returns an option/default value.
2. Normalize keys (trim/case) on both write and read sides.
3. Catch `baml.panics.MapKeyNotFound` and substitute a default when absence is acceptable.

Example fix

// before
let port = config["port"];
// after
let port = config.get("port").unwrap_or(8080);

// BAML: membership check before indexing
if (config.contains("port")) { let port = config["port"]; }

// BAML
try {
  let v = map[key];
} catch (e: baml.panics.MapKeyNotFound) {
  let v = default;
}

When it happens

Trigger: Evaluating a map indexing instruction with a key absent from the map, e.g. `map["missing_key"]` or a computed key that never matches stored keys.

Common situations: Typos in string keys, keys built from user input or external data with different formats (casing, whitespace), assuming a config/env map contains a key it does not.

Understand the failure class

Background: Record Not Found Errors: "not found", RecordNotFound, and "was not found" — what they mean and how to fix them — this error's family across 28 libraries.

Related errors

• baml.panics.AssertionFailed
• baml.panics.Cancelled
• baml.panics.Exit
• baml.panics.IndexOutOfBounds
• baml.panics.IntegerOverflow

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/37215a0c72c47c81. Report an issue: GitHub.

Appendix: source

Thrown at baml_language/crates/bex_vm_types/src/errors.rs:45

    /// An `int` (i63) arithmetic operation overflowed the representable
    /// range `[INT_MIN, INT_MAX]`. Carries a human-readable description of
    /// the operation (e.g. `"4611686018427387903 + 1"`); built only on the
    /// cold overflow path, so the `String` alloc never touches hot code.
    #[error("integer overflow: {message}")]
    IntegerOverflow { message: String },

    // Raised by array and byte-array subscripting, so the message stays generic
    // ("index", not "array index").
    #[error("index out of bounds: {index} of {length}")]
    IndexOutOfBounds { index: i64, length: usize },

    #[error("invalid field access: field {field_index} of {field_count}")]
    InvalidFieldAccess {
        field_index: usize,
        field_count: usize,
    },

    #[error("key not found in map")]
    MapKeyNotFound,

    #[error("stack overflow")]
    StackOverflow,

    #[error("assertion failed")]
    AssertionFailed,

    #[error("unreachable code executed")]
    Unreachable,

    #[error("operation cancelled")]
    Cancelled,

    /// A user-caused panic from `baml.sys.panic`, and the stdlib's panic of
    /// record for a user-violated native invariant (e.g. a reflection kind
    /// view's `_ty` field overwritten with a type of a different kind).
    #[error("baml.sys.panic: {message}")]

View on GitHub (pinned to bd85ce9dee)