BoundaryML/baml · error · StrongAstError

Expected token/node {desc}, but was unable to find it in {pa

Error message

Expected token/node {desc}, but was unable to find it in {parent:?}

What it means

StrongAstError::MissingExpectedElementDesc is the descriptive-text variant of MissingExpectedElement: the builder expected some element described by `desc` (not a single specific SyntaxKind) but the parent node had no children remaining. It reports the description and the parent's TextRange.

Source

Thrown at baml_language/crates/baml_fmt/src/ast/mod.rs:64

        expected: SyntaxKind,
        found: SyntaxKind,
        at: TextRange,
    },
    /// When an element is expected but was found to be of a different kind.
    #[error("Expected token/node {expected_desc}, but found {found:?} at {at:?}")]
    UnexpectedKindDesc {
        expected_desc: Cow<'static, str>,
        found: SyntaxKind,
        at: TextRange,
    },
    /// When an element is expected (of a specific [`SyntaxKind`]) but there were no more children left.
    #[error("Expected token/node of kind {expected:?}, but was unable to find it in {parent:?}")]
    MissingExpectedElement {
        expected: SyntaxKind,
        parent: TextRange,
    },
    /// When an element is expected (not of a single specific [`SyntaxKind`]) but there were no more children left.
    #[error("Expected token/node {desc}, but was unable to find it in {parent:?}")]
    MissingExpectedElementDesc {
        desc: Cow<'static, str>,
        parent: TextRange,
    },
    /// When the node isn't expected to have any more children (e.g. a statement found a `;`) but there are still children left.
    #[error("Unexpected additional element at {at:?} in {parent:?}")]
    UnexpectedAdditionalElement { parent: TextRange, at: TextRange },
    /// When an element is expected to be a node but it's actually a token.
    #[error("An element at {at:?} was a node when it should have been a token.")]
    ShouldBeNode { at: TextRange },
    /// When an element is expected to be a token but it's actually a node.
    #[error("An element at {at:?} was a token when it should have been a node.")]
    ShouldBeToken { at: TextRange },
}
impl StrongAstError {
    /// Checks that the given node is of the specified [`SyntaxKind`].
    ///
    /// # Errors

View on GitHub (pinned to bd85ce9dee)

Solutions

Read `desc` and `parent` to determine which element is missing and add it to the BAML source
Fix all parser-reported errors before running the formatter so trees are complete
Keep parser/baml_fmt versions in sync
Validate programmatically built trees contain all mandatory children

Defensive patterns

Strategy: validation

Validate before calling

// confirm the descriptively-required element exists before conversion
if !has_required_element(parent, desc) {
    return Err(format!("missing {desc}"));
}

Try / catch

match StrongAst::from_cst(node) {
    Ok(ast) => use(ast),
    Err(StrongAstError::MissingExpectedElementDesc { desc, parent }) => {
        report(parent, format!("missing {desc}"));
    }
    Err(e) => report_all(e),
}

Prevention

Fix truncated expressions/statements in source before formatting
Resolve parser errors first so recovery doesn't drop subtrees
Sync parser and formatter crate versions
Validate programmatically built trees for required elements

When it happens

Trigger: FromCST conversion reaches the end of a parent node's children while a required, descriptively-specified element is still needed — e.g. expecting 'an expression' or 'a type annotation' but the iterator is empty.

Common situations: Truncated BAML expressions/statements in source files; parser recovery omitting subtrees; version drift between the parser grammar and the strong-AST builder.

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/24a8de8fd57d0682. Report an issue: GitHub.