BoundaryML/baml · error

Type error: {}

Error message

Type error: {}

What it means

The BAML REPL type-checks each typed expression before evaluation; if the compiler's type diagnostics contain errors, evaluation is aborted and all type-error messages are joined into one anyhow error. This guarantees expressions only run once they pass type inference.

Source

Thrown at engine/baml-runtime/src/cli/repl.rs:514

        if type_diagnostics.has_errors() {
            eprintln!("Warning: Type errors in loaded BAML sources");
        }

        let input_expr_ast = parse_standalone_expression(input, &mut type_diagnostics)?;
        let input_expr_hir = hir::Expression::from_ast(&input_expr_ast);

        let input_expr_thir =
            typecheck_expression(&input_expr_hir, &type_context, &mut type_diagnostics);

        // Check for type errors in the user's expression
        if type_diagnostics.has_errors() {
            let error_messages: Vec<String> = type_diagnostics
                .errors()
                .iter()
                .map(|e| e.message().to_string())
                .collect();
            return Err(anyhow!("Type error: {}", error_messages.join("; ")));
        }

        // let variables: IndexMap<String, BamlValueWithMeta<TypeGeneric<TypeIR>>> = self

        let variables: IndexMap<String, BamlValueWithMeta<ExprMetadata>> = self
            .variables
            .iter()
            .map(|(k, v)| (k.clone(), v.clone()))
            .collect();

        let fn_params = self.function_parameters()?.clone();

        let runtime_clone = self.runtime.clone();
        let env_vars = self.env_vars.clone();
        let run_id = self.current_run_id.unwrap_or(0);
        let handle_llm_function = move |function_name: String,
                                        args: Vec<BamlValue>,
                                        _watch_context: Option<

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Fix the reported type error in the expression (read the joined messages after 'Type error:')
2. Run :type <expression> first to check the inferred type before evaluating
3. Use explicit casts or correct literal types in the expression
4. Re-check variable types with :env after reloading BAML sources

Example fix

// before (REPL input)
MyFn("hello")  // where MyFn expects int
// after
MyFn(42)

// Before evaluating, type-check the expression at the REPL
:type MyFn(arg1, arg2)
// In embedding code: match on the Result and inspect the anyhow message
if let Err(e) = repl.parse_and_evaluate_baml_expression(expr) {
    if e.to_string().starts_with("Type error:") { /* show diagnostics to user */ }
}

match result { Err(e) if e.to_string().starts_with("Type error:") => println!("Fix types: {e}"), Ok(v) => println!("{v:?}"), Err(e) => eprintln!("{e:#}") }

When it happens

Trigger: Calling parse_and_evaluate_baml_expression_with_status (directly or via evaluate_simple_expression_with_status / parse_and_evaluate_baml_expression) with an expression whose BAML type checking fails, e.g. passing a string to a function expecting an int.

Common situations: Typing an expression with a type mismatch at the REPL prompt, calling an LLM function with wrongly-typed positional arguments, using a variable of the wrong type after :load reloads sources with changed signatures.

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

• No BAML sources loaded. Use :load <path> to load sources.
• LLM Function {} not found
• Failed to parse function result: {}
• No parsed result available from function call
• No runtime loaded, it should be impossible to call an LLM fu

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/6a44cc08d7e28f97. Report an issue: GitHub.