gleam-lang/gleam · error

is not a built-in type.

Error message

{name} is not a built-in type.

What it means

The TypeScript prelude printer (`print_prelude_type`) maps built-in Gleam types (String, Int, Float, Bool, List, Result, BitArray, UtfCodepoint, etc.) to their TypeScript prelude equivalents. Any other type name reaching it is a compiler error, so it panics with the offending name.

Solutions

1. Check the panic message for the type name and add a matching arm in `print_prelude_type` (compiler development).
2. Ensure programmer-defined types go through the named-type printing path, not the prelude path.
3. If triggered by ordinary user code, file a Gleam compiler bug with the reported type name.

Example fix

// before (in print_prelude_type)
name => panic!("{name} is not a built-in type.")
// after (during development of a new builtin)
"MyNewType" => Ok("MyNewType".to_string()),
name => panic!("{name} is not a built-in type.")

// route only built-ins through prelude printing
const BUILTINS: &[&str] = &["Int","Float","String","Bool","List","Result","BitArray","UtfCodepoint"];
if !BUILTINS.contains(&name.as_str()) {
    return print_named_type(name); // named path instead of prelude
}

fn is_builtin_type(name: &str) -> bool {
    matches!(name, "Int" | "Float" | "String" | "Bool" | "List" | "Result" | "BitArray" | "UtfCodepoint")
}

match std::panic::catch_unwind(|| print_prelude_type(t)) {
    Ok(s) => s,
    Err(_) => fallback_to_named_type_printing(t),
}

When it happens

Trigger: A type whose name is not a recognized built-in is printed as a prelude type — typically when a new built-in type is added to the core language but not registered in `typescript.rs`, or when internals pass a non-builtin type to prelude printing.

Common situations: Gleam compiler development adding language built-ins; tooling invoking the TypeScript printer with custom/core types it wasn't designed for.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• case with no subjects
• Custom type constructor exist for type
• Custom type constructor must have custom type kind
• Invalid bit array segment type reached code generation
• invalid constants should not reach code generation

AI-assisted analysis of gleam-lang/gleam@15b07c7830 (2026-09-14). Data as JSON: /api/errors/f71a96d56452b92a. Report an issue: GitHub.

Appendix: source

Thrown at compiler-core/src/javascript/typescript.rs:1181

                    )
                ]
            }
            "Result" => {
                self.tracker.prelude_used = true;
                docvec![
                    arena,
                    UNDERSCORE_DOT_RESULT_DOCUMENT,
                    wrap_generic_arguments(
                        arena,
                        arguments
                            .iter()
                            .map(|x| self.do_print(arena, x, generic_printing))
                    )
                ]
            }
            // Getting here should mean we either forgot a built-in type or there is a
            // compiler error
            name => panic!("{name} is not a built-in type."),
        }
    }

    /// Prints a "named" programmer-defined Gleam type into the TypeScript
    /// equivalent.
    ///
    fn print_type_app(
        &mut self,
        arena: &'doc DocumentArena<'a, 'doc>,
        name: &str,
        arguments: &[Arc<Type>],
        module: &str,
        generic_printing: GenericPrinting<'_>,
    ) -> Document<'a, 'doc> {
        let name = eco_format!("{}$", ts_safe_type_name(name.to_string()));
        let name = match module == self.module.name {
            true => name.to_doc(arena),
            false => {

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