typst/typst · error

cannot mutate fields on {ty}

Error message

cannot mutate fields on {ty}

What it means

From access_dict in the evaluator: a field-mutating operation (like `obj.field = x` or `obj.field += y`) targeted a value whose type is one of symbol/content/module/function/args — types that expose read-only named fields via their own getters rather than a mutable dictionary. The at-fault input is the target value of such a type.

Source

Thrown at crates/typst-eval/src/access.rs:93

pub(crate) fn access_dict<'a>(
    vm: &'a mut Vm,
    access: ast::FieldAccess,
) -> SourceResult<&'a mut Dict> {
    match access.target().access(vm)? {
        Value::Dict(dict) => Ok(dict),
        value => {
            let ty = value.ty();
            let span = access.target().span();
            if matches!(
                value, // those types have their own field getters
                Value::Symbol(_)
                    | Value::Content(_)
                    | Value::Module(_)
                    | Value::Func(_)
                    | Value::Args(_)
            ) {
                bail!(span, "cannot mutate fields on {ty}");
            } else if typst_library::foundations::fields_on(ty).is_empty() {
                bail!(span, "{ty} does not have accessible fields");
            } else {
                // type supports static fields, which don't yet have
                // setters
                Err(eco_format!("fields on {ty} are not yet mutable"))
                    .hint(eco_format!(
                        "try creating a new {ty} with the updated field value instead"
                    ))
                    .at(span)
            }
        }
    }
}

View on GitHub (pinned to 35417aa769)

Solutions

1. Mutate a dictionary instead of these read-only field types
2. Wrap the data in your own dictionary and mutate that
3. Rebuild the value (e.g. with a show rule or constructor) instead of mutating its fields

When it happens

Trigger: Thrown at crates/typst-eval/src/access.rs:93 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of typst/typst@35417aa769 (2026-08-17). Data as JSON: /api/errors/5472e6288c66ca10. Report an issue: GitHub.