slint-ui/slint · error

Second argument not an integer

Error message

Second argument not an integer: {:?}

What it means

The interpreter's builtin ArrayRemove function requires its second argument to evaluate to a Value::Number (an integer index). If the evaluated argument is any other Value kind (string, model, struct, etc.), the interpreter panics. ArrayRemove performs no numeric coercion.

Solutions

1. Ensure the second argument is a number-typed property or expression, e.g. `property<int> idx;`
2. Convert explicitly at the call site so the expression type-checks as int (e.g. use a numeric property instead of a string field)
3. Use the LSP/compiler warnings to detect the type error in the .slint file before runtime

Example fix

// before (.slint)
property<string> selected-id;
ArrayRemove(items, selected-id); // panics
// after
property<int> selected-index;
ArrayRemove(items, selected-index);

// .slint: declare the index as an int property
property<int> selected-index; // ArrayRemove(items, selected-index) is valid

// .slint has no runtime narrowing; enforce statically by typing the index property as int

When it happens

Trigger: Calling `ArrayRemove(arr, x)` where x evaluates to a non-number — e.g. passing a string that looks like a number, a boolean, or a struct field of the wrong type instead of the numeric index.

Common situations: Passing a property declared as `string` or `bool` as the index; using a value returned by another builtin that is not a number; implicit-coversion assumptions coming from other languages.

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

• First argument not an array
• Cannot create component instance
• Cannot show component
• expect Boolean, got
• expect Number, got

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/27a26f08e46b5173. Report an issue: GitHub.

Appendix: source

Thrown at internal/interpreter/eval.rs:2197

                "push",
                log_message_location(source_location),
                model.push_row(value),
            );

            Value::Void
        }
        BuiltinFunction::ArrayRemove => {
            if arguments.len() != 2 {
                panic!("internal error: incorrect argument count to ArrayRemove")
            }

            let model = match eval_expression(ctx, &arguments[0]) {
                Value::Model(m) => m,
                _ => panic!("First argument not an array: {:?}", arguments[0]),
            };
            let index = match eval_expression(ctx, &arguments[1]) {
                Value::Number(i) => i,
                _ => panic!("Second argument not an integer: {:?}", arguments[1]),
            };

            let result = match usize::try_from(index as i64) {
                Ok(index) => model.remove_row(index),
                Err(_) => Err(i_slint_core::model::ModelError::out_of_bounds(model.row_count())),
            };
            i_slint_core::model::report_model_error(
                "remove",
                log_message_location(source_location),
                result,
            );

            Value::Void
        }

        BuiltinFunction::ArrayInsert => {
            if arguments.len() != 3 {
                panic!("internal error: incorrect argument count to ArrayInsert")

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