# E4012

Mutable constructor fields are only allowed on labelled arguments.

## Erroneous Example

```{literalinclude} /sources/error_codes/4012_error/top.mbt
:language: moonbit
```

The example above declares an unlabeled mutable field `x` in the
constructor of the `F` variant of the `E` enum, which is not allowed.

## Suggestions

Add a label to the mutable field:

```{literalinclude} /sources/error_codes/4012_fixed/top.mbt
:language: moonbit
```