# E0003

Warning name: `unused_type_declaration`

Unused type declaration.

This type is not used by any other part of your code, which means it's not
constructed nor consumed, and others can't construct nor consume due to
visibility. Type declaration is by default abstract, which means it is publicly
visible to users of this package, but they cannot construct nor destruct the
type. Therefore, this warning is only shown if the type has abstract or private
visibility, and is not constructed nor consumed by any other part of this
package.

Note that this warning might uncover other bugs in your code. For example, if
there are two types in the package that has similar name, you might just use the
other type by mistake.

## Erroneous example

```{literalinclude} /sources/error_codes/0003_error/top.mbt
:language: moonbit
```

## Suggestion

There are multiple ways to fix this warning:

- If the type is indeed useless, you can remove the definition of the type.
- If this type is not local, and is part of the public API of your module, you
  can add `pub` or `pub(all)` visibility keyword from the type.

  ```{literalinclude} /sources/error_codes/0003_fixed/top.mbt
  :language: moonbit
  :start-after: make public
  :end-before: end make public
  ```

- Check if you are referencing the type with a correct name.

It is rather rare to have a type that is private and unused at the same time;
therefore, we suggest you remove the type if it is not used.