W0059: Unavailable Type

Error

-module(main).
-spec foo() -> dep:nonexistent_type().
%%             ^^^^^^^^^^^^^^^^^^^^^^ warning: W0059: Type 'dep:nonexistent_type/0' is not available through dependencies.
foo() -> ok.

Explanation

This diagnostic warns you when your code references a type that either:

1. Does not exist (is undefined)
2. Is not accessible because it's from a module outside your application's dependencies
3. Exists but is not exported from the defining module

What This Diagnostic Checks

The diagnostic examines:

• Type specifications (-spec attributes)
• Type definitions (-type and -opaque attributes)
• Callback specifications (-callback attributes)

How to Fix

• Check if the type actually exists
• Verify the name and arity of the type
• Add the application that defines the type to your application's dependencies or move the type definition to an application that is already a dependency