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)

Which Dependencies Count

A type reference is erased at compile time, so it needs the defining application's source to be visible to the analysis, not its beam to be loadable at runtime. The reachable set is therefore wider than the one unavailable_function (W0085) uses for calls: alongside applications and included_applications, an application declared in distributed_dependencies counts as reachable. That relation describes an application reached over the network, so it carries no build-system edge and — unlike applications — is allowed to be cyclic.

Reachability is transitive across both kinds of edge, so declaring an application in distributed_dependencies also makes everything that application depends on reachable.

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