New Rule: Export All Used Types

[elbrujohalcon]

Export All Used Types

Brief Description

The idea is to emit a warning every time a spec for an exported function uses a local type that's not exported.

Should be on by default?

YES

Reasoning

If you put a type in a spec of an exported function, that means that people writing modules that call that function should know what that type is and they may as well pass instances of that type around before calling your function. So, to add their own specs, and also to keep dialyzer messages cleaner, they should put your_mod:your_type() in their specs, instead of the actual definition of the type.
To be able to do that, the type must be exported.

Refactoring Proposal

Add the non-exported types to an -export_type(…). attribute.

Examples

-module bad.

-type my_type() :: my | type.

-export [my_fun/1].

-spec my_fun(my_type()) -> my_type().
my_fun(my) -> type;
my_fun(type) -> my.

-module good.

-type my_type() :: my | type.
-export_type [my_type/0].

-export [my_fun/1].

-spec my_fun(my_type()) -> my_type().
my_fun(my) -> type;
my_fun(type) -> my.

[paulo-ferraz-oliveira]

Oh, yeah, we ARE missing this one. ❤️