In Erlang, is it possible to define a type by excluding a subtype from a supertype? As an example, how would I define a type that is "anything but pid()":
-type anything_but_pid() :: ...?
Reading the docs on supervisor data types, I came across this type spec for child_id(), which defines the type as term(), and then says Not a pid() in a comment below:
child_id() = term()Not a
pid().
Is that the best I can do if I don't want to explicitly list all possible types?
No, as of OTP 23 (and even 24) this is not supported.
Moreover, expecting Dialyzer to help is such cases is ill-advised, as Dialyzer is often over-approximating, for several reasons, and although I don't think that it would turn such a type to term() immediately, it might very well do so at the first given opportunity (e.g. wherever you call such a function).
As a possible workaround for this you may define a specification for your function like this:
-spec foo(pid()) -> ...;
(any()) -> ...
foo(data) when is_pid(data) -> error;
foo(data) -> ....
Specifically for anything_but_pid(), you could "simply" enumerate all other options:
-type anything_but_pid() :: port()
| reference()
| atom()
| bitstring()
| number()
| fun()
| maybe_improper_list()
| map()
| tuple()
Of course, this doesn't generalize...