[erlang-questions] Emulating phantom types maybe

[Tuncer Ayaz]

I've been wondering if I cannot tell Dialyzer that a function may only
accept strings produced by a certain function, in a way emulating
phantom types. While it's all a string, in such a function I expect a
certain kind of string, say, lower-cased, to give a trivial example. I
haven't figured out a way to encode that constraint. To be clear, the
string value is just an example, and this isn't exclusively about
strings.

Basically, I want to subtype structurally-equivalent values.

Why do I want this? I've run into a bug in the past, and this would
have prevented it and also documented the code more clearly.

>From what I understand, this isn't something that can be achieved with
Dialyzer, or can it?

So, here's a failed experiment trying to constrain need_lower/1 to
accept only input of type lower/0.

-type upper() :: string().
-type lower() :: string().

-spec lowercase(upper()) -> lower().
lowercase(U) -> string:to_lower(U).

-spec need_lower(lower()) -> 'ok'.
need_lower(_S) -> ok.

%% Would be great if this wouldn't dialyze.
wrong_type() ->  need_lower("UPPERCASE").