Yes! I’ve learned that Dialyzer has @opaque directive.
If you mark a type as @opaque, you can use it inside the module it is defined. Outside the module, you can pass the opaque type around, but you can’t access its fields. This directive renders @imetallica solution great! E.g.
defmodule Email do
defstcut :email
@opaque t() :: %__MODULE__{email: string()}
@spec validate(String.t()) :: t()
def validate(string_email) do
#... validations
# if this is the only place you return that struct, dialyzer will make sure nobody else creates or modifies the struct
%__MODULE__{email: string_email)
end
@spec send(t()) :: :ok | :error
def send(%__MODULE__{email: string_email}) do
#access to the field is OK because you can use @opaque type fields in the same module
:ok
end
end
def OtherModule do
def test() do
valid_email = Email.validate("email@example.com")
%Email{email | email: "something_broken"} #dialyzer will complain because the type is opaque
end
end
Combined with other approaches from this talk: https://www.youtube.com/watch?v=XGeK9q6yjsg
it gives me a pretty nice domain-driven design 
I will watch the video. For the code I’ll have to refer to the docs. May I ask, are there any patterns you apply to Phoenix specifically? 
Here’s a link to the explanation on 