ExVatcheck.check - make invalid states invalid

[stefanluptak]

Hi there. 🙂

First of all, thanks a lot for this little but very useful library.

One thing I am struggling a bit with is the return type of the ExVatcheck.check/1 function. Since it is a struct with 4 keys, the number of possible states is too big and doesn't represent the reality.

In the spirit of the "Make Illegal States Unrepresentable" I think it would be more convenient if that function returns structs that represent the only possible outcomes:

%ExVatcheck.InvalidFormatError{}
%ExVatcheck.ViesServiceError{message: String.t()}
%ExVatcheck.ViesResponseInvalid{}
%ExVatcheck.ViesResponseValid{response: map()}

Of course, those structs can be tuples or something similar. It's just a rough idea and I will be happy to implement it, if there's a positive feedback to this and once we agree on a way to do this.

What do you think?