Supertypes, subtypes, and more

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On Wednesday, 25 June 2014 at 15:33:16 UTC, bearophile wrote:
> In Phobos we have Typedef (that needs some improvements), that 
> allows to define a not compatible type.
>
> In D we have "alias this" that inside a struct allows to create 
> a new type (with instance object of size equal or larger than 
> the original type) with similar semantics of a given type. This 
> allows to define a type that acts "like" another.
>
> In Ada there is a standard way to restrict a type. In a Reddit 
> thread I have found this simple example of modern Ada code, a 
> sufficiently common need:
>
>
>> My go-to example is social-security numbers, because it's 
>> simple
>> enough that people can see what's going on as well as provides 
>> a
>> good way to illustrate how subtypes can allow one to 'ignore'1
>> the implementation.
>>
>> -- SSN format: ###-##-####
>> Subtype Social_Security_Number is String(1..11)
>>  with Dynamic_Predicate =>
>>    (for all Index in Social_Security_Number'Range =>
>>      (case Index is
>>       when 4|7 => Social_Security_Number(Index) = '-',
>>       when others => Social_Security_Number(Index) in '0'..'9'
>>      )
>>     );
>>
>> -- The above declaration can allows me to skip the validity
>> -- checks of parameters within the body of a subprogram as
>> -- the constraints are checked on subprogram-call.
>> -- I do not need to check the validity of the return value,
>> -- an attempt to return a non-conformant string will raise
>> -- and exception.
>> Function Get_SSN( Record : ID ) return Social_Security_Number;
>>
>> -- Likewise, passing a non-conformant value to SSN will raise
>> -- an exception.
>> Procedure Save_SSN( Record : ID; SSN : Social_Security_Number 
>> );
>
>
> I think where you expect a String you can send a 
> Social_Security_Number, but where it expects a 
> Social_Security_Number it doesn't accept a String. (The 
> Dynamic_Predicate and Static_Predicate of modern Ada allow to 
> perform such restrictions.) So I think there are some analogies 
> with OOP.
>
> I'd like to do something like this in D, with structs/values.
>
> Bye,
> bearophile

You can do this in C# as well:

class SocialSecurityNumber
{
     string _ssn;

     //Implicitly converts to string
     public static implicit operator string(SocialSecurityNumber 
ssn)
     {
         return _ssn;
     }
}