Optional type - how to correctly reset a wrapped immutable T

[jmh530]

On Tuesday, 27 March 2018 at 13:02:50 UTC, aliak wrote:
>
> Hmm, now that I'm explicitly trying to produce it, I feel I 
> maybe using inout incorrectly?
>
> struct Optional(T) {
>     T[] bag;
>     this(T t) {
>         bag = [t];
>     }
> }
>
> struct S {
>     Optional!(inout(int)) f() inout
>     {
>         return Optional!(inout(int))(3);
>     }
> }
>
> void main()
> {
>     auto a = S().f;
> }
>
> Above gives: Error: variable 
> `onlineapp.Optional!(inout(int)).Optional.bag` only parameters 
> or stack based variables can be inout
>
> Change inout to const e.g. and it's all good.

You may refer to the section on struct constructors. inout on a 
constructor serves as both a const and an immutable constructor. 
This would allow you to create mutable/const/immutable objects of 
whatever struct. The payload could have some different setting.

https://dlang.org/spec/struct.html#struct-constructor

How about:

struct Optional(T) {
     T[] bag;
     this(T t) {
         bag = [t];
     }
}

Optional!T optional(T)(T x)
{
     return Optional!T(x);
}

void main()
{
     int x = 3;
     const(int) xx = 3;
     immutable(int) xxx = 3;

     auto y = optional(x);
     auto yy = optional(xx);
     auto yyy = optional(xxx);
}