Blogpost about the T.init problem

[Cym13]

On Tuesday, 10 July 2018 at 13:41:56 UTC, FeepingCreature wrote:
> I've written up a short blogpost about the T.init issue.
>
> It is not very enthusiastic.
>
> https://medium.com/@feepingcreature/d-structs-dont-work-for-domain-data-c09332349f43
>
> Related links:
>
> https://github.com/dlang/phobos/pull/6594 problem with T.init 
> and toString
>
> https://github.com/dlang/phobos/pull/6619 Nullable can't work 
> with types where T.init violates invariants
>
> https://github.com/dlang/dmd/pull/8462 A somewhat sketchy PR to 
> disable invariant on struct ~this

First of all I must point that I would very much like to have 
seen a code actually producing an error in that article. Contrary 
to what is hinted just taking the struct and putting using it 
with Nullable or format() caused no error for me and worked as 
expected. Taking .init explicitely was the only thing that 
actually caused an error. I'm not saying you didn't experience 
these issues, but if you want to demonstrate a problem then 
please demonstrate it.

That said, I may be missing something obvious but what prevents 
you from overloading the init field?

     struct MyDomainData {
         string username;
         @disable this(); // don't make a MyDomainData() by 
accident!
         this(string username)
         in(!username.empty) // only non-empty usernames please!
         do { this.username = username; }
         // let's formalise the restriction.
         invariant { assert(!username.empty); }
         string toString() { ... }

         static @property MyDomainData init() {
             return MyDomainData("uninitialized");
         }

         ...
     }

     auto test = MyDomainData.init;  // There, no error

Of course that value means nothing but .init isn't meant to 
actually mean something anyway, it's just a valid value and 
that's what that init is proposing, so it shouldn't cause any 
more bugs than empty .init in a normal case.