readonly member (but assignable at constructor time)

[Jonathan M Davis]

On Monday, April 30, 2018 10:36:52 bauss via Digitalmars-d-learn wrote:
> On Saturday, 28 April 2018 at 04:56:26 UTC, lempiji wrote:
> > On Friday, 27 April 2018 at 02:59:16 UTC, Dr.No wrote:
> >> In C# you can have a readonly member assignable either at
> >> declaration or constructor time, like this:
> >>
> >> class C
> >> {
> >>
> >>   readonly myClass mc;
> >>
> >>   this()
> >>   {
> >>
> >>     mc = new myClass();
> >>
> >>   }
> >>
> >>
> >>   void doSomething()
> >>   {
> >>
> >>     mc = new myClass(); // wrong! result in compiler error, mc
> >>
> >> is readonly
> >>
> >>   }
> >>
> >> }
> >>
> >> Does D have something like this natively or there's a way to
> >> do so with traits/CTFE at runtime?
> >
> > You can use std.experimenta.typecons.Final.
> > https://dlang.org/phobos/std_experimental_typecons.html#.Final
>
> I don't know why D always swears to have library solutions for
> everything.
>
> This is clearly something that should have a DIP and a language
> implementation in my opinion, considering it's a type-system
> addition.
>
> It looks like a bad hack honestly.

Well, once upon a time, early in D2's development, it actually did have both
head and tail const, but it was deemed too complicated, and we ended up with
the transitive const that we have today that works as tail-const under some
circumstances but not all. The fact that immutable exists really complicates
things, more or less requiring tranitive const, and the fact that the type
system doesn't really distinguish between a class and its reference also
complicates things.

Personally, I think that head-const is almost worthless, so I see no real
value in adding it, but the fact that we're forced to have Rebindable to get
tail-const for classes is defintely ugly, though AFAIK, Walter and Andrei
both consider it to be a perfectly acceptable solution given the issues with
altering how the language treats classes.

Maybe a solid DIP could be constructed which would be accepted, but it's
likely going to need some good arguments given how much this has been argued
over in the past.

- Jonathan M Davis