Re: readonly member (but assignable at constructor time)
On Monday, 30 April 2018 at 10:57:51 UTC, Jonathan M Davis wrote:
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
Yeah, honestly I'm in the same opinion as you.
Usually when I have been writing C# and used readonly, what I
really wanted to use is immutable.
I'm not saying we need to implement this in the language, but
just that the library solution isn't really elegant at all,
compared to what a language implementation would have been.
Re: readonly member (but assignable at constructor time)
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
Re: readonly member (but assignable at constructor time)
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.
Re: readonly member (but assignable at constructor time)
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
Re: readonly member (but assignable at constructor time)
On Friday, April 27, 2018 02:59:16 Dr.No via Digitalmars-d-learn 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?
D has const, though that has some pretty far-reaching repercussions, since
it means that the object can't be mutated at all, and the only member
functions that can be called on it are const or inout.
If you want to specifically make it so that the object is mutable but the
reference can't be assigned to, then it would have to be protected by a
wrapper struct that @disabled opAssign and then either used alias this or
opDispatch to forward calls to the class reference inside the struct. D's
const is transitive and does not have any form of head-const, so const can't
be used in a case like that.
- Jonathan M Davis
readonly member (but assignable at constructor time)
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?
