'in' is equivalent to final const scope.
I can't understand the point of making the variable 'final' if it's already 'const' ?

Funog wrote:
> 'in' is equivalent to final const scope.
> I can't understand the point of making the variable 'final' if it's already 'const' ?
> 

In D 2.0, 'final' represents 'head-const', while 'const' means 'tail-const'.  This makes a difference when talking about arrays: 'final' will make the array reference itself const, so you can't change it to refer to a different array, while 'const' makes the contents of the array const - you can change the variable to point to another array or slice, but you can't edit any array through that reference.

Thanks,
Nathan Reed

Nathan Reed Wrote:

> Funog wrote:
> > 'in' is equivalent to final const scope.
> > I can't understand the point of making the variable 'final' if it's already 'const' ?
> > 
> 
> In D 2.0, 'final' represents 'head-const', while 'const' means 'tail-const'.  This makes a difference when talking about arrays: 'final' will make the array reference itself const, so you can't change it to refer to a different array, while 'const' makes the contents of the array const - you can change the variable to point to another array or slice, but you can't edit any array through that reference.
> 
> Thanks,
> Nathan Reed



It's true for const(uint[]) or const(uint)[], but we are talking of const uint[], which forbid any change.



void testFinal(final int[] foo)
{
    foo[0] = 10;        //OK
    foo = new int[20];  //ERROR
}
void testConst(const int[] foo)
{
    foo[0] = 10;        //ERROR
    foo = new int[20];  //ERROR
}
void testFinalConst(final const int[] foo)
{
    foo[0] = 10;        //ERROR
    foo = new int[20];  //ERROR
}


So what is the difference between 'const' and 'final const' ?

Funog wrote:
> void testFinal(final int[] foo)
> {
>     foo[0] = 10;        //OK
>     foo = new int[20];  //ERROR
> }
> void testConst(const int[] foo)
> {
>     foo[0] = 10;        //ERROR
>     foo = new int[20];  //ERROR
> }
> void testFinalConst(final const int[] foo)
> {
>     foo[0] = 10;        //ERROR
>     foo = new int[20];  //ERROR
> }
> 
> 
> So what is the difference between 'const' and 'final const' ?
> 

I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.

Thanks,
Nathan Reed

Nathan Reed Wrote:

> Funog wrote:
> > void testFinal(final int[] foo)
> > {
> >     foo[0] = 10;        //OK
> >     foo = new int[20];  //ERROR
> > }
> > void testConst(const int[] foo)
> > {
> >     foo[0] = 10;        //ERROR
> >     foo = new int[20];  //ERROR
> > }
> > void testFinalConst(final const int[] foo)
> > {
> >     foo[0] = 10;        //ERROR
> >     foo = new int[20];  //ERROR
> > }
> > 
> > 
> > So what is the difference between 'const' and 'final const' ?
> > 
> 
> I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.
> 
> Thanks,
> Nathan Reed



I agree... But then, what is the point of having 'in' equivalent to 'final const scope' rather than just 'const scope' ?

Funog Wrote:

> Nathan Reed Wrote:
> 
> > Funog wrote:
> > > void testFinal(final int[] foo)
> > > {
> > >     foo[0] = 10;        //OK
> > >     foo = new int[20];  //ERROR
> > > }
> > > void testConst(const int[] foo)
> > > {
> > >     foo[0] = 10;        //ERROR
> > >     foo = new int[20];  //ERROR
> > > }
> > > void testFinalConst(final const int[] foo)
> > > {
> > >     foo[0] = 10;        //ERROR
> > >     foo = new int[20];  //ERROR
> > > }
> > > 
> > > 
> > > So what is the difference between 'const' and 'final const' ?
> > > 
> > 
> > I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.
> > 
> > Thanks,
> > Nathan Reed
> 
> 
> 
> I agree... But then, what is the point of having 'in' equivalent to 'final const scope' rather than just 'const scope' ?
> 

The difference is easier for me to grasp when talking about class references than arrays.

class Foo { int bar; }

final Foo baz;
baz.bar = 5; // Okay
baz = quux; // Illegal

const Foo quux; // Equivilent, I _think_ to const(Foo) quux;
quux.bar = 5; // Illegal
quux = baz; //Okay

In other words, for a class reference, head-const (final) means that the reference can't be changed to point to something else, but the data can be changed. Head-const is not transitive. Tail-const (const or invariant, in the case of a class reference) means that the thing being pointed to cannot be changed, but the reference can be.

This is generalized to array references and pointers.

Robert Fraser Wrote:

> Funog Wrote:
> 
> > Nathan Reed Wrote:
> > 
> > > Funog wrote:
> > > > void testFinal(final int[] foo)
> > > > {
> > > >     foo[0] = 10;        //OK
> > > >     foo = new int[20];  //ERROR
> > > > }
> > > > void testConst(const int[] foo)
> > > > {
> > > >     foo[0] = 10;        //ERROR
> > > >     foo = new int[20];  //ERROR
> > > > }
> > > > void testFinalConst(final const int[] foo)
> > > > {
> > > >     foo[0] = 10;        //ERROR
> > > >     foo = new int[20];  //ERROR
> > > > }
> > > > 
> > > > 
> > > > So what is the difference between 'const' and 'final const' ?
> > > > 
> > > 
> > > I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.
> > > 
> > > Thanks,
> > > Nathan Reed
> > 
> > 
> > 
> > I agree... But then, what is the point of having 'in' equivalent to 'final const scope' rather than just 'const scope' ?
> > 
> 
> The difference is easier for me to grasp when talking about class references than arrays.
> 
> class Foo { int bar; }
> 
> final Foo baz;
> baz.bar = 5; // Okay
> baz = quux; // Illegal
> 
> const Foo quux; // Equivilent, I _think_ to const(Foo) quux;
> quux.bar = 5; // Illegal
> quux = baz; //Okay
> 


It isn't... Both lines are illegal. quux = baz is legal for const(Foo) quux.
If nobody can tell the difference between "const scope" and "in" ( = final const scope ), should we report it as a bug ? xD

Funog wrote:
> Robert Fraser Wrote:
> 
>> Funog Wrote:
>>
>>> Nathan Reed Wrote:
>>>
>>>> Funog wrote:
>>>>> void testFinal(final int[] foo)
>>>>> {
>>>>>     foo[0] = 10;        //OK
>>>>>     foo = new int[20];  //ERROR
>>>>> }
>>>>> void testConst(const int[] foo)
>>>>> {
>>>>>     foo[0] = 10;        //ERROR
>>>>>     foo = new int[20];  //ERROR
>>>>> }
>>>>> void testFinalConst(final const int[] foo)
>>>>> {
>>>>>     foo[0] = 10;        //ERROR
>>>>>     foo = new int[20];  //ERROR
>>>>> }
>>>>>
>>>>>
>>>>> So what is the difference between 'const' and 'final const' ?
>>>>>
>>>> I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.
>>>>
>>>> Thanks,
>>>> Nathan Reed
>>>
>>>
>>> I agree... But then, what is the point of having 'in' equivalent to 'final const scope' rather than just 'const scope' ?
>>>
>> The difference is easier for me to grasp when talking about class references than arrays.
>>
>> class Foo { int bar; }
>>
>> final Foo baz;
>> baz.bar = 5; // Okay
>> baz = quux; // Illegal
>>
>> const Foo quux; // Equivilent, I _think_ to const(Foo) quux;
>> quux.bar = 5; // Illegal
>> quux = baz; //Okay
>>
> 
> 
> It isn't... Both lines are illegal. quux = baz is legal for const(Foo) quux.
> If nobody can tell the difference between "const scope" and "in" ( = final const scope ), should we report it as a bug ? xD
> 

If I understand stuff right, "quux = baz;" should be legal.  Use of quux thereafter, including reassigning quux, does not invalidate baz's use of final.  I have not tested this with a compiler, of course...

Funog wrote:
> Robert Fraser Wrote:
> 
>> Funog Wrote:
>> 
>> > Nathan Reed Wrote:
>> > 
>> > > Funog wrote:
>> > > > void testFinal(final int[] foo)
>> > > > {
>> > > >     foo[0] = 10;        //OK
>> > > >     foo = new int[20];  //ERROR
>> > > > }
>> > > > void testConst(const int[] foo)
>> > > > {
>> > > >     foo[0] = 10;        //ERROR
>> > > >     foo = new int[20];  //ERROR
>> > > > }
>> > > > void testFinalConst(final const int[] foo)
>> > > > {
>> > > >     foo[0] = 10;        //ERROR
>> > > >     foo = new int[20];  //ERROR
>> > > > }
>> > > > 
>> > > > 
>> > > > So what is the difference between 'const' and 'final const' ?
>> > > > 
>> > > 
>> > > I suppose there's no practical difference in that case since you're using the transitive const.  Note that final const(int)[] foo is different from const(int)[] foo though.
>> > > 
>> > > Thanks,
>> > > Nathan Reed
>> > 
>> > 
>> > 
>> > I agree... But then, what is the point of having 'in' equivalent to 'final const scope' rather than just 'const scope' ?
>> > 
>> 
>> The difference is easier for me to grasp when talking about class references than arrays.
>> 
>> class Foo { int bar; }
>> 
>> final Foo baz;
>> baz.bar = 5; // Okay
>> baz = quux; // Illegal
>> 
>> const Foo quux; // Equivilent, I _think_ to const(Foo) quux;
>> quux.bar = 5; // Illegal
>> quux = baz; //Okay
>> 
> 
> 
> It isn't... Both lines are illegal. quux = baz is legal for const(Foo) quux.
> If nobody can tell the difference between "const scope" and "in" ( = final const scope ), should we report it as a bug ? xD
> 

I tried this:

---
void f(const int* p)
{
	int y = 9;
	
	*p = 7;	 // line 5
	p = &y;  // line 6
}

void main()
{
	int x = 5;

	f(&x);
}
---
dmd -run inargs
inargs.d(5): Error: *p is not mutable
inargs.d(6): variable inargs.f.p cannot modify final/const/invariant variable 'p'


Adding 'final' doesn't change anything.  So it would seem that this feature isn't finalized yet.  'scope' doesn't seem to work either.  If I use 'final const(int)*' instead, I get the same errors.

Barring optimizations, a function's arguments are not known a compile-time.  So I don't know if giving them the const or invariant storage class makes much sense.  Maybe the idea was that const in a parameter list would always mean const as a type constructor.  For other declarations, 'final const(int)*' allows run-time (by a constructor) initialization, while 'const int*' does not.  So maybe the latter one should be disallowed in parameter lists.