const confusion

[Witold Baryluk]

Hi,

i haven't been here for so long time. So hi all. I'm back again.

I was considering myself as hardcore D hacker, but
in one point I fail completly. Constness.

Consider this simple code:

module ct;

class C {
	const int a;
	const C b;

	this(int a_) {
		a = a_;
		b = null;
	}
	this(int a_, in cord b_) {
		a = a_;
		b = b_;
	}
	C m() const { // line 16
	    return b;
	}
}

void main() {
	C c1 = new C(1);
	C c2 = new C(2, c1);
	c2 = c2.m(); // 23
}

So I have class C which all methods are "const". So it is completly immutable object,
one can say.

The problem is with line 16, I define function m which is const (because it
doesn't change anything). But i'm returing some part of it, so actually somone
else can change it hypotetically

Actually it is impossible for two reasons:
  * all fields are const,
  * there is no non-consts methods beside constructors.

Of course compiler says here: (dmd 2.028)
ct.d(16): Error: cannot implicitly convert expression (this.b) of type const(C) to ct.C

  

so we change line 16 to:
const(C) m() const { // line 16

And I can agree that this is correct solution.

but then there is another problem, now in main() function:
ct.d(23): Error: cannot implicitly convert expression (c2.m()) of type const(C) to ct.C

So now, i need to change all occurence of C to const(C), ok no problem:
void main() {
	const(C) c1 = new C(1);
	const(C) c2 = new C(2, c1);
	c2 = c2.m();  // line 23
}

Ok, nice. I can create some alias so it will be easier to use, also.

But now is another problem:
ct.d(23): Error: variable ct.main.c2 cannot modify const


But I wan't to have possibility to change local reference to this class!
Shouldn't const be about content of object?

Ok, go and just create new variable:
void main() {
	const(C) c1 = new C(1);
	const(C) c2 = new C(2, c1);
	auto c3 = c2.m();  // line 23
}

compiler now will be happy.

Ok, it is good for functional style of programing, but consider this:

void main() {
	const(C) c1 = new C(1);
	const(C) c2 = new C(2, c1);
	while (c2 !is null) {
		c2 = c2.m();  // line 23
	}
	writefln(c2.a);
}


So now i need to write tail recursive version of this loop, because compiler
doen't allow me to reuse variable.

I can write tail recursive loop, but it's:
  * not so efficient
  * users of my library are not familiary with functional programing
  * it's unnatural.

Horiblle.

How to ensure constness of data, and still have possibility of changing references of local variables?


-- 
Witold Baryluk <baryluk at smp.if.uj.edu.pl>
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