const Propagation

[Julian Kranz via Digitalmars-d]

Hi all,

I've got a little problem regarding const. In fact, my problem 
originates from C++ where I've tried to implement a visitor that 
deals with both const and non-const objects. As this did not work 
out, I checked whether it would work in D. Unfortunately, I could 
not figure out how. Maybe someone can help.

Basically, I would like to propagate constness to a lambda 
function. Here's the example code:

import std.stdio;

class Hugo {
  public int x = 42;

  void blah(void function(Hugo h) f) {
    f(this);
  }
}

void main() {
    Hugo hugo = new Hugo();
    void function(Hugo h) f = function(Hugo h) {
      h.x = 99;
    };
    hugo.blah(f);

    const Hugo inge = hugo;
    void function(Hugo h) g = function(Hugo h) {
      writeln("foobar");
    };
    inge.blah(g);
}

This does not compile. The D compiler complains that I must not 
call a mutable method on a const object (inge). However, if I 
make the method const, I have to also make the parameter of the 
lambda const which results in the compiler rejecting the call 
"hugo.blah(f)" since this lambda actually modifies the object.

I hope you get what I want. I want to be able to call blah() on a 
const instance of Hugo with a lambda that does not modify its 
argument and otherwise call blah() with any function, even one 
that modifies the object. Is that possible?

Thank you very much!