Extended Type Design: further examples

[Andrei Alexandrescu (See Website For Email)]

Derek Parnell wrote:
> Hmmm ... so the difference between 'const' and 'invariant' is that const is
> only effective for one level deep, but 'invariant' is effective for all
> levels, right?

Nonono. Const means: it might have originated as a variable, and 
somebody, somewhere, might hold a mutating reference to this and call it 
"my precious".

Invariant is: nobody ever holds a mutating reference to it.

Example:

void Fun(const char[] a, char[] b)
{
   char c = a[0];
   b[0] = 'z';
   assert(a[0] == c); // will fail
}

void main()
{
   char[] x = "Wyda".dup;
   Fun(x, x);
}

Notice how the data referenced to by a is changing from under a's feet. 
Now let's rewrite Fun:

void Gun(invariant char[] a, char[] b)
{
   char c = a[0];
   b[0] = 'z';
   assert(a[0] == c); // never, ever, ever, ever fails
}

void main()
{
   char[] x = "Wyda".dup;
   Fun(x, x); // can't compile!
       // mutating references can't be bound to invariant references
}


Andrei