const?? When and why? This is ugly!

[Walter Bright]

Derek Parnell wrote:
> import std.stdio;
> void main()
> {
>   int [] a = new int [1];
> 
>    a [0] = 1;
> 
>    invariant (int) [] b = cast (invariant (int) []) a;
> 
>    writef ("a=%s b=%s\n", a [0], b[0]);
>    a [0] += b [0];
>    writef ("a=%s b=%s\n", a [0], b[0]);
>    a [0] += b [0];
>    writef ("a=%s b=%s\n", a [0], b[0]);
> }
>    
> The problem is that we have declared 'b' as invariant, but the program is
> allowed to change it. That is the issue.
> 

The following also compiles:

char c;
int* p = cast(int*)&c;
*p = 5;

and is clearly buggy code. Whenever you use a cast, the onus is on the 
programmer to know what they are doing. The cast is an escape from the 
typing system.