Asking a const-related fix [Was: Re: DMD workforce contribution]

[Jonathan M Davis]

On Monday, November 07, 2011 05:49:52 bearophile wrote:
> Given how much often I find this problem in D coding, is someone willing and
> able to write a patch to allow code like this (it's in Bugzilla, with
> normal priority)?
> 
> 
> const struct Foo {
>     const int[5] a;
>     const int[] b;
>     const int[int] aa;
>     this(in int n) pure {
>         this.a[] = n; // line 6
>         this.b = new int[5];
>         this.b[0] = n; // line 8
>         this.aa[1] = 2; // line 9
>     }
> }
> void main() {}
> 
> 
> The latest DMD gives:
> 
> test.d(6): Error: slice this.a[] is not mutable
> test.d(8): Error: this.b[0] isn't mutable
> test.d(9): Error: this.aa[1] isn't mutable

I don't think that any of those are _supposed_ to work. When dealing with a 
const or immutable variable, it can only be initialized, not assigned to. For

this.a[] = n;

you're assigning all of the elements of n to all of the corresponding elements 
in a. That's an assignment, so of course it doesn't work. If you want to copy 
the array, then use

this.a = n.dup;

or

this.a = n.idup;

For

this.b[0]

you're assigning an element in the array. It's const, so that's not legal. The 
same goes with

this.aa[1] = 2;

That would be altering aa, which isn't legal. const variables can be 
initialized but _can't_ be assigned to. If you want to do something fancier 
than a simple initialization, you're going to need to create another array 
which you do everything to and _then_ initialize the const array with that. 
The errors that you're running into are completely by design.

- Jonathan M Davis