A const idiom + a different 'delete'

[Andrej Mitrovic]

The scope trick won't work. You can't modify a const after the declaration.

On Wed, Aug 11, 2010 at 7:26 PM, Jonathan M Davis <jmdavisprog at gmail.com>wrote:

> On Wednesday, August 11, 2010 08:59:29 bearophile wrote:
> > This is an little idiom that is getting common in my code, it's similar
> to
> > the 'transients' of Clojure language.
> >
> > Often I have to build a simple data structure like an array associative
> or
> > another kind of array, it needs to be mutable because I fill it in few
> > lines of code code. When it's complete I never need to change it again,
> so
> > now I'd like it to be const. I can't freeze it, so I have to create a new
> > variable:
> >
> >
> > void main() {
> >     int[int] aa_;
> >     foreach (i; 0 .. 10)
> >         aa_[i] = i * i;
> >
> >     // now I'd like to freeze aa
> >     const(int[int]) aa = aa_;
> > }
> >
> >
> > If the array is fixed-sized the situation is worse because the last line
> > copies the whole array.
> >
> > Sometimes I use a small function to do this.
> >
> > This is an alternative way to write it that I've never used because I
> don't
> > like it much:
> >
> > void main() {
> >     const(int[int]) aa = {
> >         int[int] result;
> >         foreach (i; 0 .. 10)
> >             result[i] = i * i;
> >         return result;
> >     }();
> > }
> >
> >
> > In Python I sometimes use "delete" to remove a name from the local
> > namespace that I don't want to use any more. This cleaning purpose may be
> > a replacement purpose for the delete keyword, but I don't know if you
> like
> > it:
> >
> >
> > void main() {
> >     int[int] aa_;
> >     foreach (i; 0 .. 10)
> >         aa_[i] = i * i;
> >
> >     // now I'd like to freeze aa
> >     const(int[int]) aa = aa_;
> >     delete aa_; // from now on the 'aa_' name can't be seen/used.
> > }
> >
> >
> > Here delete doesn't clean the aa_, it just removes the aa_ name from the
> > local namespace. If there's another reference to aa_ it is unchanged and
> > it sees an unchanged associative array. The point of removing the aa_
> name
> > is to keep the namespace tidy.
> >
> > As with variable definitions a goto can't jump over one delete. In Python
> > you can later create a new variable with the same name, but in D the
> > variable doesn't actually vanish from the stack, so I think it's better
> to
> > disallow successive redefinitions of it.
> >
> > Bye,
> > bearophile
>
> I think that's a total no-go because it would depend on program flow.
> You're
> trying to alter a compile-time property at runtime. All it takes is having
> delete within an if statement, and all of a sudden, depending on how your
> program runs, the name may or may not be visible. You kind hide the
> variable
> creation inside another scope if you want this sort of behavior:
>
> void main()
> {
>    const(int[int]) aa;
>
>    {
>         int[int] aa_;
>        foreach(i; 0 .. 10)
>            aa_[i] = i * i;
>         aa = aa_;
>    }
> }
>
>
> That naturally removes the name from the namespace just fine. It's possible
> that
> this particular idiom calls for some sort of language addition or having
> something added to Phobos, but removing symbols from a namespace with
> statements
> is not going to work very well - if at all - in a compiled,
> statically-typed
> language like D.
>
> - Jonathan M Davis
>
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