The magic behind foreach (was: Re: Descent 0.5.3 released)

[Bill Baxter]

On Fri, Jan 23, 2009 at 11:15 AM, Jarrett Billingsley
<jarrett.billingsley at gmail.com> wrote:
> On Thu, Jan 22, 2009 at 7:00 PM, Bill Baxter <wbaxter at gmail.com> wrote:
>>
>> I posted a proposal for how to hide the magic int from the user a
>> while back, but my conclusion was that my approach would require AST
>> macros in order to give it a reasonable syntax.  With the ast macros
>> you'd be able to do something like  yield(i) in the body of your
>> foreach, where yield is an appropriately defined macro.
>>
>
> Consider the case where you return a value inside a foreach loop:
>
> int find(char[] value)
> {
>    foreach(k, v; someAA)
>        if(v == value)
>            return k;
> }
>
> How is this implemented?  It puts a local variable in the stack frame
> of find(), called __result.  Then the foreach delegate just writes
> into that local when it returns (__result = k; return
> IM_RETURNING_NOW;), and the compiler-generated cruft returns the value
> of that local from find() (switch(...) { case IM_RETURNING_NOW: return
> __result; }).
>
> The delegate return status code doesn't have to be any different.
> Just have the delegate return a bool (should iteration stop?) and put
> its actual return status in the stack frame of the enclosing function.

Yep, that was the gist of it.  Put the return value on the stack of
the calling frame.
Nothing all that fancy really, just you want to be able to hide that
__result = blah ugliness from the user.

--bb