Control Structures Proposal

[nazo]

I think that FunctionStatement is better than opBlock. (the appended 
patch is for dmdfe.)

Example:
import std.stdio;

void main(){
   int i=void;
   For (i=0, i<10, ++i){
     writefln(i);
   }
   For (i=0, i<10, ++i)writefln(i);
}

void For(lazy void ini, lazy bool cond, lazy void inc, void delegate() dg) {
   for (ini(); cond(); inc()) dg();
}

janderson wrote:
> I know creating user control structures has been talked about before.
> 
> Anyways something to chew on, it occurs to me that D almost has the 
> power to create these structures.
> 
> For example you can currently do something like this in D (untested).
> 
> For({int i=0;}, i!=0, {++i})
> ({
> 
> 
> });
> 
> 
> or
> 
> If (X)
> ({
> 
> }).
> Else
> ({
> 
> ));
> 
> I was thinking if D provided a block operator like opCall then we would 
> be much closer to getting this flexibility.
> 
> the syntax would be:
> 
> struct [name of struct]
> {
>     static [returntype] opBlock(void delegate() block, [, parm, param, 
> ...]);
> }
> 
> ie:
> 
> struct For
> {
>     static void opBlock(void delegate() block, void delegate() init, 
> lazy bool condition, void delegate() iteration)
> {
>     ...
> }
> 
> };
> 
> //Then we could write:
> 
> For({int i=0;}, i!=0, {++i})
> {
> 
> }
> 
> The next thing that could be done to improve this is to allow all single 
> terms to be turned into delegates automatically, like lazy works so we 
> don't have to provide the {}.  ie this should then work:
> 
> For(int i=0;, i!=0, ++i)
> {
> 
> }
> 
> The last thing that would be necessary to complete the picture is to 
> allow contacted statements (like Else in for loop).  This is the most 
> difficult syntax to figure out (Any ideas?).  I think the return type 
> from the opBlock is one idea, however it suffers from the fact that all 
> the processing is done in the second struct its pretty odd.  Perhaps
> 
> 
> struct [name of struct]
> {
>     static [returntype] opBlock(void delegate() block, [, void 
> delegate() blockname2, void delegate() blockname3, ...] [, parm, param, 
> ...]);
> }
> 
> //ie If Else
> 
> struct If
> {
>     static void opBlock(void delegate() block, void delegate() Else);
> }
> 
> Of course that would mean someone could write my forloop example like 
> this as well :(
> 
> For (i!=0, ++i)
> {
> 
> }
> init
> {
>     int i = 0;
> }
> 
> Ug.
> 
> 
> (PS I know lazy may be removed in the future, I hope normal delegates 
> will work as its replacement).
> 
> -Joel

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