foreach

[H. S. Teoh via Digitalmars-d]

On Fri, Jun 13, 2014 at 10:15:59PM +0000, monarch_dodra via Digitalmars-d wrote:
> On Friday, 13 June 2014 at 22:07:53 UTC, H. S. Teoh via Digitalmars-d wrote:
> >>The very idea of a loop without a condition seems very, very wrong
> >>to me.
> >
> >Why would it be "very, very wrong"? Perpetual cycles are ubiquitous
> >in nature
> 
> Hum... you guys seem to be forgetting about break statements.
> 
> You know when you want to do something, and also do something
> *between* each iteration, but don't want to check your end condition
> twice per loop? It usually ends up looking something like this:
> 
> T[] arr = ... ;
> auto len = arr.length;
> write("[");
> if ( len != 0 ) {
>   size_t i = 0
>   for ( ;  ;  ) {
>     write(arr[i]);
>     if (++i == len) break;
>     write(", ");
>   }
> }
> write("]");

I'm of the opinion that today's ubiquitous for/while/do loops are all
defective caricatures of the "true" looping construct:

	loop {
		// body1
	} while (condition) {
		// body2
	}

where body1 runs first, then the condition is checked, then body2 runs
followed immediately by body1 again, etc.. Either body1 or body2 can be
elided as needed, of course:

	// do-loop
	loop {
		// body1
	} while(condition);

	// while-loop
	loop while(condition) {
		// body2
	}

	// the real infinite loop ;-)
	loop {
		// body1
	}

	// busy wait loop
	loop while (!externallySetFlag);

And here's an example the full loop in action:

	write("[");
	if (!range.empty) {
		loop {
			writef("%s", range.front);
			range.popFront();
		} while (!range.empty) {
			writef(", ");
		}
	}
	writeln("]");

The fact that this is the One True Loop is backed up by the necessity of
%| in order to complete the functionality of %( and %) in std.format. %(
and %) wouldn't be halfway as cool as they are, if %| didn't exist.


T

-- 
Questions are the beginning of intelligence, but the fear of God is the beginning of wisdom.