spec doesn't define this behavior as far as I can tell

import std;
auto loop(){
	struct foreach_{
		int opApply(int delegate(int i) dg){
			dg(0).writeln;//1
			dg(1).writeln;//2
			dg(2).writeln;//3
			return 3;
	}}
	return foreach_();
}
unittest{
	a: while(true){
	b: foreach(i;loop()){
		if(i==0){break;}
		if(i==1){break b;}
		if(i==2){break a;}
}}}

rn I believe you could pretty reasonably overload the behavior of break in a crafted opApply to make a 2d iteration

it should be possible to modify ref indexs and other stateful things that are tricky with pure ranges

slight errors produce infinite loops tho and this is like the 3rd opApply ive ever written, no idea on stable patterns

On Wednesday, 27 August 2025 at 20:34:22 UTC, monkyyy wrote:

import std;
auto loop(){
	struct foreach_{
		int opApply(int delegate(int i) dg){
			dg(0).writeln;//1
			dg(1).writeln;//2
			dg(2).writeln;//3
			return 3;
	}}
	return foreach_();
}
unittest{
	a: while(true){
	b: foreach(i;loop()){
		if(i==0){break;}
		if(i==1){break b;}
		if(i==2){break a;}
}}}

For a valid opApply you need to continue looping as long as the delegate returns 0.

If the delegate returns something other than 0, then you have to return that value from opApply. If you don't, the behavior is not defined. Even what the different values do is not defined or specified.

If you reach the end of your data that you are looping, and the final delegate call returned 0, then you return 0.

How you loop is defined by you. So calling the delegate in different ways is OK, but continuing on a non-zero value is not valid.

Relevant spec information (see paragraph 5):

https://dlang.org/spec/statement.html#foreach_over_struct_and_classes

-Steve

On Thursday, 28 August 2025 at 00:49:24 UTC, Steven Schveighoffer wrote:

should vs could

The pattern seems to be 1 is a standard break, 2 and greater incrementing by 1 other labels in reverse order. I expect 0, 1 vs >1 could be justified, maybe the compiler does[offset-result] list of address pointers and outside, hostile, changes I think this will survive and maybe could've been in the compiler the entire time.