Block Lambdas: break and continue

Allen Wirfs-Brock allen at wirfs-brock.com
Sat Jan 14 13:53:12 PST 2012


On Jan 14, 2012, at 1:31 PM, François REMY wrote:

> Thanks for your reply. I think you misunderstood the whole concept. The idea would not to implement that for Arrays. Arrays already have their own  semantics.
>  
> I think the idea about Block Lamdba is to introduce functionnal programming concepts to ECMAScript. It’s it’s the case, developers will want to implement their own kind of Head|Queue stream. For exemple, a webworker computing values and calling the callback with the computed value each time he output a new one (using a postMessage channel). The problem is that you may need to have a way to say to the WebWorker “OK, I want to break the loop, I’ve what I need, please stop computing and free memory.” You could use your own “throw” pattern but using a standard one could be useful for code reuse.

Actually, I think having a standard patten is a hazard.  The reason is that there may be multiple additional and dynamically determined control abstraction layers (and activation records) between the "WebWorker" iteration abstraction the actual evaluation of the Block Lambda that does the "throw".  If you have a standard exception that is used for this purpose then you have to worry about the intermediate layer using that same standard.

As Dave and I both mentioned.  This sort of escape mechanism really needs to be uniquely defined as part of the contract of each iterator abstraction that supports.  Defining a specific (and unique) exception type is a fine way to do it.  For example, my forNum function could define a privateName valued property that does that:

forNum.breaker = Name.create();  //sometimes hoisting is nice
function forNum(start, end,increment=1,body={||}) {
   if (start > end) return;
   try {
      body(start);
   } catch (e if e is forNum.breaker) {return}; // probably better to factor this hander into a non-recursive wrapper
   return forNum(start+increment,end,increment,body);
}

which could be used like:

forNum(first,last,1) {|n|
   if (n==0) throw forNum.breaker;
   ...
}

The above style forces the iteration abstraction to explicitly define its "break" contract and forces the user of the iteration abstraction to use the contract specified mechanism.  No tripping over undocumented or unexpected usage of defaults.

Allen














>  
> I hope I was clear,
> François
>  
>  
> From: Brendan Eich
> Sent: Saturday, January 14, 2012 10:16 PM
> To: François REMY
> Cc: Herby Vojčík ; es-discuss at mozilla.org
> Subject: Re: Block Lambdas: break and continue
>  
>> <postbox-contact.jpg>	François REMY	January 14, 2012 1:01 PM
>> If we want to avoid to break TCP, we can go with “throw break;” and “throw continue;”.
> 
> This doesn't address Herby's TCP-violating wish for a non-return that  completes the block-lambda's control flow normally with a value (the message to which I was replying). But you're right that it wouldn't violate TCP either if we support it the same in a block statement as in a block-lambda downward funarg.
> 
>> It would throw a new BreakException or a new ContinueException, from the place where they are executed. If it’s outside a block lambda, it’s outside a block lambda. It doesn’t matter.
> 
> Yes, this would avoid TCP violations but not carry a return value -- Herby's wish.
>>  
>> But it would set a “standard” for breaking throug ‘function loops’.
> 
> I considered this in drafting the block-lambda revival strawman. Other languages have gone here. Nevertheless, I would like to leave it out (remember N. Wirth on language design). It adds more complexity for a use-case that I bet is rare (in any case it needs credible demonstration of being quite common).
> 
> The complexity in the semantics is one issue Dave raised. This corresponds to complexity for optimizing engines, compared to the purely static break/continue semantics in the strawman.
> 
> Finally, the Array extras ship sailed. People already have to use some or every in lieu of a break-from-forEach. Using a function callback with forEach, one needs only to return to simulate continue. Now if we do standardize block-lambdas and throw break or throw continue, we certainly can elaborate the extras to catch these exceptions.
> 
> Such a more complex design seems workable with the costs noted above. But will the benefits really outweigh those costs? I doubt it. First, Array forEach and other uses will continue to use functions for quite a while, or else a compiler from new standard JS to old. In the compiler case, throw and try/catch will be required, and the compiler will have to monkey-patch the extras to deal with the new exceptions. This will be a performance killer, and no fun to debug.
> 
> So my thinking remains that we are better off, when in doubt, leaving reified break and continue exceptions "out".
> 
> /be
> 
>> François
>>  
>> From: Brendan Eich
>> Sent: Saturday, January 14, 2012 9:51 PM
>> To: Herby Vojčík
>> Cc: es-discuss at mozilla.org
>> Subject: Re: Block Lambdas: break and continue
>>  
>>> <postbox-contact.jpg>	Herby Vojčík	January 14, 2012 10:42 AM
>>> === David Herman wrote === 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>>> === 
>>> 
>>> What about the exception-less suggestion I put in? It should work in any loop construct with lambda-block, even if you must know a little about the loop implementation itself. That is, to be able to put: 
>>> 
>>>    continue |expression|; 
>> 
>> Who says the block-lambda is being called from a loop at all? Why should use-cases that want an early result and completion have to use continue, which is for loops?
>> 
>> Worse, this violates TCP. Now you copy and paste this block-lambda code back into a block statement to refactor the other direction, and no such "here is the completion value, do not flow past this point in the block" semantics obtain.
>>> 
>>> as a statement in lambda block which instructs the lambda-block itself (not the outer function) to return the expression? This is the de-facto continue semantics (lambda-block, do return a value and the enclosing loop will continue to the next iteration (possibly stopping the loop if it chooses not to have more iterations)).
>> 
>> No it's not. There is no de-facto continue semantics for block-lambdas because they haven't been prototyped. For block statements, no such continue semantics exists.
>> 
>>> It is not possible to enforce break in the same manner, but for continue, it is possible. 
>> 
>> It's possible to abuse any existing keyword, but first: why must there be a new TCP violation? Block-lambda bodies are often expressions, or if statements, then short/functional-style statements, not large bodies demonstrating early-normal-completion opportunities.
>> 
>> We should not eliminate TCP violations only to add new ones, especially without any evidence they're needed and pay their way. Otherwise we'll get an infinite regress of TCP-pure-then-add-new-exceptions-and-repeat additions.
>> 
>> /be
>>> 
>>> Herby 
>>> 
>>> -----Pôvodná správa----- From: David Herman 
>>> Sent: Saturday, January 14, 2012 6:12 PM 
>>> To: Axel Rauschmayer 
>>> Cc: Brendan Eich ; es-discuss at mozilla.org 
>>> Subject: Re: Block Lambdas: break and continue 
>>> 
>>> On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote: 
>>> 
>>> 
>>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as: 
>>> 
>>>    Array.prototype.forEach = function(f) { 
>>>        for (let i = 0, n = this.length; i < n; i++) { 
>>>            try { 
>>>                f.call(this, this[i], i); 
>>>            } catch (e) { 
>>>                if (e instanceof BreakException) 
>>>                    break; 
>>>                else if (e instanceof ContinueException) 
>>>                    continue; 
>>>                else 
>>>                    throw e; 
>>>            } 
>>>        } 
>>>    }; 
>>> 
>>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able. 
>>> 
>>> Did I understand your suggestion correctly? 
>>> 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>>> 
>>> Dave 
>>> 
>>> _______________________________________________ 
>>> es-discuss mailing list 
>>> es-discuss at mozilla.org 
>>> https://mail.mozilla.org/listinfo/es-discuss 
>>> _______________________________________________ 
>>> es-discuss mailing list 
>>> es-discuss at mozilla.org 
>>> https://mail.mozilla.org/listinfo/es-discuss 
>>> <postbox-contact.jpg>	David Herman	January 14, 2012 9:12 AM
>>> 
>>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as:
>>> 
>>> Array.prototype.forEach = function(f) {
>>> for (let i = 0, n = this.length; i < n; i++) {
>>> try {
>>> f.call(this, this[i], i);
>>> } catch (e) {
>>> if (e instanceof BreakException)
>>> break;
>>> else if (e instanceof ContinueException)
>>> continue;
>>> else
>>> throw e;
>>> }
>>> }
>>> };
>>> 
>>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able.
>>> 
>>> Did I understand your suggestion correctly?
>>> 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do.
>>> 
>>> Dave
>>> 
>>> 
>>> <postbox-contact.jpg>	Axel Rauschmayer	January 13, 2012 9:04 PM
>>> I think it’s a valid concern. The idea is: If I can implement my own loops (the nice-looking paren-free syntax feeds that illusion!) then I also want those loops to have break and continue. You could statically determine what construct, say, a break applies to and either throw a BreakException (if it applies to a lambda) or TCP-break (if it applies to an enclosing non-lambda loop). In the examples below, when I see a continue, I look for the innermost enclosing loop braces and the ones belong to list[i].forEach are definitely candidates.
>>>  
>>>  
>>>  
>>> --
>>> Dr. Axel Rauschmayer
>>> axel at rauschma.de
>>>  
>>> home: rauschma.de
>>> twitter: twitter.com/rauschma
>>> blog: 2ality.com
>>>  
>>> <postbox-contact.jpg>	Brendan Eich	January 13, 2012 8:54 PM
>>>> <compose-unknown-contact.jpg>	Grant Husbands	January 13, 2012 7:29 PM
>>>> Block lambdas have been a hot topic, recently, but there's a point of significant divergence between Ruby (which appears to be the inspiration)
>>> 
>>> Not Ruby alone, and not in any chauvinist my-language-is-better sense. Smalltalk is the original inspiration for Ruby blocks, and the correspondence principle has deep roots.
>>> 
>>>> and the proposed solution, in the handling of continue (called 'next', in Ruby) and 'break'.
>>>>  
>>>> To whit: In Ruby, 'next' will end the current run (iteration) of the block, and 'break' will (somehow) terminate the method lexically connected with the block. It can be claimed that this is more intuitive than the current proposal, which aims to make 'break' and 'continue' propagate through block lambdas in the same way 'return' would.
>>> 
>>> "Intuitive" depends on intuition, which is not well-defined. Do you mean a Rubyist might expect different behavior for break? That is possible but JS ain't Ruby and break should not change to do something like what it does in Ruby (and we aren't defining a next equivalent for JS).
>>> 
>>>> Ruby does also support syntactic loops and the same keywords therein and so directly violates Tennent's Correspondence Principle, even though such has been touted as a core reason for the construct. Instead, I believe it reasonable to invoke intuition in this matter. It is intuitive for 'return' to return a value from the lexically enclosing method and it is intuitive for 'continue' to commence the next iteration of the current loop,
>>> 
>>> Wait, why do you think break and continue without label operands do anything other than break from the nearest enclosing loop (or switch or labeled statement if break), or continue the nearest enclosing loop? The proposal specifies this.
>>> 
>>> function find_odds_in_arrays(list,        // array of arrays
>>>                              skip)        // if found, skip rest
>>> {
>>>   let a = [];
>>>   for (let i = 0; i < list.length; i++) {
>>>     list[i].forEach {
>>>       |e|
>>>       if (e === skip) {
>>>         continue;                         // continue the for loop
>>>       }
>>>       if (e & 1) {
>>>         a.push(e);
>>>       }
>>>     }
>>>   }
>>>   return a;
>>> }
>>> 
>>> function find_more_odds(list, stop) {
>>>   let a = [];
>>>   for (let i = 0; i < list.length; i++) {
>>>     list[i].forEach {
>>>       |e|
>>>       if (e === stop) {
>>>         break;                      // break from the for loop
>>>       }
>>>       if (e & 1) {
>>>         a.push(e);
>>>       }
>>>     }
>>>   }
>>>   return a;
>>> }
>>> 
>>>> however that loop is constructed.
>>> 
>>> What do you mean by this? The spec talks about nearest enclosing loop or relevant control structure in the source code. Are you talking about internal loops in implementations (dynamically dispatched at that) of methods that take block-lambdas as arguments? I.e.
>>> 
>>> 
>>> function find_first_odd(a) {
>>>   a.forEach { |e, i|
>>>               if (e & 1) return i; }  // returns from function
>>>   return -1;
>>> }
>>> 
>>> 
>>> The Array.prototype.forEach method's internal implementation is its business, and a break instead of the return would be a static error in this example. It would not be a dynamic throw-like construct that is caught by forEach's implementation.
>>> 
>>> /be
>>> <compose-unknown-contact.jpg>	Grant Husbands	January 13, 2012 7:29 PM
>>> Block lambdas have been a hot topic, recently, but there's a point of significant divergence between Ruby (which appears to be the inspiration) and the proposed solution, in the handling of continue (called 'next', in Ruby) and 'break'.
>>>  
>>> To whit: In Ruby, 'next' will end the current run (iteration) of the block, and 'break' will (somehow) terminate the method lexically connected with the block. It can be claimed that this is more intuitive than the current proposal, which aims to make 'break' and 'continue' propagate through block lambdas in the same way 'return' would.
>>>  
>>> Ruby does also support syntactic loops and the same keywords therein and so directly violates Tennent's Correspondence Principle, even though such has been touted as a core reason for the construct. Instead, I believe it reasonable to invoke intuition in this matter. It is intuitive for 'return' to return a value from the lexically enclosing method and it is intuitive for 'continue' to commence the next iteration of the current loop, however that loop is constructed.
>>>  
>>> Note that the label-based break/continue could still have the desired effect, if the proposal was updated to be more like Ruby's blocks.
>>>  
>>> I don't have a strong opinion on the subject, but I hadn't noticed the above being discussed, elsewhere, and thought it worth raising. If there is a better place for me to raise this, please let me know where and accept my apologies.
>>>  
>>> Regards,
>>> Grant Husbands.
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>> <postbox-contact.jpg>	Brendan Eich	January 14, 2012 12:51 PM
>>> <postbox-contact.jpg>	Herby Vojčík	January 14, 2012 10:42 AM
>>> === David Herman wrote === 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>>> === 
>>> 
>>> What about the exception-less suggestion I put in? It should work in any loop construct with lambda-block, even if you must know a little about the loop implementation itself. That is, to be able to put: 
>>> 
>>>    continue |expression|; 
>> 
>> Who says the block-lambda is being called from a loop at all? Why should use-cases that want an early result and completion have to use continue, which is for loops?
>> 
>> Worse, this violates TCP. Now you copy and paste this block-lambda code back into a block statement to refactor the other direction, and no such "here is the completion value, do not flow past this point in the block" semantics obtain.
>>> 
>>> as a statement in lambda block which instructs the lambda-block itself (not the outer function) to return the expression? This is the de-facto continue semantics (lambda-block, do return a value and the enclosing loop will continue to the next iteration (possibly stopping the loop if it chooses not to have more iterations)).
>> 
>> No it's not. There is no de-facto continue semantics for block-lambdas because they haven't been prototyped. For block statements, no such continue semantics exists.
>> 
>>> It is not possible to enforce break in the same manner, but for continue, it is possible. 
>> 
>> It's possible to abuse any existing keyword, but first: why must there be a new TCP violation? Block-lambda bodies are often expressions, or if statements, then short/functional-style statements, not large bodies demonstrating early-normal-completion opportunities.
>> 
>> We should not eliminate TCP violations only to add new ones, especially without any evidence they're needed and pay their way. Otherwise we'll get an infinite regress of TCP-pure-then-add-new-exceptions-and-repeat additions.
>> 
>> /be
>>> 
>>> Herby 
>>> 
>>> -----Pôvodná správa----- From: David Herman 
>>> Sent: Saturday, January 14, 2012 6:12 PM 
>>> To: Axel Rauschmayer 
>>> Cc: Brendan Eich ; es-discuss at mozilla.org 
>>> Subject: Re: Block Lambdas: break and continue 
>>> 
>>> On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote: 
>>> 
>>> 
>>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as: 
>>> 
>>>    Array.prototype.forEach = function(f) { 
>>>        for (let i = 0, n = this.length; i < n; i++) { 
>>>            try { 
>>>                f.call(this, this[i], i); 
>>>            } catch (e) { 
>>>                if (e instanceof BreakException) 
>>>                    break; 
>>>                else if (e instanceof ContinueException) 
>>>                    continue; 
>>>                else 
>>>                    throw e; 
>>>            } 
>>>        } 
>>>    }; 
>>> 
>>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able. 
>>> 
>>> Did I understand your suggestion correctly? 
>>> 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>>> 
>>> Dave 
>>> 
>>> _______________________________________________ 
>>> es-discuss mailing list 
>>> es-discuss at mozilla.org 
>>> https://mail.mozilla.org/listinfo/es-discuss 
>>> _______________________________________________ 
>>> es-discuss mailing list 
>>> es-discuss at mozilla.org 
>>> https://mail.mozilla.org/listinfo/es-discuss 
>>> <postbox-contact.jpg>	David Herman	January 14, 2012 9:12 AM
>>> 
>>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as:
>>> 
>>> Array.prototype.forEach = function(f) {
>>> for (let i = 0, n = this.length; i < n; i++) {
>>> try {
>>> f.call(this, this[i], i);
>>> } catch (e) {
>>> if (e instanceof BreakException)
>>> break;
>>> else if (e instanceof ContinueException)
>>> continue;
>>> else
>>> throw e;
>>> }
>>> }
>>> };
>>> 
>>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able.
>>> 
>>> Did I understand your suggestion correctly?
>>> 
>>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do.
>>> 
>>> Dave
>>> 
>>> 
>>> <postbox-contact.jpg>	Axel Rauschmayer	January 13, 2012 9:04 PM
>>> I think it’s a valid concern. The idea is: If I can implement my own loops (the nice-looking paren-free syntax feeds that illusion!) then I also want those loops to have break and continue. You could statically determine what construct, say, a break applies to and either throw a BreakException (if it applies to a lambda) or TCP-break (if it applies to an enclosing non-lambda loop). In the examples below, when I see a continue, I look for the innermost enclosing loop braces and the ones belong to list[i].forEach are definitely candidates.
>>>  
>>>  
>>>  
>>> --
>>> Dr. Axel Rauschmayer
>>> axel at rauschma.de
>>>  
>>> home: rauschma.de
>>> twitter: twitter.com/rauschma
>>> blog: 2ality.com
>>>  
>>> <postbox-contact.jpg>	Brendan Eich	January 13, 2012 8:54 PM
>>>> <compose-unknown-contact.jpg>	Grant Husbands	January 13, 2012 7:29 PM
>>>> Block lambdas have been a hot topic, recently, but there's a point of significant divergence between Ruby (which appears to be the inspiration)
>>> 
>>> Not Ruby alone, and not in any chauvinist my-language-is-better sense. Smalltalk is the original inspiration for Ruby blocks, and the correspondence principle has deep roots.
>>> 
>>>> and the proposed solution, in the handling of continue (called 'next', in Ruby) and 'break'.
>>>>  
>>>> To whit: In Ruby, 'next' will end the current run (iteration) of the block, and 'break' will (somehow) terminate the method lexically connected with the block. It can be claimed that this is more intuitive than the current proposal, which aims to make 'break' and 'continue' propagate through block lambdas in the same way 'return' would.
>>> 
>>> "Intuitive" depends on intuition, which is not well-defined. Do you mean a Rubyist might expect different behavior for break? That is possible but JS ain't Ruby and break should not change to do something like what it does in Ruby (and we aren't defining a next equivalent for JS).
>>> 
>>>> Ruby does also support syntactic loops and the same keywords therein and so directly violates Tennent's Correspondence Principle, even though such has been touted as a core reason for the construct. Instead, I believe it reasonable to invoke intuition in this matter. It is intuitive for 'return' to return a value from the lexically enclosing method and it is intuitive for 'continue' to commence the next iteration of the current loop,
>>> 
>>> Wait, why do you think break and continue without label operands do anything other than break from the nearest enclosing loop (or switch or labeled statement if break), or continue the nearest enclosing loop? The proposal specifies this.
>>> 
>>> function find_odds_in_arrays(list,        // array of arrays
>>>                              skip)        // if found, skip rest
>>> {
>>>   let a = [];
>>>   for (let i = 0; i < list.length; i++) {
>>>     list[i].forEach {
>>>       |e|
>>>       if (e === skip) {
>>>         continue;                         // continue the for loop
>>>       }
>>>       if (e & 1) {
>>>         a.push(e);
>>>       }
>>>     }
>>>   }
>>>   return a;
>>> }
>>> 
>>> function find_more_odds(list, stop) {
>>>   let a = [];
>>>   for (let i = 0; i < list.length; i++) {
>>>     list[i].forEach {
>>>       |e|
>>>       if (e === stop) {
>>>         break;                      // break from the for loop
>>>       }
>>>       if (e & 1) {
>>>         a.push(e);
>>>       }
>>>     }
>>>   }
>>>   return a;
>>> }
>>> 
>>>> however that loop is constructed.
>>> 
>>> What do you mean by this? The spec talks about nearest enclosing loop or relevant control structure in the source code. Are you talking about internal loops in implementations (dynamically dispatched at that) of methods that take block-lambdas as arguments? I.e.
>>> 
>>> 
>>> function find_first_odd(a) {
>>>   a.forEach { |e, i|
>>>               if (e & 1) return i; }  // returns from function
>>>   return -1;
>>> }
>>> 
>>> 
>>> The Array.prototype.forEach method's internal implementation is its business, and a break instead of the return would be a static error in this example. It would not be a dynamic throw-like construct that is caught by forEach's implementation.
>>> 
>>> /be
>>> <compose-unknown-contact.jpg>	Grant Husbands	January 13, 2012 7:29 PM
>>> Block lambdas have been a hot topic, recently, but there's a point of significant divergence between Ruby (which appears to be the inspiration) and the proposed solution, in the handling of continue (called 'next', in Ruby) and 'break'.
>>>  
>>> To whit: In Ruby, 'next' will end the current run (iteration) of the block, and 'break' will (somehow) terminate the method lexically connected with the block. It can be claimed that this is more intuitive than the current proposal, which aims to make 'break' and 'continue' propagate through block lambdas in the same way 'return' would.
>>>  
>>> Ruby does also support syntactic loops and the same keywords therein and so directly violates Tennent's Correspondence Principle, even though such has been touted as a core reason for the construct. Instead, I believe it reasonable to invoke intuition in this matter. It is intuitive for 'return' to return a value from the lexically enclosing method and it is intuitive for 'continue' to commence the next iteration of the current loop, however that loop is constructed.
>>>  
>>> Note that the label-based break/continue could still have the desired effect, if the proposal was updated to be more like Ruby's blocks.
>>>  
>>> I don't have a strong opinion on the subject, but I hadn't noticed the above being discussed, elsewhere, and thought it worth raising. If there is a better place for me to raise this, please let me know where and accept my apologies.
>>>  
>>> Regards,
>>> Grant Husbands.
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>> 
>> <postbox-contact.jpg>	Herby Vojčík	January 14, 2012 10:42 AM
>> === David Herman wrote === 
>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>> === 
>> 
>> What about the exception-less suggestion I put in? It should work in any loop construct with lambda-block, even if you must know a little about the loop implementation itself. That is, to be able to put: 
>> 
>>    continue |expression|; 
>> 
>> as a statement in lambda block which instructs the lambda-block itself (not the outer function) to return the expression? This is the de-facto continue semantics (lambda-block, do return a value and the enclosing loop will continue to the next iteration (possibly stopping the loop if it chooses not to have more iterations)). It is not possible to enforce break in the same manner, but for continue, it is possible. 
>> 
>> Herby 
>> 
>> -----Pôvodná správa----- From: David Herman 
>> Sent: Saturday, January 14, 2012 6:12 PM 
>> To: Axel Rauschmayer 
>> Cc: Brendan Eich ; es-discuss at mozilla.org 
>> Subject: Re: Block Lambdas: break and continue 
>> 
>> On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote: 
>> 
>> 
>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as: 
>> 
>>    Array.prototype.forEach = function(f) { 
>>        for (let i = 0, n = this.length; i < n; i++) { 
>>            try { 
>>                f.call(this, this[i], i); 
>>            } catch (e) { 
>>                if (e instanceof BreakException) 
>>                    break; 
>>                else if (e instanceof ContinueException) 
>>                    continue; 
>>                else 
>>                    throw e; 
>>            } 
>>        } 
>>    }; 
>> 
>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able. 
>> 
>> Did I understand your suggestion correctly? 
>> 
>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do. 
>> 
>> Dave 
>> 
>> _______________________________________________ 
>> es-discuss mailing list 
>> es-discuss at mozilla.org 
>> https://mail.mozilla.org/listinfo/es-discuss 
>> 
>> <postbox-contact.jpg>	David Herman	January 14, 2012 9:12 AM
>> 
>> If I understand your suggestion, you're proposing that non-local break and continue should be exposed as standard exceptions, and then implementors of loop-like abstractions could choose to catch them. E.g. you could implement forEach as:
>> 
>> Array.prototype.forEach = function(f) {
>> for (let i = 0, n = this.length; i < n; i++) {
>> try {
>> f.call(this, this[i], i);
>> } catch (e) {
>> if (e instanceof BreakException)
>> break;
>> else if (e instanceof ContinueException)
>> continue;
>> else
>> throw e;
>> }
>> }
>> };
>> 
>> Whereas a function that does *not* want to expose whether it's using loops would simply do nothing with BreakException and ContinueException, and they would propagate out and you'd get the lexical scoping semantics. Meanwhile, break/continue with an explicit target would never be catch-able.
>> 
>> Did I understand your suggestion correctly?
>> 
>> This *may* not violate TCP (I'm not quite sure), but I'm not enthusiastic about the idea. The semantics is significantly more complicated, and it requires you to understand whether a higher-order function like forEach is catching these exceptions or not. So it becomes an additional part of the API of a function. If someone doesn't document what they do with BreakException and ContinueException, then writing callbacks you won't actually be able to predict what `break` and `continue` will do.
>> 
>> Dave
>> 
>> 
>> <postbox-contact.jpg>	Axel Rauschmayer	January 13, 2012 9:04 PM
>> I think it’s a valid concern. The idea is: If I can implement my own loops (the nice-looking paren-free syntax feeds that illusion!) then I also want those loops to have break and continue. You could statically determine what construct, say, a break applies to and either throw a BreakException (if it applies to a lambda) or TCP-break (if it applies to an enclosing non-lambda loop). In the examples below, when I see a continue, I look for the innermost enclosing loop braces and the ones belong to list[i].forEach are definitely candidates.
>>  
>>  
>>  
>> --
>> Dr. Axel Rauschmayer
>> axel at rauschma.de
>>  
>> home: rauschma.de
>> twitter: twitter.com/rauschma
>> blog: 2ality.com
>>  
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> https://mail.mozilla.org/listinfo/es-discuss

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