Block Lambdas: break and continue

François REMY fremycompany_pub at yahoo.fr
Sat Jan 14 13:14:01 PST 2012


What you said about ‘large-scale’ try/catch seems irrevelant to me. If you make a function that takes a callback as an argument, you already HAVE to use a try-catch, since you have to prevent any faillure in the code of the callback since you can’t trust it, even if he’s empty (as you noted). So, the BreakException has no chance to go outside the function who called the block lambda.

If you want to implement a function that loops, you just need to add some code to handle BreakException and ContinueException. If you don’t want, you just don’t use it. It’s up to you.
From: Brendan Eich 
Sent: Saturday, January 14, 2012 10:00 PM
To: Axel Rauschmayer 
Cc: es-discuss at mozilla.org 
Subject: Re: Block Lambdas: break and continue


  Axel Rauschmayer
  January 14, 2012 12:35 PM

  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.


See Allen's post for how this does not compose well.


  I don’t think it’s a must-have, but whenever you catch exceptions, you have similar issues.

Sorry, termination style exception handling is pretty much a failure outside of local protocols with automated catching (for/of in Harmony loops, comprehensions, and generator expressions).

People build local ad-hoc try/catch machines, e.g. for generator schedulers, and those can be managed even without syntax, but these "kernels" require expertise and careful API design.

But in general when you see a try/catch, the catch is empty and the try is because of a call out of module or ownable unit of code, into some hostile subsystem or "other" that has in the past thrown a random exception.

IOW, termination-style exception handling does not scale. It's also a dynamic thing, like magic return codes. Intervening functions on the call stack with no necessary static relation to one another may have to try and catch (or at least try and finally) to unwind-protect. Not for memory management, of course, but for other mandatory protocols such as RAII patterns.

So I'm against adding BreakException and ContinueException and defining block-lambda break and continue in terms of throw. As Dave and Allen argued, this enlarges the contract of every possible intervening function to include exceptions. Exceptions are usually ignored at high level, used only locally (across shallow continuation calls).

We should avoid adding dependencies on exceptions that must be managed by arbitrarily large codebases and arbitrarily deep callstacks.

/be


  -- 
  Dr. Axel Rauschmayer
  axel at rauschma.de

  home: rauschma.de
  twitter: twitter.com/rauschma
  blog: 2ality.com


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  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




  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


  Brendan Eich
  January 13, 2012 8:54 PM

    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


  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.
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