More on continuations
Don Box offered a few answers to my questions in his latest post, but I am still unsatisfied. Here’s why.
First of all, the various comments on our respective blog entries taught me that C# doesn’t really support continuations, which explains the awkward return yield syntax. This particular C# feature is probably closer to generators, which are themselves a more restricted form of closures (but still more useful than anything we have in Java).
Don’s answer to the question “how useful are continuations?” is the following:
I’ve been programming in C# 2.0 for over a year now. I regularly find myself using the following two pre-defined delegates from mscorlib in my programs:
namespace System { public delegate bool Predicate<T>(T item); public delegate void Action<T>(T item); }
I am quite puzzled by this code snippet which doesn’t seem to have much to do with continuations. Don’s remaining text is a pretty convincing argument of the usefulness of delegates in C#, something I am in full agreement with. Years ago, when Microsoft came up with its own JVM and IDE, my fondness for delegates was immediate. They appeared to me as a typesafe and reasonably object-oriented way to provide method callbacks.
Back then, I spent hours on emails and discussions trying to convince Java developers around me (including at JavaSoft, and more particularly on the Swing team) that we should have delegates in Java. But my words fell on deaf ears, creating cascades of flame wars that never accomplished anything. The problem was, of course, that the reactions were clouded by politics and not driven by pragmatism. So Java drifted away from delegates and will probably never recover. This is quite sad and probably the cause for thousands of wasted objects every day as we create and destroy interfaces for every click of a button.
Anyway.
Don shows a pretty good example of predicates and first-order logic that allows him to neatly isolate logic between callers and callees. It’s a fairly common idiom that you also find in many places in Java (comparators, file filters, etc…). If you are not familiar with this kind of trick, I strongly encourage you to read up on the STL (Standard Template Library) which, even though it is written in C++ and makes heavy use of advanced template techniques, contains a lot of very important concepts that you will undoubtedly find useful in your daily Java programming.
So I’m back to my original question: how useful are continuations, really? I can’t shake off the idea that they are nothing more than a glorified way to do goto. A cleaner way, sure, since it remembers contexts and frames, but I am still looking for this one example where a continuation-based code is more readable than a loop-based one.
Will anyone take up the challenge?
Comments (17)
Cedric, of course this is in the context of a web application, but compare: http://rifers.org/#D.3 with http://rifers.org/#D.4
BTW I completely agree that people often get confused with generators, delegates, closures and the like and use the term continuations which are quite different (e.g. they're more like setjmp/longjmp in C). For me the big win of continuations are in workflow style applications or wizard style web applications; where programs can be written with a procedural style (i.e. nice and simple from the programmers perspective) but the container can pause execution of threads and turn an apparently synchronous application into a purely event based asynchronous application. i.e. for me continuations are away for developers to write sync looking code but deploy it in a purely async way. There are serious issues to address with continuations though - like how to properly implement the serialisation (suspend/resume) of the stack.
I think when you say continuation you actually mean closures. These two I believe are completely different. Example of closure which is more readable: (Groovy like syntax) list.exists(elm | elm.name == "Cedric") You don't see the loop here, while the equivalent foreach/for loop would be slightly verbose. I think the power of the closure is in writing functions like 'exists' which eliminate the loop. There are bunch of such standard functions like 'forall', 'collect', 'iterate' which more or less eliminate the need for foreach loop. Other typical usage of closure is do some pre and post processing, like executing the code within a transaction etc.
If you agree that Java should have type-safe closures, vote! http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5061325
krishnan, what you've shown would be satisfied by a simple anonymous subroutine. It need not capture ("close over") its environment to serve that purpose.
<strong>C# Continuations useful? Hell yeah!</strong> Last week, Sam Ruby posted a very interesting article on continuations for "people older than dirt" (a category which I, according to his definition, fall into). The topic became even more interesting when Don Box posted how you can use a very similar ...
The challenge Cedric posed was "one example where a continuation-based code is more readable than a loop-based one". I thought the example I gave satisfied that.Would it have captured ("close over") its environment if it had used a local variable in scope instead of the literal. list.exists(elm | elm.name == someLocalVar) I personally view closures as a more powerful and compact anonymous class. I understand that you can change the local variables, and it works even after the method completes (meaning the variable cannot be stored in stack or the local frame itself should be captured in heap). From the example I have seen, I am not yet convinced that such powerful constructs are really used extensively in practice. I would be quite happy to see a compact anonymous class syntax (as a special case) rather than full fledged support for closures (I can live with final variables). What would be nice is, btn.addActionListener(ActionEvent e | dothis()); And the compiler should automatically synthesize a ActionListener (anonymous impl) class, insert my code in the class's single method. Intellij does something similar as part of code completion, but it would be better if compiler can do this. This of course would work only for a special case where the interface has just one method. C# delegates also fall in this category (single method), but it still seems to be very popular (even with Cedric :)
<strong>Closures and Continuations</strong>
Cedric, While I doubt that this will statisfy you, you might check out my post from this morning: http://www.pluralsight.com/blogs/dbox/archive/2005/04/27/7780.aspx DB
I think inner classes are better than delegates, closures, what ever you want to call them, because: 1. They are just a single concept - a class 2. They are easy to refactor into a standaolone class 3. They can inherit from a base class - code reuse 4. They can have fields 5. They can have more than one method Their disadvantage of read-only access to a local primitive is not major; you can wrap it, e.g. in an array. If an object has set methods or equivalent then you effectively have write access, therefore the read-only access is probably only relevent for primitives. I have some sympathy with the shorter syntax, I even proposed some changes: http://forums.java.net/jive/thread.jspa?threadID=508&tstart=30 The change I proposed was: 1. new keyword optional 2. If a class or method only had one line then curly braces not needed, i.e. like for, if, etc. 3. If there was only one method to implement from an interface or abstract class then you didn't need to give method visibility, method return type, method name, or argument types. Using these 3 rules, the following: textField.addActionListner( new ActionListener() { public void actionPerformed( ActionEvent notUsed ) { textArea.append( textField.getText() ); } }); Becomes: textField.addActionListner( ActionListener() ( notUsed ) textArea.append( textField.getText() ); );
system calls & interrupts, does this rings a bell?
It need not capture ("close over") its environment to serve that purpose - no that
It seems like continuations would be useful for implementing cooperative multithreading. There is also a class of problems with navigating multiple recursive structures, that are well served by continuations. I believe the canonical example was efficiently detecting if two binary trees have the same values at their leaf nodes (you have to do it without first collecting each tree's leaf nodes into a list). You can do it, but only by manually doing what continuations give you for free (i.e. capturing and restoring the state of the stack). Or by using two threads. :) Ah, found it, it's called the "Same Fringe Problem", apparently coroutines are enough (although not if you are comparing more than two trees, IIUC): http://c2.com/cgi/wiki?SameFringeProblem
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Multimedia scheduling is a great application for coroutines or continuations. A good example of a language that exploits coroutines is the CHuCK programming language. An example of a use of coroutines would be: ~ { // create a new "shred"/fiber/coroutine stack PlayNote(C); now += 1.5; // advance the scheduler 1.5 beats. PlayNote(EFlat); now += 0.5; // advance the scheduler 0.5 betas. } ~ { PlayNote(E); now += 1.0; // advance the scheduler 1 beat. PlayNote(D); now += 1.0; PlayNote(C); } In the intended implementation, the the now property set method would capture the execution stack, and push a delegate into a priority queue for later execution. A more modest implementation, requiring continuations, but not full coroutine implementation would be: ~ { // start closure of outer variables PlayNote(CSharp); // language extension that calls // Scheduler.At(double time, Action resumeDelegate); // where invocation of resumeDelegate // continues execution on the following // statement. yield Scheduler@ now + 1.5; // (or yeild now+1.5), which executes this.At(time, delegate);