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Continuations are extremely useful in functional programming languages (e.g. the Cont monad in Haskell) because they allow a simple and regular notation for imperative-style code. They're also useful in some older imperative languages because they can be used to implement missing language features (e.g. exceptions, coroutines, green threads). But for a modern object-oriented language with support built in for these features, what arguments would there be for also adding support for first-class continuations (whether the more modern delimited style reset and shift or scheme-like call-with-current-continuation)?

Are there arguments against adding support other than performance and complexity of implementation?

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Let's let Eric Lippert answer this one:

The obvious question at this point is: if CPS is so awesome then why don’t we use it all the time? Why have most professional developers never heard of it, or, those who have, think of it as something only those crazy Scheme programmers do?

First of all, it is simply hard for most people who are used to thinking about subroutines, loops, try-catch-finally and so on to reason about delegates being used for control flow in this way. I am reviewing my notes on CPS from CS442 right now and I see that in 1995 I wrote down a profQUOTE: “With continuations you have to sort of stand on your head and pull yourself inside out”. Professor Duggan (*) was absolutely correct in saying that. Recall from a couple days ago that our tiny little example of CPS transformation of the C# expression M(B()?C():D()) involved four lambdas. Not everyone is good at reading code that uses higher-order functions. It imposes a large cognitive burden.

Moreover: one of the nice things about having specific control flow statements baked in to a language is that they let your code clearly express the meaning of the control flow while hiding the mechanisms – the call stacks and return addresses and exception handler lists and protected regions and so on. Continuations make the mechanisms of control flow explicit in the structure of the code. All that emphasis on mechanism can overwhelm the meaning of the code.

In the next article, he explains how asynchrony and continuations are exactly equivalent to one another, and goes over a demonstration of taking a simple, (but blocking,) synchronous network operation and rewriting it in asyncronous style, making sure to cover all the hidden gotchas that have to be covered in order to get it right. It turns into a massive mess of code. His summary at the end:

Holy goodness, what a godawful mess we’ve made. We've expanded two lines of perfectly clear code into two dozen lines of the most godawful spaghetti you've ever seen. And of course it still doesn’t even compile because the labels aren’t in scope and we have a definite assignment error. We;d still need to further rewrite the code to fix those problems.

Remember what I was saying about the pros and cons of CPS?

  • PRO: Arbitrarily complex and interesting control flows can be built out of simple parts – check.
  • CON: The reification of control flow via continuations is hard to read and hard to reason about – check.
  • CON: The code that represents the mechanisms of control flow completely overwhelms the meaning of the code – check.
  • CON: The transformation of ordinary code control flow into CPS is the kind of thing that compilers are good at, and almost no one else is – check.

This is not some intellectual exercise. Real people end up writing code morally equivalent to the above all the time when they deal with asynchrony. And, ironically, even as processors have gotten faster and cheaper, we spend more and more of our time waiting for stuff that isn’t processor-bound. In many programs, much of the time spent is with the processor pegged to zero waiting for network packets to make the trip from England to Japan and back again, or for disks to spin, or whatever.

And I haven't even talked about what happens if you want to compose asynchronous operations further. Suppose you want to make ArchiveDocuments an asynchronous operation that takes a delegate. Now all the code that calls it has to be written in CPS as well. The taint just spreads.

Getting asynchronous logic right is important, it’s only going to be more important in the future, and the tools we have given you make you “stand on your head and turn yourself inside out” as Professor Duggan wisely said.

Continuation passing style is powerful, yes, but there's got to be a better way of making good use of that power than the code above.

Both articles, and the following series on a new C# language feature that moves all this mess into the compiler and lets you write your code as normal control flow with a special keyword to mark certain parts as asynchronous, are well worth reading even if you're not a C# developer. I'm not, but it was still quite the enlightening experience when I ran across it for the first time.

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    It's worth noting that if your language supports syntax extensions like macros, then continuations become far more useful because you can hide the mechanisms for implementing various kinds of interesting control flow inside the syntax extensions. Which is basically how "await" works anyway, it's just defined in the language since C# doesn't provide the tools to define those sorts of syntax extensions yourself. – Jack Apr 11 '15 at 3:11
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    Good article, although I'll note that CPS and first-class continuations aren't the same thing exactly (CPS is what you do in a language with no support for continuations when you find you do need them). It sounds like C# is going down exactly the same route I'm thinking about (although I can't decide if I want to automate tyre transform to CPS or implement full-blown stack-copying delimited continuations). – Jules Apr 11 '15 at 18:12
  • @Jack - that's exactly what I'm considering right now: the language I'm designing has a macro facility that could support the CPS transform automatically. But full native continuations might give better performance... the question is, how often would they be used in a performance-critical situation? – Jules Apr 11 '15 at 18:15
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The article referenced in the accepted answer is mostly true about the fact of the difficulties on reading/reasoning the CPS'd code by humans, but it is not quite convincing as the answer to this question. Moreover, it may be actually a counter argument to avoid including first-class continuations in the language designs.

CPS transformation is a general way to make the code able to express some higher-order control effects in a language without first-class continuations or other appropriate control operators, in a somewhat portable flavor. That is, if you have no chance to use them in such a language, you may choose to manually do the CPS transformation to translate the source code you have written in the direct style to some spaghetti-like one in the same language (once you really have the resources to do it correctly and manually).

On the other hand, if the language has already the ability to express such control effects, you can just write code directly using such features, without the need to transform your source code. Instead, the implementation (usually a compiler) of the language will do this (unlikely in the same target language, though) for you on demand. Note there are certain other ways to implement first-class continuations, but they usually rely on more assumptions on the non-portable details of the underlying systems (to make themselves practically implementable), so such ways are less well-known to most people.

The code with first-class continuations and higher-order control operators is indeed difficult to reason about for people without sufficient training. (Experts may suggest users to have knowledge of some formal methods to reason about the things to avoid trickery.) However, it is still not difficult as CPS'd code in general. Not only the process of CPS transformation itself, but also the result of the transformation is error-prone to read/verify to humans, even including experts. Such nature is a strong argument supporting to provide the features directly in the language to enable the implementations doing the proper works, instead of enforcing users to do unintuitive manual translations of their code.

The remained argument against first-class continuations and higher-order control operators is mainly based on the fact that such features are too low-level for most application tasks. Higher-level mechanisms like exception handling and cooperative multitasking should be more usual to average programmers (if such "high-level" features are still not too difficult to learn and to use). This is fair. However, to include a feature is not to encourage to abuse it; it is more about to avoid the embarrassment when other features are insufficient. Once the users meet the cases where the built-in high-level features are insufficient, they will sooner or later reach such low-level features, by requiring or reinventing them. Otherwise, either the need cannot be satisfied, or they have to invent a whole language implying the capability of such low-level wheels.

The conclusions:

  • They are still useful to users with needs to get the low-level works done portably, but probably not that useful to others.
  • I'm not sure the ratio of former in a typical OOP PL, but likely quite a few.
  • However, for the specific language features, the amount of users using them directly does not matter its usefulness much, because other users will usually still rely on them indirectly. Especially in the cases of low-level features, the amount of "other" users will be potentially large.
    • The only exception is that those features are too badly designed so no one ever relies on them indirectly.
    • The operator call/cc may be superseded by delimited continuations and higher-level features in practice. But the whole family of (first-class) continuations... No, I think.
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I’m using this in shipping Swift Code, and since it’s not done to show my continuation skills but to solve a genuine problem, it’s quite easy to understand and would be very difficult to do without continuations.

Problem: Write a function that checks whether internet connectivity is there, and when it has decided, calls a continuation with a parameter indicating if the network is available. Obviously asynchronous because it can take a while, especially in the failure case.

Complication: This method is called from multiple threads and also repeatedly from the same thread. And having code that checks for internet access running several times is inefficient, difficult or both.

Solution: The method has an array of continuations, initially empty. First thing a call it checks whether the array is empty, otherwise there’s a call running already. If another call is running, just add the continuation to the array and return. Otherwise store the continuation as the sole array element, decide whether the network is available, and when decided extract all the continuations, set the array to empty, and call all the continuations.

Synchronised {
    If array != empty {
        Add continuation to array, return
    } else {
        Store continuation into empty array
    }
}
In the background {
    Determine success/failure
    Synchronised {
        Make copy of array
        Array = empty
    }
    For each continuation in copy of array {
        Call continuation with success / failure
    }
}

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