Do any non-Lisp/Scheme languages have continuations?
Ruby has `#callcc` which produces first-class continuations AFAIK https://ruby-doc.org/core-2.2.0/Continuation.html
Writing a Lisp: Continuations
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Re: Writing a Lisp: Continuations
#12Continuations are strangely underused, as they enables writing long-living processes in a simple way, without having to keep them in running in a thread, or even in memory.
Then a real programming language can replace all "business processing" crap languages. Let's say you write a framework that escapes to a continuation whenever the "process" is waiting for Futures or Promises to complete, and returns the thread to a pool.
Depending on what you are waiting for, such as webhooks, asynch events/messages, or with some work even outstanding network requests ( if it's meaningful ), it would be possible to serialize the continuation to storage, potentially restore it on another machine, weeks later.
With some bookkeeping, it would be possible to also store what Futures have been completed, and what the continuation is currently waiting for, so it would be possible to keep track of the progress of the long running process, and use that information to show a nice status report.
Re: Writing a Lisp: Continuations
#13Are there any continuation implementations in Common Lisp?
Re: Writing a Lisp: Continuations
#14Continuations are strangely underused, as they enables writing long-living processes in a simple way, without having to keep them in running in a thread, or even in memory. Then a real programming language can replace all "business processing" crap languages. Let's say you write a framework that escapes to a continuation whenever the "process" is waiting for Futures or Promises to complete, and returns the thread to…
We are doing exactly this in my current company. There is a quite nice language called Orc for this. A site can do IO, we can serialize
the state and return to it weeks later when we have a triggering event.
Re: Writing a Lisp: Continuations
#15Do any non-Lisp/Scheme languages have continuations?
C# has async/await and TPL, e.g. .ContinueWith(...)
Javscript has promises and async/await
Re: Writing a Lisp: Continuations
#16I remember trying to learn continuations during my CS degree, and evidently even today I still don't understand them. The examples don't seem to help either – how exactly does the control flow function?
Re: Writing a Lisp: Continuations
#17I remember trying to learn continuations during my CS degree, and evidently even today I still don't understand them. The examples don't seem to help either – how exactly does the control flow function?
You know how in Python the "yield" statement is actually an expression and can be sent values using "next"? Imagine there is a special "yield" called "call-with-current-continuation" that can be used anywhere (not just in generators) which bundles everything up and makes a generator-like thing called a "continuation." This continuation object can be called later on, like using "next" in Python. Unlike generators, though, continuations can usually be resumed repeatedly, always resuming at the same point where the call-with-current-continuation expression was.
A weird thing is that call-with-current-continuation doesn't yield back to something; it yields to the function given to call-with-current-continuation.
Re: Writing a Lisp: Continuations
#18Do any non-Lisp/Scheme languages have continuations?
Here's the obligatory "Haskell has a monad for that" comment http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Mo...
Speaking of monads, one of the themes in Prof. Sussman's "Adventures in advanced symbolic programming" course was that many metacircular evaluator experiments can be set up by using mostly the same core evaluator but swapping out the underlying monad. I'm pretty sure I implemented the "ambivalent operator" that way, using the list monad (in Scheme), but when we lifted amb to the host Scheme, it needed call/cc to be able to use the built-in control flow.
One thing I like about Haskell is that it's designed so that adding new control flow is not a big deal -- just design a monad. In Scheme it seems to be more work to get it all right.
Re: Writing a Lisp: Continuations
#19Continuations are strangely underused, as they enables writing long-living processes in a simple way, without having to keep them in running in a thread, or even in memory. Then a real programming language can replace all "business processing" crap languages. Let's say you write a framework that escapes to a continuation whenever the "process" is waiting for Futures or Promises to complete, and returns the thread to…
Continuations have a context which takes up space in memory. That context keeps a reference on an environment (or chain of environments) containing all the resources which must be there when the continuation is invoked.
Re: Writing a Lisp: Continuations
#20Do any non-Lisp/Scheme languages have continuations?
Some Smalltalks have them and they are used by Seaside but made optional in Seaside 3.0 because of the platforms that don't have them.