5 ms·

Continuations by example: Exceptions, time-traveling search, threads, and more

151 points by alschwalm 6y ago

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verytrivial 6y ago
I always start Matt Might articles thinking "Okay, THIS is the one I'm going to understand all the way to the very end without unlearning what I just read, else I die," and alas, I have again died.
- vincnetas 6y ago
  Me to. Level failed, but then i found this cheat code from apache and at lest i now understand basics. http://commons.apache.org/sandbox/commons-javaflow/ http://commons.apache.org/sandbox/commons-javaflow/
  - offbynull 6y ago
    AFAIK Javaflow's been inactive for over a decade. Newer implementations for the same concept include https://github.com/offbynull/coroutines https://github.com/offbynull/coroutines and https://github.com/vsilaev/tascalate-javaflow/ https://github.com/vsilaev/tascalate-javaflow/. Full disclosure: I'm the one who built / owns the first project.
    - dlo 6y ago
      Love it. Lmk if you never need a hand. I was on the core analysis team at Fortify.
    - sjtindell 6y ago
      Thanks for sharing this background and cool project.
    - joel_dice 6y ago
      On the topic of inactive continuation implementations for Java: I implemented native support for both delimited (shift/reset) and non-delimited (call/cc) continuations in the Avian JVM. See https://github.com/ReadyTalk/avian/blob/master/test/extra/ComposableContinuations.java https://github.com/ReadyTalk/avian/blob/master/test/extra/Co... for some mind-bending examples.
- azeirah 6y ago
  I'm having an ADHD night and I had the same thought, except I haven't died yet. Felt like giving up three times already, but I did have a click moment after playing around with a scheme repl for a couple of minutes. I understand the first example of call/cc now! (display (call/cc (lambda (cc) (display "I got here.\n") (cc "This string was passed to the continuation.\n") (display "But not here.\n")))) Edit: Tried playing around with mit-scheme, but it doesn't work with the ctrl-v'ing the examples. Chezscheme does work.
  - hencq 6y ago
    It seems that MIT-Scheme needs the cc function to be a function of 1 argument (the return value for the continuation)[0] Changing it to the following should work: (let ((start #f)) (if (not start) (call/cc (lambda (cc) (set! start cc)))) (display "Going to invoke (start)\n") (start #f)) [0] https://www.gnu.org/software/mit-scheme/documentation/stable/mit-scheme-ref/Continuations.html#Continuations https://www.gnu.org/software/mit-scheme/documentation/stable...
  - DonaldPShimoda 6y ago
    For what it's worth, almost any time Matt writes about Scheme he's likely using Racket so it'd be worth trying that for his posts!
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emmanueloga_ 6y ago
I was actually relieved when I found this article [1] which argues against call/cc as a core language feature. Alas, I think the argument does not dismiss _delimited continuations_, so now my burden has shifted from mastering call/cc to mastering shift/reset :-/ [2]. 1: http://okmij.org/ftp/continuations/against-callcc.html http://okmij.org/ftp/continuations/against-callcc.html 2: https://en.wikipedia.org/wiki/Delimited_continuation https://en.wikipedia.org/wiki/Delimited_continuation
jolmg 6y ago
I think I've found a good use for continuations in a parser combinator library I'm writing, though I haven't gotten around to working on the corresponding feature yet. Has anyone used continuations in a serious/non-academic way?
- gus_massa 6y ago
  I used to implement an equivalent of exit or return statement, like in this example. #lang racket ; implement sqrt in a very inefficient way (let/ec exit (for ([i 10]) (when (= (* i i) 49) (exit i))) "not found :(") ;==> 7 There are more idiomatic and easy ways to write this, but when you have to many nested and not nested for's, sometimes it is useful to have a magical exit-everything-now.
- a1369209993 6y ago
  You could dispute "serious", but I've used it in a not!lisp for various control flow purposes. func a0 a1 = callcc \return: # ... callcc (again =) # ... x ?? return y again again
- kazinator 6y ago
  Continuations can be implicitly used. If you have a language that is compiled using transformation to CPS. Then continuations are everywhere under the hood.
Google234 6y ago
Wow, imagine writing in such an unreadable language

151 points by alschwalm 6y ago

deleted 6y ago

[deleted]

I always start Matt Might articles thinking "Okay, THIS is the one I'm going to understand all the way to the very end without unlearning what I just read, else I die," and alas, I have again died.

vincnetas 6y ago
Me to. Level failed, but then i found this cheat code from apache and at lest i now understand basics. http://commons.apache.org/sandbox/commons-javaflow/ http://commons.apache.org/sandbox/commons-javaflow/
- offbynull 6y ago
  AFAIK Javaflow's been inactive for over a decade. Newer implementations for the same concept include https://github.com/offbynull/coroutines https://github.com/offbynull/coroutines and https://github.com/vsilaev/tascalate-javaflow/ https://github.com/vsilaev/tascalate-javaflow/. Full disclosure: I'm the one who built / owns the first project.
  - dlo 6y ago
    Love it. Lmk if you never need a hand. I was on the core analysis team at Fortify.
  - sjtindell 6y ago
    Thanks for sharing this background and cool project.
  - joel_dice 6y ago
    On the topic of inactive continuation implementations for Java: I implemented native support for both delimited (shift/reset) and non-delimited (call/cc) continuations in the Avian JVM. See https://github.com/ReadyTalk/avian/blob/master/test/extra/ComposableContinuations.java https://github.com/ReadyTalk/avian/blob/master/test/extra/Co... for some mind-bending examples.
azeirah 6y ago
I'm having an ADHD night and I had the same thought, except I haven't died yet. Felt like giving up three times already, but I did have a click moment after playing around with a scheme repl for a couple of minutes. I understand the first example of call/cc now! (display (call/cc (lambda (cc) (display "I got here.\n") (cc "This string was passed to the continuation.\n") (display "But not here.\n")))) Edit: Tried playing around with mit-scheme, but it doesn't work with the ctrl-v'ing the examples. Chezscheme does work.
- hencq 6y ago
  It seems that MIT-Scheme needs the cc function to be a function of 1 argument (the return value for the continuation)[0] Changing it to the following should work: (let ((start #f)) (if (not start) (call/cc (lambda (cc) (set! start cc)))) (display "Going to invoke (start)\n") (start #f)) [0] https://www.gnu.org/software/mit-scheme/documentation/stable/mit-scheme-ref/Continuations.html#Continuations https://www.gnu.org/software/mit-scheme/documentation/stable...
- DonaldPShimoda 6y ago
  For what it's worth, almost any time Matt writes about Scheme he's likely using Racket so it'd be worth trying that for his posts!

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deleted 6y ago

[deleted]

emmanueloga_ 6y ago

I was actually relieved when I found this article [1] which argues against call/cc as a core language feature. Alas, I think the argument does not dismiss _delimited continuations_, so now my burden has shifted from mastering call/cc to mastering shift/reset :-/ [2]. 1: http://okmij.org/ftp/continuations/against-callcc.html http://okmij.org/ftp/continuations/against-callcc.html 2: https://en.wikipedia.org/wiki/Delimited_continuation https://en.wikipedia.org/wiki/Delimited_continuation

jolmg 6y ago

I think I've found a good use for continuations in a parser combinator library I'm writing, though I haven't gotten around to working on the corresponding feature yet. Has anyone used continuations in a serious/non-academic way?

gus_massa 6y ago
I used to implement an equivalent of exit or return statement, like in this example. #lang racket ; implement sqrt in a very inefficient way (let/ec exit (for ([i 10]) (when (= (* i i) 49) (exit i))) "not found :(") ;==> 7 There are more idiomatic and easy ways to write this, but when you have to many nested and not nested for's, sometimes it is useful to have a magical exit-everything-now.
a1369209993 6y ago
You could dispute "serious", but I've used it in a not!lisp for various control flow purposes. func a0 a1 = callcc \return: # ... callcc (again =) # ... x ?? return y again again
kazinator 6y ago
Continuations can be implicitly used. If you have a language that is compiled using transformation to CPS. Then continuations are everywhere under the hood.

Google234 6y ago

Wow, imagine writing in such an unreadable language