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I've been playing around with delimited continuations in a toy programming language of mine for a while now and I seriously think that its a paradigm that will

I've been playing around with delimited continuations in a toy programming language of mine for a while now and I seriously think that its a paradigm that will eventually take over the scripting language space. First-class control flow is so powerful once you get the hang of it!

xfer 9y ago
they already exist even though in a limited form: async/await, coroutines, generators etc. I doubt whether first-class support is that important.
- fao_ 9y ago
  "First class functions exist in a limited form: Function pointers, etc. I doubt whether first-class support is that important".
  - xfer 9y ago
    Well, i would love to see some applications of delimited continuations other than those mentioned here: http://matt.might.net/articles/programming-with-continuations--exceptions-backtracking-search-threads-generators-coroutines/ http://matt.might.net/articles/programming-with-continuation... . Most languages have support for these applications. The only thing other than that i have seen work is continuation based web servers.
    - tyoverby 9y ago
      The page that you linked to shows off some features that are not really found in any major programming langauge. Time-traveling search is a big one for me. Many languages have exceptions, but with shift/reset, you can implement your own exception handling behavior trivially, including things like resumable exceptions. Some languages have async/await, but almost all of those implementations are state-machine based, which means that they don't compose well when used with more complex language features like generics and higher order functions. But more than that, in continuation-based asynchronous programming, "async" functions are just normal functions that execute in an async prompt. Here's an example of a multi-user asynchronous TCP echo server in my toy language with tagged delimited continuations: async { loop { let conn = accept_connection("localhost", 9999) async { loop { let data = conn.next_data() conn.send(data) } } } } Where all of those asyncrhonous constructs (including `async` itself) are really short wrappers around delimited continuation machinery. You'll notice that the nested calls to `async` indicate that there are two asyncrhonous contexts 1. Connection acceptance happen on their own "thread" of control 2. Each connection gets their own context in which everything is internally asyncrhonous
      - xfer 9y ago
        time-travelling search can be done with recursion. Otherwise, i agree that they don't compose well with each other in some languages. However, it is harder to implement efficiently as far as i understand; time will tell if they are widely understood to be implemented in future languages.
    - tyoverby 9y ago
      Another thing that I should mention is that most langauges, features like generators / async / exceptions don't compose well at all. In a language where these features are implemented through continuations, it's really easy to pick how they interact, leading to some really powerful tricks!
    - srean 9y ago
      Could you list some these (main stream) languages. I know a few but I am surely missing the others you know of.
aidenn0 9y ago
As hinted at in this paper, continuations are directly at odds with dynamic unwinding (e.g. "finally" blocks).

xfer 9y ago

they already exist even though in a limited form: async/await, coroutines, generators etc. I doubt whether first-class support is that important.

fao_ 9y ago
"First class functions exist in a limited form: Function pointers, etc. I doubt whether first-class support is that important".
- xfer 9y ago
  Well, i would love to see some applications of delimited continuations other than those mentioned here: http://matt.might.net/articles/programming-with-continuations--exceptions-backtracking-search-threads-generators-coroutines/ http://matt.might.net/articles/programming-with-continuation... . Most languages have support for these applications. The only thing other than that i have seen work is continuation based web servers.
  - tyoverby 9y ago
    The page that you linked to shows off some features that are not really found in any major programming langauge. Time-traveling search is a big one for me. Many languages have exceptions, but with shift/reset, you can implement your own exception handling behavior trivially, including things like resumable exceptions. Some languages have async/await, but almost all of those implementations are state-machine based, which means that they don't compose well when used with more complex language features like generics and higher order functions. But more than that, in continuation-based asynchronous programming, "async" functions are just normal functions that execute in an async prompt. Here's an example of a multi-user asynchronous TCP echo server in my toy language with tagged delimited continuations: async { loop { let conn = accept_connection("localhost", 9999) async { loop { let data = conn.next_data() conn.send(data) } } } } Where all of those asyncrhonous constructs (including `async` itself) are really short wrappers around delimited continuation machinery. You'll notice that the nested calls to `async` indicate that there are two asyncrhonous contexts 1. Connection acceptance happen on their own "thread" of control 2. Each connection gets their own context in which everything is internally asyncrhonous
    - xfer 9y ago
      time-travelling search can be done with recursion. Otherwise, i agree that they don't compose well with each other in some languages. However, it is harder to implement efficiently as far as i understand; time will tell if they are widely understood to be implemented in future languages.
  - tyoverby 9y ago
    Another thing that I should mention is that most langauges, features like generators / async / exceptions don't compose well at all. In a language where these features are implemented through continuations, it's really easy to pick how they interact, leading to some really powerful tricks!
  - srean 9y ago
    Could you list some these (main stream) languages. I know a few but I am surely missing the others you know of.

aidenn0 9y ago

As hinted at in this paper, continuations are directly at odds with dynamic unwinding (e.g. "finally" blocks).