Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
On the general notion of continuations, I believe Matt Might's blog explains
it quite well using Javascript.
http://matt.might.net/articles/by-example-continuation-passing-style/
In the way of a simple example, he suggests that instead of writing
function id(x) {
return x ;
}
a CPS version might write:
function id(x,ret) {
ret(x) ;
}
etc...
IMO things appear confusing to newbies (it happened to me once too) when
people dont use intuitive names for obvious things like continuations
On Fri, Sep 30, 2011 at 11:42 PM, Mark Spezzano
mark.spezz...@chariot.net.au wrote:
Hi,
Can someone please give me a _lucid_ and _simple_ explanation of exactly
how continuations can be used in Haskell?
I've already had a look at most of the tutorials and explanations on the
web, but I'm still confused. Continuations and CPS have me baffled. (I have
most of the Haskell textbooks and even these are sketchy on Continuations)
I don't understand the notion of the Cont monad and how it can be used for
multitasking, backtracking and interrupting computations. I understand that
functions take in a (continuation) function that represents the work
remaining to do, but al of the explanations on the web and in technical
papers seems to trip over themselves in explaining the fundamentals to a
CPS-newbie.
If anyone could explain such concepts to me in unambiguous, clear English
then this would be very helpful.
Thanks in advance for your help,
Mark
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Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Ozgur Akgun wrote: On 1 October 2011 11:55, Yves Parès limestr...@gmail.com wrote: BTW Heinrich, the evalState (sequence . repeat . State $ \s - (s,s+1)) 0 at the end doesn't work anymore. It should be replaced by : evalState (sequence . repeat . StateT $ \s - Identity (s,s+1)) 0 Or equivalently: evalState (sequence . repeat . state $ \s - (s,s+1)) 0 Thanks, I've changed it. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Really Simple explanation of Continuations Needed
Hi, Can someone please give me a _lucid_ and _simple_ explanation of exactly how continuations can be used in Haskell? I've already had a look at most of the tutorials and explanations on the web, but I'm still confused. Continuations and CPS have me baffled. (I have most of the Haskell textbooks and even these are sketchy on Continuations) I don't understand the notion of the Cont monad and how it can be used for multitasking, backtracking and interrupting computations. I understand that functions take in a (continuation) function that represents the work remaining to do, but al of the explanations on the web and in technical papers seems to trip over themselves in explaining the fundamentals to a CPS-newbie. If anyone could explain such concepts to me in unambiguous, clear English then this would be very helpful. Thanks in advance for your help, Mark ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Mark Spezzano wrote: Can someone please give me a _lucid_ and _simple_ explanation of exactly how continuations can be used in Haskell? I've already had a look at most of the tutorials and explanations on the web, but I'm still confused. Continuations and CPS have me baffled. (I have most of the Haskell textbooks and even these are sketchy on Continuations) I don't understand the notion of the Cont monad and how it can be used for multitasking, backtracking and interrupting computations. I understand that functions take in a (continuation) function that represents the work remaining to do, but all of the explanations on the web and in technical papers seems to trip over themselves in explaining the fundamentals to a CPS-newbie. If you just want to implement multitasking, backtracking or interrupting computations, without continuations, I recommend my Operational Monad Tutorial http://apfelmus.nfshost.com/articles/operational-monad.html The link to the Cont monad is explained at the very end. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Hi Heinrich, I'm really looking to use the Cont monad itself--but the link you gave me is also helpful, so thank you. If anyone else has words of wisdom to add to this thread please feel free to pitch in. Thanks, Mark On 01/10/2011, at 5:08 PM, Heinrich Apfelmus wrote: Mark Spezzano wrote: Can someone please give me a _lucid_ and _simple_ explanation of exactly how continuations can be used in Haskell? I've already had a look at most of the tutorials and explanations on the web, but I'm still confused. Continuations and CPS have me baffled. (I have most of the Haskell textbooks and even these are sketchy on Continuations) I don't understand the notion of the Cont monad and how it can be used for multitasking, backtracking and interrupting computations. I understand that functions take in a (continuation) function that represents the work remaining to do, but all of the explanations on the web and in technical papers seems to trip over themselves in explaining the fundamentals to a CPS-newbie. If you just want to implement multitasking, backtracking or interrupting computations, without continuations, I recommend my Operational Monad Tutorial http://apfelmus.nfshost.com/articles/operational-monad.html The link to the Cont monad is explained at the very end. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Having worked with the operational-package, I only can recommend it. In fact I was trying to do the same thing you are now. The only thing is that operational needs the use of GADTs, which come as an extension, but still are a useful and heavily used feature. BTW Heinrich, the evalState (sequence . repeat . State $ \s - (s,s+1)) 0 at the end doesn't work anymore. It should be replaced by : evalState (sequence . repeat . StateT $ \s - Identity (s,s+1)) 0 2011/10/1 Mark Spezzano mark.spezz...@chariot.net.au Hi Heinrich, I'm really looking to use the Cont monad itself--but the link you gave me is also helpful, so thank you. If anyone else has words of wisdom to add to this thread please feel free to pitch in. Thanks, Mark On 01/10/2011, at 5:08 PM, Heinrich Apfelmus wrote: Mark Spezzano wrote: Can someone please give me a _lucid_ and _simple_ explanation of exactly how continuations can be used in Haskell? I've already had a look at most of the tutorials and explanations on the web, but I'm still confused. Continuations and CPS have me baffled. (I have most of the Haskell textbooks and even these are sketchy on Continuations) I don't understand the notion of the Cont monad and how it can be used for multitasking, backtracking and interrupting computations. I understand that functions take in a (continuation) function that represents the work remaining to do, but all of the explanations on the web and in technical papers seems to trip over themselves in explaining the fundamentals to a CPS-newbie. If you just want to implement multitasking, backtracking or interrupting computations, without continuations, I recommend my Operational Monad Tutorial http://apfelmus.nfshost.com/articles/operational-monad.html The link to the Cont monad is explained at the very end. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Hi. On 1 October 2011 11:55, Yves Parès limestr...@gmail.com wrote: BTW Heinrich, the evalState (sequence . repeat . State $ \s - (s,s+1)) 0 at the end doesn't work anymore. It should be replaced by : evalState (sequence . repeat . StateT $ \s - Identity (s,s+1)) 0 Or equivalently: evalState (sequence . repeat . state $ \s - (s,s+1)) 0 Ozgur ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Really Simple explanation of Continuations Needed
Hello, Oleg has a introduction for delimited continuations which he presented during ICFP: http://okmij.org/ftp/continuations/index.html#tutorial Of course, it's worth mentioning that the Cont monad is actually doing delimited continuations, cf.: http://okmij.org/ftp/continuations/ContExample.hs Ultimately, the idea is simple, which may be part of the reason why it's so hard to explain (cf., monads). The short version is that at any point in the execution of a program we have some part of the program which has already run, and some part which has yet to run; the latter is the continuation, which is also often called the calling context. All the hoopla about continuations comes from the tricksy things we can do once we realize that programs have these two parts instead of only thinking about the history of the execution. Operationally, the way the Cont monad works is that we explicitly build up the continuation as a function to be called, and then runCont actually applies that function in order to yield a result. What use is this? Well, it gives us a version of the goto statement, namely call/cc. Another part of the problem (other than the simplicity) is that the term continuation has many different meanings in computer science. On the one hand we have the call/cc notion of continuations, which is what's captured by Scheme's call/cc and by the Cont monad. On the other hand we have the CPS transformation that many compilers use when optimizing code. But the topics of discussion for call/cc and CPS are very different, and so it's easy to get confused if you try to think of them as the same thing. They are closely related, but they're different enough that you should keep them separate until you have some understanding of what they're about. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
