Comments on: Continuation monad in Scala http://blog.tmorris.net/continuation-monad-in-scala/ The weblog of Tony Morris Sat, 04 Feb 2012 12:49:10 +0000 http://wordpress.org/?v=2.6 By: λ Tony’s blog λ » Blog Archive » Nothing returns anything, ever! http://blog.tmorris.net/continuation-monad-in-scala/#comment-57656 λ Tony’s blog λ » Blog Archive » Nothing returns anything, ever! Tue, 28 Dec 2010 09:39:12 +0000 http://blog.tmorris.net/?p=512#comment-57656 [...] another battle Further, there are some interesting properties about continuations, in particular, they are a a monad, perhaps even the mother of all monads, but we’ll leave all that for another day [...] [...] another battle Further, there are some interesting properties about continuations, in particular, they are a a monad, perhaps even the mother of all monads, but we’ll leave all that for another day [...]

]]> By: Peter http://blog.tmorris.net/continuation-monad-in-scala/#comment-29940 Peter Mon, 26 Jan 2009 21:26:35 +0000 http://blog.tmorris.net/?p=512#comment-29940 Is it possible to express something like the following Scheme snippet with this continuation formulation? <code> (+ 1 (call/cc    (lambda (k)    (+ 2 (k 4))))) </code> Is it possible to express something like the following Scheme snippet with this continuation formulation?


(+ 1 (call/cc
   (lambda (k)
   (+ 2 (k 4)))))

]]> By: Tony Morris http://blog.tmorris.net/continuation-monad-in-scala/#comment-27223 Tony Morris Sat, 03 Jan 2009 21:34:45 +0000 http://blog.tmorris.net/?p=512#comment-27223 Thanks Martin, I wasn't aware of Responder. It looks like Responder applies the first type parameter of my example as <code>Unit</code>. That is, <code>Continuation[Unit, A]</code> ~= <code>Responder[A]</code>. Thanks Martin, I wasn’t aware of Responder. It looks like Responder applies the first type parameter of my example as Unit. That is, Continuation[Unit, A] ~= Responder[A].

]]> By: Martin http://blog.tmorris.net/continuation-monad-in-scala/#comment-27211 Martin Sat, 03 Jan 2009 16:18:50 +0000 http://blog.tmorris.net/?p=512#comment-27211 Hi Tony, Not sure whether this is known: There is something very similar to what you describe in Scala's standard library: It's called Responder, and it is in the scala package. Cheers -- Martin Hi Tony,

Not sure whether this is known: There is something very similar to what you describe in Scala’s standard library: It’s called Responder, and it is in the scala package.

Cheers

— Martin

]]> By: Vadim http://blog.tmorris.net/continuation-monad-in-scala/#comment-26734 Vadim Mon, 29 Dec 2008 10:46:40 +0000 http://blog.tmorris.net/?p=512#comment-26734 Thanks, I finally got it ) Thanks, I finally got it )

]]> By: Tony Morris http://blog.tmorris.net/continuation-monad-in-scala/#comment-26699 Tony Morris Mon, 29 Dec 2008 01:26:12 +0000 http://blog.tmorris.net/?p=512#comment-26699 Hi Vadim, It doesn't get evaluated because the continuation acts as a function to the value. It is when that function's arguments are applied that all the evaluation occurs. Consider this simple Java snippet: <pre lang="Java"> int method() { return a + b; } </pre> This is slightly different to: <pre lang="Java"> int method = a + b; </pre> This is because of the evaluation semantics. The use of the <code>def</code> keyword in Scala effectively makes the code like the former (while <code>val</code> would make it like the latter). I hope this helps. Hi Vadim,
It doesn’t get evaluated because the continuation acts as a function to the value. It is when that function’s arguments are applied that all the evaluation occurs.

Consider this simple Java snippet:

int method() {
  return a + b;
}

This is slightly different to:

int method = a + b;

This is because of the evaluation semantics. The use of the def keyword in Scala effectively makes the code like the former (while val would make it like the latter).

I hope this helps.

]]> By: Vadim http://blog.tmorris.net/continuation-monad-in-scala/#comment-26688 Vadim Sun, 28 Dec 2008 22:52:21 +0000 http://blog.tmorris.net/?p=512#comment-26688 Hey, great examples. If only I could understand them as easily as you seem to be writing. I'm fairly new to Scala (it's been a week) and this example you provided (squares), which is supposed to be trivial by your measures, confuses me even after an hour of beating my head against the interpreter. Why doesn't <code>square(n)</code> in <code>def squarec[R](n: Int) = unit[R](square(n))</code> get evaluated immediately after <code>squarec[Unit](n)</code> is called? Instead, it gets evaluated 2 times upon the last call to the resulting Continuation .apply (<code>k(println)</code>), before executing the function supplied to the .apply (<code>println</code>). I'm sure it's something very simple, but it's driving me nuts. Hey, great examples. If only I could understand them as easily as you seem to be writing. I’m fairly new to Scala (it’s been a week) and this example you provided (squares), which is supposed to be trivial by your measures, confuses me even after an hour of beating my head against the interpreter.

Why doesn’t square(n) in def squarec[R](n: Int) = unit[R](square(n)) get evaluated immediately after squarec[Unit](n) is called? Instead, it gets evaluated 2 times upon the last call to the resulting Continuation .apply (k(println)), before executing the function supplied to the .apply (println).

I’m sure it’s something very simple, but it’s driving me nuts.

]]>