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1 point by devlinsf 5450 days ago | link | parent

Hmm... I'm interested in points 4 & 5 specifically. Could someone offer an explanation/post some good links?


4 points by fallintothis 5450 days ago | link

First-class continuations & call/cc are notoriously difficult to explain (and indeed, can be hard to understand in code that uses them). Your best bet is to Google around for info, seeing if you can make sense of the (vast) material available. http://en.wikipedia.org/wiki/Call-with-current-continuation and http://community.schemewiki.org/?call-with-current-continuat... might be good starts.

As for point 5, Arc uses certain characters as syntax abbreviations iff the special characters occur in what would otherwise be normal symbols (to the Scheme reader). So far, there's

  .a    ; is the same as (get a)
  a.b   ; is the same as (a b)
  !a    ; is the same as (get 'a)
  a!b   ; is the same as (a 'b)
  f&g   ; is the same as (andf f g)
  f:g   ; is the same as (compose f g)
  ~f    ; is the same as (complement f)
where

  ((get a) b)        ; is the same as (b a) by Arc's indexing
                     ; e.g., ("abc" 0) is #\a
  ((andf f g) x)     ; is the same as (and (f x) (g x))
  ((compose f g) x)  ; is the same as (f (g x))
  ((complement f) x) ; is the same as (no (f x))
See arc.arc for the actual definitions of these operators.

There are precedence and associativity rules, such as

  a!b.c ; is the same as ((a 'b) c) because ! and . are 
        ; left-associative
  f:g:h ; is the same as (compose f g h)
  ~f:g  ; is the same as (compose (complement f) g)
To explore these more, you can use the ssexpand function in Arc:

  arc> (ssexpand 'a:b.c!d)
  (compose a b.c!d)
  arc> (ssexpand 'b.c!d)
  ((b c) (quote d))

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2 points by conanite 5450 days ago | link

call/cc, (or ccc in arc) isn't actually used very much in the official distribution, except to provide escape routines, à la

  (catch
    (each x xs 
      (if (foo x) 
          (throw x))))
so you don't need to go all the way to the end of a loop.

The last time I looked, ccc isn't used to capture and re-use first-class continuation objects except for this escape-clause purpose.

Here's a fine article on the subject - http://www.intertwingly.net/blog/2005/04/13/Continuations-fo...

And here's a long-winded, verbose, java-oriented article I wrote last year in an attempt to explain ccc in java terms - http://www.fnargs.com/2008/05/continuations-arc-vs-java.html

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1 point by conanite 5450 days ago | link

On top of that, while we're on the topic, I don't know what happens if you jump into another continuation in the following circumstances:

1. you're inside atomic-invoke, call-w/stdout, call-w/stdin, or anything that creates context that needs to be undone on the way out. The problem is that a continuation from in here can be exited multiple times although it was entered only once.

2. similarly, you're inside protect, (equivalent to "finally" in java) - is the "after"-fn called, and if so, only once or for each re-entered continuation?

3. the continuation you jump into was originally running on another thread.

I should write tests to figure out what happens in all these cases, but if anybody knows the answers I can live with being lazy :)

With all these questions, I can only echo fallintothis' remark that continuations are notoriously difficult to explain, and while they make for some great intellectual amusement (see Mondo Bizarro - http://arcfn.com/2008/03/arc-continuation-puzzle.html and http://arcfn.com/2008/03/continuations-made-difficult.html ), they're not practical for everyday coding.

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1 point by tumult 5447 days ago | link

This is what dynamic-wind is for. http://docs.plt-scheme.org/reference/cont.html#(def._((quote...)

(dynamic-wind is one of the coolest things ever)

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