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Re: Wormholes

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On 04/09/12 02:12, Roedy Green wrote:
> Somehow I must pass information down the long chain from x to y.  This
> means changing the signatures of all the intermediate methods, and
> adjusting code to the new way.  This can cause ripples incommensurate
> with the triviality of the change.

Assuming that you can't improve your structure or refactor, a 
ThreadLocal might be appropriate as your wormhole.

I've had to do this lately.  The intermediate steps were recursive calls 
through a hierarchy.  y was a custom implementation of a node nested in 
that hierarchy.  x started off the recursive call.

If I was only using the hierarchy once, for one call by x, I would have 
simply embedded the context in y.  But the hierarchy was to be re-used, 
and y needed a different context for each call by x.  Also, calls by x 
could be concurrent.

However, I could guarantee that any single call by x would invoke y by 
the same thread.  That allowed the context to be passed by a 
ThreadLocal, set by x just before the call.

I suppose you could do something similar without threads, e.g.:

   class Context {
     Map<ContextLocal<?>, Object> items = new IdentityHashMap<>();
   }

   class ContextLocal<T> {
     void set(Context ctxt, T val) {
       ctxt.put(this, val);
     }

     T get(Content ctxt) {
       return (T) ctxt.get(this);
     }
   }

Then you'd refactor once to pass Context through the chain.  x creates a 
static ContextLocal<MyContext> myContext, and one Context ctxt for each 
call, and sets up myContext.set(ctxt, value) before the call.  y calls 
myContext.get(ctxt) to get the value.

-- 
ss at comp dot lancs dot ac dot uk

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Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-03 18:12 -0700
  Re: Wormholes eric@invalid.com (EricF) - 2012-09-04 02:06 +0000
    Re: Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-04 10:08 -0700
    Re: Wormholes Wanja Gayk <brixomatic@yahoo.com> - 2012-09-09 13:55 +0200
  Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-03 22:34 -0400
    Re: Wormholes Stuart <DerTopper@web.de> - 2012-09-04 12:15 +0200
  Re: Wormholes markspace <-@.> - 2012-09-03 21:00 -0700
    Re: Wormholes Robert Klemme <shortcutter@googlemail.com> - 2012-09-04 07:08 +0200
  Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 08:47 +0200
    Re: Wormholes Lew <noone@lewscanon.com> - 2012-09-04 03:10 -0700
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 16:09 +0200
        Re: Wormholes Gene Wirchenko <genew@ocis.net> - 2012-09-04 08:34 -0700
          Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 17:46 +0200
            Re: Wormholes Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-09-04 09:31 -0700
        Re: Wormholes Lew <lewbloch@gmail.com> - 2012-09-04 11:14 -0700
        Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-04 19:59 -0400
    Re: Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-04 13:20 -0700
      Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-04 20:00 -0400
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:01 +0200
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:02 +0200
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:02 +0200
        Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:03 +0200
      Re: Wormholes markspace <-@.> - 2012-09-05 17:34 -0700
  Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-04 14:18 +0100
    Re: Wormholes Robert Klemme <shortcutter@googlemail.com> - 2012-09-05 21:51 +0200
      Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-05 23:26 +0100
        Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 09:32 +0000
          Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-06 11:47 +0100
            Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 15:10 +0000
              Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-06 20:12 +0100
                Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 21:08 +0000
  Re: Wormholes Nigel Wade <nmw@ion.le.ac.uk> - 2012-09-04 17:26 +0100
  Re: Wormholes Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-09-04 09:28 -0700
  Re: Wormholes David Lamb <dalamb@cs.queensu.ca> - 2012-09-04 16:54 -0400

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