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Re^6: How A Technique Becomes Over-rated

by QM (Parson)
on Sep 23, 2005 at 14:48 UTC ( #494558=note: print w/replies, xml ) Need Help??

in reply to Re^5: How A Technique Becomes Over-rated
in thread How A Function Becomes Higher Order

...they are mostly structured and organized - not 50 line anonymous subroutines defined in the middle of another subroutine, with access to the parent's local variables even after it has returned.
I don't think that's a fair characterization (the sub shown is only 5 lines, and 2 of those are comments), and certainly isn't the point.

The parents "local" variables are lexical, and they are created explicitly for the closure. If I bake you a turnover, serve it on a paper plate, and give you a plastic fork to eat it with, should I be annoyed that you still have the plate and fork, or worse, have recycled them when you finished?

What if each object needs a unique compare function?
Then what are you comparing it to? I would have thought similarity was a prerequisite for comparison.
Sorry, that should say each pair of objects. More descriptively, what if each object is a collection, and needs a unique next method? But there are so many of unique collections that you can't precode classes for all of them? Which suggests similarities to Streams, which comes back to HOP.

What if these functions aren't known at compile time?
Closures are compiled too. That is, whether you use gt or > (or whatever) is decided at compile time. Its just the variables that change.
Ah, you see the gap approaching? Whether to use gt or > can be decided at compile time, but doesn't need to be. The leap of insight is that some higher caller can pass in a coderef to use in place of any predefined notions of how a comparison function should work. If the closure is coded well -- no unnecessary assumptions, just the fundamental "give me a coderef that I can pretend is a comparison function" -- and then the closure is more general and more useful than was ever imagined.

For an example, see the opening chapters of HOP, where the <code>dir_walk<code> directory walker function is gradually transmogrified into a generic traversal function, then used as a web spider.

In the end, it's the tool you know that gets used. It took me 3 days to put together one of those Walmart entertainment centers. Then I discovered electric screwdrivers, and never looked back.

Quantum Mechanics: The dreams stuff is made of

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Re^7: How A Technique Becomes Over-rated
by Ctrl-z (Friar) on Sep 23, 2005 at 17:56 UTC
    I don't think that's a fair characterization

    You are right (though it might be the point), I had changed it while you were responding. Regarding your analogy, you might be annoyed that I leave with your plate and fork and carry it wherever I go, even after you move out - and noone believes you as the plate and fork are now invisible to everyone ;-)

    I appreciate what you are saying, and to quote from HOP

    ...I think the functional style leads to a lighter-weight solution...that keeps the parameter functions close to the places they are used instead of stuck off in a class file. But the important point is that although the styles are different, the decomposition of the original function into useful components has exactly the same structure.

    The point about closesness is a good one. Its not possible in OO Perl and only kind of possible in Java, as I tried to show in an earlier post. But, by limiting your customization technique to callbacks, all shared state must be stored in the lexical scope of the caller. By limiting your object to a coderef, theres only one thing you can do with it - call it. Thats fine for simpler stuff - number generators, map, grep etc - and I like those kind of ideas. But its definitely not the tool I know for building relatively complicated software.

    time was, I could move my arms like a bird and...
      But, by limiting your customization technique to callbacks, all shared state must be stored in the lexical scope of the caller.
      ($foo, $bar) = incrementByOneAndTwo(42); ($baz, $quux) = incrementByOneAndTwo(0); $\="\n"; print $foo->(); print $bar->(); print $quux->(); print $foo->(); print $baz->(); print $bar->(); print $quux->(); sub incrementByOneAndTwo { my $shared_state = shift; return (sub{$shared_state+=1}, sub{$shared_state+=2}); }

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