|Just another Perl shrine|
Re^6: How A Technique Becomes Over-ratedby QM (Parson)
|on Sep 23, 2005 at 14:48 UTC||Need Help??|
...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?
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 each object needs a unique compare function?Then what are you comparing it to? I would have thought similarity was a prerequisite for comparison.
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.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.
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.