I Finally Understand Closures

http://www.iode.co.uk/blog/pdrummond/i_finally_understand_closures.html

by kirubakaran • 18 years ago

24 21 18 years ago

iode.co.uk

sophist• 18 years ago

Why are there so many of these "I finally understand [cloures | pointers | misc CS concept]" posts? I mean these are pretty fundamental concepts that any first year CS student should be able to grasp pretty quickly.

ragaskar• 18 years ago

I don't think it's so much "I finally understand X", but more of "I finally have found an application for X".

I think it's interesting to see how other people solve problems.

swombat2• 18 years ago

"I've finally found an application for closures" is almost as daft as "I've finally found an application for variables".

If you haven't found an application for such-and-such basic programming concept yet, chances are that's because your experience is so limited that you should probably spend more time coding instead of writing ejaculatory blog posts.

(not directed at the parent directly, more at the article, which wasted a minute of my precious life)

tlrobinson• 18 years ago

I think the problem arises when people hear about a concept and try to understand it without knowing the foundation required to really understand it.

Pointers are easy if you understand the basics of how a computer works and know a language like C.

Likewise, closures are easy when you know a little about functional programming and understand first class functions.

a-priori• 18 years ago

Monads are another high-theoretical concept that needs a real-world metaphor. So far, the best one I've heard is the "nuclear waste" one. I've been trying to grok monads for about two years now.

DaniFong• 18 years ago

Speaking of which, I don't know why someone hasn't hooked a closure system into html. Think of it, you can bind links to an instance of the website on archive.org -- if eventually it goes down, you can at least recover what the author meant to point to.

gaius3• 18 years ago

Because many of us never were CS undergrads. We formally learnt to program in the context of another field (Mech Eng in my case - FORTRAN, MATLAB and DCL), and prior to that were entirely self-taught. Many of us have many years of experience with procedural and object-oriented styles and are only recently coming to the functional style, either out of pure interest, or out of necessity. So bear with us :-)

aguha• 18 years ago

This example demands that you understand some domain-specific needs (of a text editor), so it's a little annoying.

But, the main idea is right on the mark. The point is that the natural structure of the code (that uses closures) is completely impossible to achieve without closures.

Great stuff! I think the author gets it. :-)

richcollins3• 18 years ago

Maybe now he also understands why objects are often preferable. Closures are liked handicapped objects. Names are resolved when the closures is created, not when it is called. This is not nearly as flexible as an object, where names are resolved when the function is called.

henning• 18 years ago

OK, have fun implementing an interface just to manipulate a collection when a one-liner would have sufficed.

richcollins3• 18 years ago

Why do you assume you can't use a one liner in an OO language?

Counter example:

list(1, 2, 3, 4) map(+(1))

http://iolanguage.com/

khafra• 18 years ago

The venerable master Qc Na was walking with his student, Anton. Hoping to prompt the master into a discussion, Anton said "Master, I have heard that objects are a very good thing - is this true?" Qc Na looked pityingly at his student and replied, "Foolish pupil - objects are merely a poor man's closures."

Chastised, Anton took his leave from his master and returned to his cell, intent on studying closures. He carefully read the entire "Lambda: The Ultimate..." series of papers and its cousins, and implemented a small Scheme interpreter with a closure-based object system. He learned much, and looked forward to informing his master of his progress.

On his next walk with Qc Na, Anton attempted to impress his master by saying "Master, I have diligently studied the matter, and now understand that objects are truly a poor man's closures." Qc Na responded by hitting Anton with his stick, saying "When will you learn? Closures are a poor man's object." At that moment, Anton became enlightened.

swombat2• 18 years ago

Brilliant. Just brilliant.

richcollins3• 18 years ago

I've never seen an implementation of closures with dynamic lookups. Perhaps you can't point me in the right direction.

ajkirwin• 18 years ago

To be honest, I still don't get it. He says, "Such and such will help you understand closures" and then a few lines of python, then suddenly, "because start_pos is a closure".

Which, really, still doesn't explain what he means. That and for the love of god, I hate things such as Clojure. Call me a stick in the mud, but I like languages in which I don't have to mentally expand 80% of words.

dominik• 18 years ago

A closure is an implementation technique for representing procedures with free variables. -- SICP @ http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-15.html...

I Finally Understand Closures

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