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Metaprogramming Ruby
scottl writes "Metaprogramming Ruby is the first book to give an in-depth and readable explanation of how dynamic programming works in Ruby. It is both readable and accurate and will be a valuable resource to intermediate and probably advanced Ruby programmers for some time to come." Keep reading for the rest of scottl's review.
Metaprogramming Ruby
author
Paolo Perrotta
pages
261
publisher
Pragmatic Bookshelf
rating
9
reviewer
scottl
ISBN
978-1-93435-647-0
summary
Metaprogramming Ruby is an excellent introduction to dynamic and metaprogramming using Ruby. It will be useful to intermediate and advanced users of Ruby and potentially even to beginners with some programming experience.
Metaprogramming Ruby is not a book for programmers new to Ruby, but would make an excellent follow on book to either Programming Ruby by Dave Thomas, Chad Fowler, and Andy Hunt or The Ruby Way by Hal Fulton. Both of the above books have chapters on or at least mention metaprogramming, but this is the first full length exposition of it. The book itself is broken into two sections and four appendices. In the first section, you (the reader) is paired with a more experienced programmer for a week, a chapter for each day, and as the week progresses you cover different topics in metaprogramming after being given tasks by your boss. The second section has two chapters on the design and implementation of ActiveRecord from Rails as well as a chapter on safe metaprogramming. Finally, there are three very useful appendices and a bibliography. One other note is that the book has the concept of "spells". Each concept as it is discussed in the book will have a spell associated with it. There will be a note in the sidebar with the name of the spell and the third appendix has the spells listed in alphabetical order, a short example of the spell, and a page number linking back to the extended discussion of the spell.
The first chapters are an informal introduction to metaprogramming where the reader, just starting a new job, is paired with Bill, an experienced developer, for a week. Each day is a chapter and each chapter covers a specific topic. The topics, as might be expected, increase in complexity as the week goes on. This more informal style actually works quite well. I had expected to be a bit irritated with the "schtick", but it turns out that Perrotta does not use it to excess. The topics covered here are Monday: The Object Model, Tuesday: Methods, Wednesday: Blocks, Thursday: Class Definitions, and Friday: Code That Writes Code.
Chapter 1 on Monday starts out with you meeting Bill, your experienced pair programmer, on a Monday morning and then goes straight into the Ruby object model. It discusses reopening classes to add additional methods including predefined Ruby classes such as String. It follows up with Monkey Patching (opening an existing class and redefining an existing method) and the pitfalls therein.
In Chapter 2, methods are examined. Perrotta goes over a problem with duplicated code and how to reduce this code by generating and calling code dynamically. He then moves to showing the same example using method_missing(), followed by adding a change to respond_to? to make sure the methods show up.
Wednesday's topic in Chapter 3 covers blocks, lambdas, and procs all of which are really just "callable objects". The chapter starts with a review of blocks. This is followed by a discussion of scopes and how to carry variables through scopes using blocks as closures. Perratto shows how to use instance_eval() to receive blocks and then use them to manipulate scopes. Next, converting blocks into "callable objects", lambdas and Procs, and then calling them later is covered. Finally, there's a short introduction to domain specific languages (DSL) using the the techniques from this chapter.
Chapter 4 or Thursday covers class definitions. A discussion of self and the current class open the chapter. There's also a section on singleton methods and eigenclasses. There are class instance variables, variables that belong to the class and not to a particular object. Examples of class macros, such as attr_reader, attr_writer, and attr_accessor, and how to write them are shown. Finally he covers around aliases where method names can be renamed and then redefined but the original is still available.
The final day of the week, Friday, Chapter 5, goes deep into Ruby metaprogramming with code that writes code. In this chapter, Perrotta shows how to implement an "attr_checked" attribute in a few different ways starting with a kernel method using eval and moving on to passing the validation attributes in a block. Then this gets moved to a class macro (from Chapter 4), and finally moving on to using a module with hooks. This last is a pattern that's seen in many Ruby projects including Rails and two I'm familiar with, Sequel and Ramaze.
The second section, Metaprogramming in Rails, consists of two chapters on ActiveRecord and a final chapter on metaprogramming safely. In the first two chapters, Perrotta takes a tour through
ActiveRecord, the Rails object relational mapper (ORM) and shows how ActiveRecord uses the tips and techniques from the previous chapters. The final chapter on safe metaprogramming discusses how to test metaprogramming and working around and with to make sure that monkey patching doesn't cause problems.
Finally, there are three appendices. The first shows common Ruby idioms that are seen pretty much in all Ruby code. They provide a good review, but I'm not sure how useful they really are for the audience that this book is aimed at. The second appendix is one DSLs. This is also a nice to have, but there's probably not enough to let you program a DSL if you don't have additional help from somewhere. The final appendix really is almost worth the price of the book. It contains a list of metaprogramming "spells". Each of the spells contains a short programming example as well as the page number with the longer explanation. This is incredibly useful when looking at code from any of the major frameworks (some mentioned above) and you don't understand a piece of it. Just by scanning through the spells you can often find a simple version of what you're looking at and then read a longer explanation.
All in all, this is one of the better, more readable programming books that I've read in a long while. Perrotta keeps it to around 250 pages including the appendices and it's packed full of useful information. As I noted above, this book is highly recommended as a second Ruby book. The presentation of metaprogramming is both enjoyable and useful and the book itself is well written.
You can purchase Metaprogramming Ruby from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
The first chapters are an informal introduction to metaprogramming where the reader, just starting a new job, is paired with Bill, an experienced developer, for a week. Each day is a chapter and each chapter covers a specific topic. The topics, as might be expected, increase in complexity as the week goes on. This more informal style actually works quite well. I had expected to be a bit irritated with the "schtick", but it turns out that Perrotta does not use it to excess. The topics covered here are Monday: The Object Model, Tuesday: Methods, Wednesday: Blocks, Thursday: Class Definitions, and Friday: Code That Writes Code.
Chapter 1 on Monday starts out with you meeting Bill, your experienced pair programmer, on a Monday morning and then goes straight into the Ruby object model. It discusses reopening classes to add additional methods including predefined Ruby classes such as String. It follows up with Monkey Patching (opening an existing class and redefining an existing method) and the pitfalls therein.
In Chapter 2, methods are examined. Perrotta goes over a problem with duplicated code and how to reduce this code by generating and calling code dynamically. He then moves to showing the same example using method_missing(), followed by adding a change to respond_to? to make sure the methods show up.
Wednesday's topic in Chapter 3 covers blocks, lambdas, and procs all of which are really just "callable objects". The chapter starts with a review of blocks. This is followed by a discussion of scopes and how to carry variables through scopes using blocks as closures. Perratto shows how to use instance_eval() to receive blocks and then use them to manipulate scopes. Next, converting blocks into "callable objects", lambdas and Procs, and then calling them later is covered. Finally, there's a short introduction to domain specific languages (DSL) using the the techniques from this chapter.
Chapter 4 or Thursday covers class definitions. A discussion of self and the current class open the chapter. There's also a section on singleton methods and eigenclasses. There are class instance variables, variables that belong to the class and not to a particular object. Examples of class macros, such as attr_reader, attr_writer, and attr_accessor, and how to write them are shown. Finally he covers around aliases where method names can be renamed and then redefined but the original is still available.
The final day of the week, Friday, Chapter 5, goes deep into Ruby metaprogramming with code that writes code. In this chapter, Perrotta shows how to implement an "attr_checked" attribute in a few different ways starting with a kernel method using eval and moving on to passing the validation attributes in a block. Then this gets moved to a class macro (from Chapter 4), and finally moving on to using a module with hooks. This last is a pattern that's seen in many Ruby projects including Rails and two I'm familiar with, Sequel and Ramaze.
The second section, Metaprogramming in Rails, consists of two chapters on ActiveRecord and a final chapter on metaprogramming safely. In the first two chapters, Perrotta takes a tour through
ActiveRecord, the Rails object relational mapper (ORM) and shows how ActiveRecord uses the tips and techniques from the previous chapters. The final chapter on safe metaprogramming discusses how to test metaprogramming and working around and with to make sure that monkey patching doesn't cause problems.
Finally, there are three appendices. The first shows common Ruby idioms that are seen pretty much in all Ruby code. They provide a good review, but I'm not sure how useful they really are for the audience that this book is aimed at. The second appendix is one DSLs. This is also a nice to have, but there's probably not enough to let you program a DSL if you don't have additional help from somewhere. The final appendix really is almost worth the price of the book. It contains a list of metaprogramming "spells". Each of the spells contains a short programming example as well as the page number with the longer explanation. This is incredibly useful when looking at code from any of the major frameworks (some mentioned above) and you don't understand a piece of it. Just by scanning through the spells you can often find a simple version of what you're looking at and then read a longer explanation.
All in all, this is one of the better, more readable programming books that I've read in a long while. Perrotta keeps it to around 250 pages including the appendices and it's packed full of useful information. As I noted above, this book is highly recommended as a second Ruby book. The presentation of metaprogramming is both enjoyable and useful and the book itself is well written.
You can purchase Metaprogramming Ruby from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
I don't think most people give a crap about Ruby in general. Why not just use a programming language that does it right?
the narrative style can drive you nuts.
The summary mentions dynamic programming; but this book contains nothing about dynamic programming. (A particular method suitable to problems with optimal substructure that can be used in a subset of cases where recursion can be used and typically generates results very quickly.)
The author, who I've seen speak, instead writes about "metaprogramming" which in my personal opinion is a silly catch phrase to sell talks and books when actually he's just talking about using some of the advanced functionality present in Ruby (and also JavaScript) to do things that would be done with macros or can't be done at all in traditional object oriented and procedural languages.
If you're buying this book thinking it contains some new breakthrough paradigm, and you're already familiar with the ins and outs of Ruby prepare to be disappointed. If your background is Java or C++ and you've just learned enough to get by until now it'll probably be an eye-opener.
These are some carefully-chosen words. Ruby is evolving faster than any other language, it seems. You blink your eyes and the XML parser you were using is out-of-favor; everyone has switched to the new one which really is much improved. This can be frustrating in that you must work to keep up with the Ruby world, but all these changes really to seem to be bringing the best of the best ideas to the surface.
Combine this rapid change with Ruby's metaprogramming ability and you see a programming language ecosystem in which evolution is sure to favor that little OO language from Japan...
A slashdotter who didn't build his own computer is like a Jedi who didn't build his own lightsaber.
These "metaprogramming" techniques, regardless of whether they're done in Ruby or JavaScript or Lua or some other language, cause nothing but problems when it comes to maintainability.
Sure, they might help the programmer write the code in slightly less time, but then they make it damn near impossible to debug later on.
Errors are missed because they only happen in obscure cases at runtime, but would have otherwise been caught when using less-dynamic techniques. Even working code becomes unnecessarily complex, and really can only be understood by stepping through it, line by line, using a debugger.
When I catch my developers using these techniques, we have a nice little chat about writing maintainable code, and then their code gets reworked. Far too often, what small amount of time might be saved in the first place ends up being lost 50 or 100 times over while debugging. We don't play that game.
Pragprog has an AWESOME book coming out that I can't wait to see reviewed called "SQL Antipatterns". Check out the sample chapter on selecting a random row from the database. Freaking brilliant stuff.
"Fighting the underpants gnomes since 1998!" "Bruce Schneier knows the state of schroedinger's cat"
and finish their sentence in the paragraph?
When I catch my developers using these techniques, we have a nice little chat about writing maintainable code, and then their code gets reworked.
God, nothing is more annoying than an opinionated boss that starts sticking his fingers in your code.
cause nothing but problems when it comes to maintainability
Yes, because it's much more maintainable to keep repeating the same code everywhere and having it scattered throughout the source than to use some metaprogramming to make a useful (and efficient) macro.
Yes, because it's much more maintainable to keep repeating the same code everywhere and having it scattered throughout the source than to use some metaprogramming to make a useful (and efficient) macro.
Then extract the repeated code into another method. Much more maintainable than a macro.
When did programming become something that is "manly"? How does sitting at a computer bashing out code (Or more likely in your case, copy/pasting code) constitute being manly in any sense of the word?
Go be a lumberjack/pirate and then we can talk manly, boy.
I agree with you for most cases, which is why you only use metaprogramming in extreme cases. But sometimes speed or some other factor (maybe the code is just cleaner by adding a new language construct) and metaprogramming is the correct method.
God, nothing is more annoying than an opinionated boss that starts sticking his fingers in your code.
Except programmers who leave you with a mess once they're gone or do things in a way that is resistant to future changes, or don't follow your coding guidelines and produce code very different from your other programmers, or, ...... I could go on all day with good, valid reasons why a boss (or someone) should be doing code reviews.
You clearly haven't worked on any major projects or in a real development shop.
I develop in Ruby more than I'd like now and here's what I've seen as a past and present programmer who's used Java, C#, PHP, Perl, Smalltalk, LISP, C++, C, Python, Lua, and Visual Basic amongst others on real projects. Generally, I prefer Ruby to most languages, but only because it reminds me of other languages I'd yet still rather work with instead of Ruby ironically.
It's good to see Ruby improve and gain popularity, but like so many other languages lately (C#, Java, etc.), it seems to want to imitate Smalltalk poorly. Rather I'd like to see Ruby library authors, language devs, and framework zealots pay a little more attention to what is already out there. It reminds me of high school when you hang out with that friend who thinks whenever they discover something for the first time, they must have invented it. Ruby on Rails unfortunately gives the language a lot of bad press and needs to turn down the idiot volume.
Ruby also seems to have some diarrhea of the programming language where it wants to mix paradigms from so many other languages. It is like the kid who can't make up its mind. Even when it tries to emulate, it does it poorly, for example with hashes as hacks on named parameters in Obj C and Smalltalk (and .NET 4 now). This leads to wildly inconsistent libraries, methodologies, etc. Are you Perl? Are you Python? Are you CL or Haskell? I love a lot of things from all the languages I work with, but at some point you need focus, direction, and a firm point of view. This is one reason I love Smalltalk and Common Lisp. The two are wildly different in many ways, but share a strong stance of what is acceptable and useful in the language. In other words, these languages are well designed and have a good crap filter.
Ruby comes off amateurish at best in comparison and makes me feel like I'm working with the new web 3.0 PHP crowd. It's really sad because I love a good language or two, but it's just dumb to try to accomplish such lofty tasks without proper knowledge, leadership, and vision. When you think about it, you have to be kind of an asshole to want to create a new language with all that we have out there. It'd be one thing if it was so radically different as that would represent revolution, but Ruby is so derivative that I often wonder what value added. I tell myself when people do something like this, it must be curiosity + the inner nerd, but that still does not make it ok in my mind. What I wouldn't give if those same people would learn to actually understand and grasp more established languages in Smalltalk and Lisp, or hell even C++. It's not that I fear change, it's just that there's something to be said for experience and well tested languages that have gone through the growing pains already for better or worse. If there was something truly different, that made even functional programming seem trite, then I'd definitely hop on it to learn if nothing else, but Ruby isn't like that.
Finally, I have a problem in the fact that Ruby is not defined in itself. There's a lot of discussion about this if you google so I won't go into it, but historically, strong languages are able to do this because the internal mechanisms are fast enough and adequately expressive to allow for this. It is definitely a strength of a language if it can be built from the ground up in itself due to elegant design. Smalltalk is probably the best example of this, but owes a lot to earlier effort. Why can't Ruby do the same (too late)?
Generally, I am rather happy people are discovering Ruby. It allows me to keep certain languages like Lisp and Smalltalk largely my secret. While people build glorified Microsoft Access apps and then later are let down by scalability, performance, inconsistencies, lack of proper internationalization support, deployment tools, stability, etc., I'm happily being 10x more productive in my languages of choice. Unfortunately the world is a cruel place and employers love jumping on the bandwagon. I had the pleasure of seeing this with Java, where everyone thought
Boy am I glad I don't work for you. While metaprogramming certainly isn't right for every case, it is literally perfect for some. Best tool for the job, and all.
How else would be compensate for his unusually small penis?
So can you give a example of an extreme case where metaprogramming (i.e. dynamically generating code) has an advantage over just using a good old-fashioned precompiled abstraction? (Does it really only work in extreme cases?)
There are multiple kinds of development shops - i assume that you are referring to the kind of shop where you are given ample time (or at least sufficient time) to complete projects and where quality and few bugs are valued over having it done in a short period of time. There are other kinds of shops (like the one I've worked in for quite a while), where the management is less organized and goals include - get it done yesterday! Code tends to become a little less maintainable anyway in these types of situations...to the developers working on these types of projects, sometimes, well-documented macros and other such meta-programming aids become the only means of keeping the code-base they maintain organized for themselves under their time constraints. Unfortunately for management, the skilled programmers who are capable of succeeding in these types of environments tend to want to leave as they feel abused, overworked, and underpaid. Sadly, I think these kinds of shops are actually becoming more and more common. so, these techniques may, one day, even become almost the norm...
That's easy. Put it next to your penis. Everyone looks like Mandingo in comparison to you.
I think an example (I just started learning Ruby a few weeks ago, coming from Perl and C but I'm already in love with it) is when setting 'attr_accessor' in a class definition to automatically create getters and setters if you don't intend on doing anything fancy with regards to an attribute but don't want it accessed directly. Also, I think Rails pretty much relies on this as well, as is my understanding, but as I said, I've only had a brief association with the language.
Then extract the repeated code into another method. Much more maintainable than a macro.
But this *is* a case of extracting repeated code into another method already.
Ezekiel 23:20
These "metaprogramming" techniques, regardless of whether they're done in Ruby or JavaScript or Lua or some other language, cause nothing but problems when it comes to maintainability.
If done poorly, sure.
Sure, they might help the programmer write the code in slightly less time, but then they make it damn near impossible to debug later on.
Then your debugging tools suck, or the metaprogramming was abused. Banning it isn't like banning goto, it's like banning exception handling. Just because you can abuse it and ignore all errors doesn't mean you should never be able to intercept an exception.
Errors are missed because they only happen in obscure cases at runtime,
Can you give an example of such an "obscure" case?
I can see errors being missed because they only happen at runtime, but guess what? Them's the breaks. If you don't like it, you need to stop using Ruby or JavaScript or Lua, and stick to languages like Java, and make sure you always used checked exceptions, and you still won't catch all of them.
Runtime errors happen. That's what unit tests are for.
Even working code becomes unnecessarily complex,
Metaprogramming can make code much shorter, more readable, and more maintainable. If there's some additional complexity at runtime, so be it.
When I catch my developers using these techniques, we have a nice little chat about writing maintainable code, and then their code gets reworked.
If I was working for you (and I'm glad I'm not), I'd look forward to educating you on maintainability.
Let's look at an example: attr_accessor. It wouldn't be Ruby without it. You probably allow it, and it probably makes sense to you to allow it. It's also an example of metaprogramming -- while it's built into the language, and probably hand-tweaked in C for speed, it really makes much more sense once you understand that logically, attr_accessor itself could have been written using define_method.
How about another example: Any decent ORM. Either it's going to use metaprogramming to define your schema (like DataMapper does with 'property'), or it's going to use metaprogramming to expose your existing schema (like Rails does by reading your database). Tell me that's not more "maintainable" than hundreds of lines of XML config files.
Or I'll use a real-world example from the appengine-jruby project:
And in actual code:
That defines a method 'parent_id' which returns the current id of the 'parent' of the current record (an Appengine-specific concept), and a method 'parent' which returns the actual entity of the parent. But notice I passed the symbol :parent explicitly -- I could just as easily have passed something else. (Why would I want to? Because maybe I need a parent method for some other reason, but I don't want to lose this functionality.)
Metaprogramming techniques can be abused. So can anything. Goto is harmful, but sometimes it's useful to be able to return a value from the middle of several nested loops without having to terminate each explicitly. Perl can look like line noise, and while it won't ever really be beautiful, you can write maintainable, readable Perl. You can write COBOL in any language.
So yes, it can be abused, but it's also an extremely powerful technique, and you are crippling your programmers and taking a hit on both productivity and maintainability by banning it.
Don't thank God, thank a doctor!
I'm glad you don't work for me, too.
We hired a Ruby guy once. He was the most immature "professional" I've ever had to deal with. He couldn't take any form of criticism, he was always "right", and couldn't work well with others at all.
Aside from his lack of interpersonal skills, his work was complete shit. Instead of just writing code to get it done, he'd insist on using every design pattern he could. 10-line functions became 50-class "frameworks" using every Ruby trick available.
We will NEVER hire a Ruby fellow like that again. We've had much better success hiring Perl and Python programmers, and having them learn Ruby on the job.
Sure, they might help the programmer write the code in slightly less time, but then they make it damn near impossible to debug later on. Errors are missed because they only happen in obscure cases at runtime, but would have otherwise been caught when using less-dynamic techniques. Even working code becomes unnecessarily complex, and really can only be understood by stepping through it, line by line, using a debugger.
I guess that when you're working with shitheads who don't know the limits of their competency, you're screwed anyway. No matter what language is involved, they always creep up. I once noted an apparently incompetent programmer - that is incompetent in general as a programmer - who, despite this fact, never forgot to remark that he "absolutely loves C++ templates and tries to do anything with them". You can imagine the results. Java incompetents will probably abuse threads. Good programmers know that there are both safe tools in their language meant to be used daily, and powerful, but attention requiring tools to solve difficult problems when they arise. Sane people don't just go about abusing Lisp macros, Forth parsing words and Java reflection "because they can".
Ezekiel 23:20
Ruby's ecosystem evolves very quickly, sure, but the underlying language doesn't. Ruby written 10 years ago isn't significantly different to that written now. It's the libraries that are being used that have changed, but Perl has become susceptible to that in the last few years too (consider Perl5i and all the Perl 6-isms that are entering Perl 5).
ActiveRecord. Let's say you have a database table called "products" with three fields, "name," "description," and "price." This code:
:)
class Product < ActiveRecord::Base
end
Instantiates a class called 'product' which is a subclass of ActiveRecord::Base (AR::Base). As soon as the <i>act</i> of subclassing AR::Base, occurs, AR::Base has code that infers the name of the database table (in this case 'products') as the singular, lower case version of the subclass' name (Product). Then, it reads the table 'products' from the database, and dynamically adds getters and setters for the name, description, and price attributes.
In addition, AR::Base knows how to respond to a lot of methods whose names aren't known until runtime. Product.find_by_name("Table") will internally invoke "SELECT * FROM PRODUCTS WHERE name = 'Table'" and fetch a Product whose name, description, and price are reflected in the output of that SQL query.
Hence, you can do this on the Ruby console:
>> product = Product.find_by_name("Table")
>> puts product.name
"Table"
>> puts product.description
"This is a table (my description was living inside the database)"
>> puts product.price
$4.29
It's greatly simplified, but metaprogramming allows you to construct classes and objects who <i>present</i> behavior based on run time state.
You really have to dig in and use a language like Ruby for a while to see how powerful this is. If you're worried about the lack of maintainability, then write good tests
Slashdot: Where people pretend to be twice as smart as they really are by behaving like children.
isn't long enough for what they're typing?
Hmm, I've always thought of metaprogramming as code that produces code, not dynamically interpolating objects in an ORM (as nifty as that might be).
C++ template classes are a Turing-complete meta-language that, to me, really sum up metaprogramming. See this very interesting and horrifying book: http://www.amazon.com/Modern-Design-Generic-Programming-Patterns/dp/0201704315
Why mention "Ruby" so explicitly? It appears that you just had bad luck hiring a overzealous developer not capable of writing good enough software precisely fit for the problem, with the best tools for the job.
You'll find these guys in every type of language or programming paradigm...
Right, but *this* book review is about a book about Ruby. So it's probably pretty suitable to talk about Ruby here.
If the book was about LISP then you'd probably see a lot more LISP discussion going on.
Q: Why is starting a post in the "Subject" line incredibly annoying?
Shutting down free speech with violence isn't fighting fascism. It IS fascism!
you are crippling your programmers and taking a hit on both productivity and maintainability by banning it.
I think it'd be a bit off-putting too. You're hired for a certain expertise, then you have that expertise overridden for arguably factually incorrect reasons.
I recently read an article about why video games are addictive (http://www.cracked.com/article_18461_5-creepy-ways-video-games-are-trying-to-get-you-addicted_p2.html); short story, they fill a void that would be much better filled by our jobs, if our jobs gave us (1) autonomy; (2) complexity and (3) feedback. It sounds like a good idea to not strip any of these away if it can be avoided.
What's particularly nice about Ruby, over the theoretically nicer LISP/Smalltalk/etc, is that Ruby is actually used fairly often...
Every move-to-column you make
Every gdb-point you break
Every file you save
Every font-lock-keyword you say
I'll be ru-u-unning you
From the Song of Emacs
Your example doesn't seem like metaprogramming in the typical sence, I don't see anything special about it. But that might be due to excessive exposure to Python.
I suppose you could see any ORM layer as a form of metaprogramming (as you're generating and executing SQL at runtime), but I'd argue that SQL isn't a programming language but rather a database language.
Thus this book isn't about metaprogramming, but "metadatabasing" (ugh).
This sig is intentionally left blank
The book only uses ActiveRecord as one example of many. Please don't let a single example define an entire work. :/
So can you give a example of an extreme case where metaprogramming (i.e. dynamically generating code) has an advantage over just using a good old-fashioned precompiled abstraction? (Does it really only work in extreme cases?)
Meta-programming can help when creating frameworks and platforms for a generalized audience (.ie. Rails WRT developers that want to develop apps following a specific architectural pattern). The variables of generality can be captured in *meta-patterns* (I just puked a little by typing that clique - useful, but still a clique).
Meta-programming should not be used for application-specific code where more traditional approaches should suffice.
In other words, if your team is developing a platform or a framework, then meta-programming is ok (perhaps necessary.)
For developing business-specific code, application-specific functionality, then yeah, I'd agree with the anonymous poster above in that I would not want them to be meta-hacking away instead of using less esoteric means, not unless it is something very specific and required (.ie. a meta-programming artifact expected by the underlying framework.)
Tell me that's not more "maintainable" than hundreds of lines of XML config files.
Now that I think about it, those hundreds of lines of XML config files are going to have to be interpreted by something to turn them into method calls -- so again, it's going to result in metaprogramming, unless you define your schema again by repeating all those methods and properties inside your model source as method and property definitions.
Don't thank God, thank a doctor!
The actual implementation of the functionality I described involves tons of calls to instance/module_eval, define_method, etc. that are available to every Ruby object which allow you to modify an object's methods at runtime. The amount of code that is generated on-the-fly is both substantial and essential. How can that not be metaprogramming?
Slashdot: Where people pretend to be twice as smart as they really are by behaving like children.