Metaprogramming Ruby 84
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 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.| 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. |
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.
Re:Not dynamic programming... (Score:3, Insightful)
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.
"Metaprogramming" [wikipedia.org] is an old term with a well-established definition, and if simplified to C/C++ audience, that definition would by and large look like "things that are done with macros". So I'm not sure what your problem is. So far as I can see, he is using the term correctly.
Re:These techniques are horrid for maintainability (Score:1, Insightful)
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.
Smalltalk and LISP for the History Major (Score:3, Insightful)
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 it was thor's magic unicorn wand made of skittles. We had to convert a 10-year old Smalltalk system that had experienced only downtime once in its entire existence to Java. There was no point in this exercise, no net gain. A few years later, we got everything working but the system was far more complex, slower, and really was 0 net gain. We ended up porting it back again to Smalltalk and it runs to this day beautifully. It has nothing to do with the language, but rather the process and buzz. I see the same ridiculousness around Ruby.
So to all you Ruby'ers out there....I hope you enjoy it. I ask you to not get stuck in your existing paradigms because it is just that much like php or java to make you feel comfortable. Learn something new and pick the right language for the task. Don't trust the buzz and instead, branch out and learn some new paradigms as it will help you the next time reality forces you to dive in to some client's old PHP mess. In the end, it's the programmer, not the language and sadly, most people suck at programming period.
And Good job improving Ruby, the 1980s (or even 50s) welcomes you!
Re:These techniques are horrid for maintainability (Score:5, Insightful)
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.
Re:Smalltalk and LISP for the History Major (Score:3, Insightful)
Ruby has its (serious) faults, but the multi-paradigm nature of it is not one of them. It's probably the main reason I like the language. Yes, it's not perfect in any given programming style, but the mix of styles is a huge convenience. It's nice to be able to use LISP-like features at times, but still be able to bank out quick, traditional imperative-style code when it's appropriate.
This isn't to say they shouldn't take more from Smalltalk and LISP. Those rough edges are just slightly bad enough to bet in the way. I have high hopes that these problems will be fixed in the future, though.
What's particularly nice about Ruby, over the theoretically nicer LISP/Smalltalk/etc, is that Ruby is actually used fairly often...
A: Because it breaks the flow of a message (Score:3, Insightful)