Let's Do Some Engineering Pt. 1: Design Patterns
gnuu.org
gnuu.org
http://norvig.com/design-patterns/
Sort of funny that he mentioned dependency injection as a benefit of one of these approaches; building things around DI was one of the reasons net-ssh got rewritten. When you find yourself writing Java in Ruby or Python, stop, take a step back, and reconsider.
But hey, I guess the Ruby stdlib includes the "Singleton" pattern for fun right? Not because you'd use it, or anything. Same with delegates. And I guess Rails didn't actually implement Martin Fowler's Active Record, Action Controller and MVC patterns? You just dreamed those.
Wake up. Your comment is painful and a little embarrassing. Design patterns are language independent. They are not implementation, they are descriptions. They can be used in any language, and they are used in Ruby.
The point is, most GoF patterns(like factory, builder, strategy, facade etc.) are trivial in "dynamic" languages like ruby and lisp and strictly applying those static-language-origin patterns cause nothing but unneeded complexity in ruby.
The problem with your argument is that the "triviality" of their implementation has nothing to do with whether or not you use them.
class X < DelegateClass(String); end
Yes, you can implement the Adapter pattern in Ruby without doing anything explicit-- that doesn't mean you didn't use the Adapter pattern. This is where denial kicks in. If you say "I don't use strategy patterns, I use DYNAMIC PROGRAMMING!" you're seriously deluding yourself.
The original idea behind the "Design Patterns movement" was to provide a common language for describing recurring concepts in software design. It's the same idea behind Alexander's "A Pattern Language", which is something else you should read.
Nobody is arguing that describing things a little more rigorously is bad. But a lot of people have problems with the way "engineers" and "architects" abuse the pattern concept.
Alexander did not write a book that claimed you could build an entire city by picking out a "44. Local Town Hall" and mixing it with a "69. Public Outdoor Room". He had an idea, that the cart was dragging the horse in architecture, and attempted to use vocabulary to reframe the discussion about why specific choices were being made.
The Gang of Four introduced a minor corruption in that approach --- they had no fundamental thesis about how software should be designed, only a notion that there was folkloric knowledge that should be captured and named --- but their followers completed the corruption wholeheartedly. Instead of a language describing software, the GoF patterns became a perscription for building it.
And so we have Ruby code that includes a "singleton" module into classes, and SOAP4R code that includes "factory" classes (note again how ruby already transparently supports the abstract factory concept), and people argung that we should start "doing a little engineering" by pulling these clumsy ideas back off the shelf.
I can't argue this any better than Norvig can. In fact, I'll invoke him again by suggesting that you don't want to end up like Norvig's "opponent" in the sudoku-solving contet.
In Java, I guess it makes sense to have a separate concept for such a thing, since you can't pass around a function or a function pointer, so you need some wrapper code and some extra architecture in order to do it. But in a normal language, who cares about the pattern?
You really, really ought to follow that Norvig link I posted. There is more insight and utility in that 10 minute read than in everything Fowler wrote.
There is a "Singleton" in the Ruby standard library, but since every Ruby class inherently includes a "singleton", you'd be embarassing yourself to use it. Note that it occurs nowhere in current Rails. Note also that "Singleton" is notoriously derided as an example of how superficial some of the GoF patterns are; it is, at its core, a puffed-up way of saying "global variable". Maybe a better name for it would be "very global variable".
Regardless, the twin problems with this blog post remain:
* The idea behind design patterns isn't that they give you a library of pre-built components that you can pick up off the shelf and use to assemble complete programs. But that's exactly how this blog post uses them.
* The GoF design patterns in particular describe ways to patch around the weaknesses of C++ and Java; they are, as Norvig writes, transparently implemented in modern languages, and reimplementing them is something akin to writing your own string matching or filename globbing code.
The better name for it already exists: it is the Singleton pattern, because OOP software developers already understand this concept. You don't need to invent new names, this is why patterns exist. Also note that whether or not you use the "Singleton" mixin in Ruby, you are still using the Singleton pattern whenever you define a class with no constructor. Yes, you can do this without "include Singleton"; no, this does not mean you're magically using some new concept nobody's thought of before. You're still using Singletons.
* The blog post doesn't use any patterns at all. Given that there are no code examples or suggestions, I don't see how you came to this conclusion. The blog post never tells you how to use patterns, it tells you why you use patterns. Where does it advise you to "pick up components and use them to assemble complete programs"?
* Java did not exist when GoF wrote Design Patterns, so it clearly was not addressing any weaknesses of that language. Your point is DOA and you seem to be pulling this out of your ass. But, to elaborate: GoF isn't describing ways to patch any languages, it's describing ways to attack software problems. The idea of using a Strategy pattern (for instance) is universal among any programming language, it has absolutely nothing to do with code. So is using a factory method. Ever used `ActiveRecord.build` or 'create_with_...'? That's a factory method. The same concept applies with adapters, facades, et al. How you implement the pattern is completely irrelevant to the entire book. You could do it with dynamic dispatch or classes, the book doesn't define the patterns by their implementations. The point is you're using them. Pretending you're not is some kind of childish posturing so you can tell yourself you're "not doing anymore Java". I'm sorry you experienced your Java nightmares, but design patterns have nothing to do with that language.
Similarly, you don't write "Strategy" code in a language that has first-class functions, and you don't write "Factory" code in a language that in which classes are first-class objects like everything else. You don't need a "Command" pattern in a language with closures and lambdas. And you don't need an "Adapter", "Facade", "Bridge" or "Proxy" in a language where function calls are already messages.
You would have to deliberately write a book with no truth in it whatsoever to come up with a list of "patterns" that have no applicability at all. The Gang of Four didn't do that. It's an interesting book with valid ideas. But the approach of constructing programs out of "patterns" has been discredited, particularly and on account of the case where people use the Gang of Four's patterns as their template.
Incidentally, you think I'm blazing new trails of ignorance here, but I am very late to the party with these comments, which I've mostly shoplifted from much smarter developers.
Here's one such comment, from a (probably) better developer and (definitely) better writer than me. Tell me what you think of it:
This practice is not only common, but institutionalized. For example, in the OO world you hear a good deal about "patterns". I wonder if these patterns are not sometimes evidence of case (c), the human compiler, at work. When I see patterns in my programs, I consider it a sign of trouble. The shape of a program should reflect only the problem it needs to solve. Any other regularity in the code is a sign, to me at least, that I'm using abstractions that aren't powerful enough-- often that I'm generating by hand the expansions of some macro that I need to write.
# This is a singleton class
class MyClass
class << self
private :new
def instance; @instance ||= new end
end
end
And guess what? I just used the Singleton pattern.
Same straw man argument: that anybody who points out that the GoF patterns are 'considered harmful' is arguing against concise, regular descriptions of design constructs.
What's your point? The GoF patterns already lead you astray; just a few moments ago, upthread, you had to waste cycles considering whether to use a library to implement a "singleton pattern". The endpoint of this thinking is the SOAP4R code.
Design patterns are code smells.
I have good news for you. You're working in a modern language. You don't need to think about patterns. You don't need to spend even 15 seconds up front thinking about how you might decompose your problem into "Proxies" and "Factories". You can get to work right now on your problem domain.
Because you're in a modern language, when, down the road, you discover that you need, say, to move a component into its own address space to collect calls and cache them, Ruby already gives you a "Proxy" construct. It's "method_missing" and it's idiomatic to the language. You will not need to dramatically alter your architecture to remote the method calls to a cache process; you'll just change the name of the class you call, from ExpensiveCollection to CachedExpensiveCollection, which will be about 15 lines of code long.
And so there's another problem with using the names that OOPSLA dorks came up with 15 years ago. You're fighting the language's own idioms. Ruby doesn't want you to think in terms of "singletons" and "factories" and "commands". It wants you to know how closures work, and how messages are different from simple function calls.
Another good article to read is that post by Wil Shipley about not writing too much code up front. Someone else can find the URL for you.
> Patterns are still patterns whether you acknowledge them or not, and they are not defined by the LOC or number of classes they require in language X.
> Refusing to admit that patterns are used hurts your development team and the community, because patterns (simply identifying them) serve a very important purpose.
> Well, design patterns are just like [agile] metaphors, but they are meant for code, not UI.
> For instance, consider the factory pattern versus the builder pattern. Both are concerned with creating objects, but a builder is also concerned with the assembling of the object. This is a relatively subtle but pretty noticeable difference. Telling someone you are using a "builder pattern" should immediately ring a bell in their heads saying, "oh, it’s not just a factory".
Patterns are specific nomenclature not implementation.