# Are my metaprogramming underpants showing? **URL:** \ **Category:** Ruby\ **Created:** [December 6, 2005, 12:54am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549 "2005-12-06T00:54:43Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 12:54am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/1 "2005-12-06T00:54:43Z")
Hello all, I’ve done a quick little project to see if I can wrap my head around Ruby metaprogramming, and I’d like to run it by some more-experienced minds. I picked the quick little task of coding up the web API for flickr ([Flickr Services](http://flickr.com/services/API)). Not necessarily for practical usage (though I may use it myself), but more because it struck me as a good example since there are a lot of methods with extremely similar behaviour on the client side (send parameters to this URL). In any case, here’s the code: require ‘pp’ class Flickr def method\_missing(method\_id, \*params) # Find the desired class name class\_name = method\_id.to\_s.capitalize # Find the corresponding instance variable ivar = :“@#{method\_id}” ``` # have we already made this particular class before? unless self.class.const_defined?(class_name) # new class which inherits from this one. new_class = self.class.const_set(class_name, Class.new ``` (self.class)) # new instance variable instance\_variable\_set(ivar, new\_class.new) end ``` # if we have parameters, execute the appropriate method (returning # what, though?) otherwise return the instance we just made so # that the next thing can be called correctly. return instance_variable_get(ivar) unless params.length > 0 the_method = instance_variable_get(ivar).class.name.downcase.gsub ``` (/::/, ‘.’) # abstract out the actual API call for the moment. puts “call: [http://flickr.com/services/rest/?method=](http://flickr.com/services/rest/?method=)‘# {the\_method}’” puts "with these other params: " pp \*params end end # envisaged usage flickr = Flickr.new result = flickr.test.echo({“api\_key” =\> “something long”, “foo” =\> “bar”}) The idea is that the usage should mirror how the methods are defined in the flickr docs. In general I’m asking (like the subject suggests) if my metaprogramming underpants are showing? Have I defied any particular conventions? Does this seem like a sensible approach, and if so, a sensible implementation? Have I set myself up for some rather spectacular failures? Specifically, though, there are three aspects of the code that I’m particularly curious whether anyone has any alternate approaches: 1. encoding the method name as a class heirarchy. e.g. ‘flickr.test.echo’ is implicit in the class definition that results from that call (Flickr::Test::Echo), then the method name gets reconstructed from that when the eventual call is made. Any other ways to do this? 2. relying on params.length to determine the ‘end’ of the call seems a little funny. On the other hand, every method call takes at least one parameter. One idea I had was to inherit from Proc, and define #call, which would let flickr methods get passed around as, well, methods, though this seems to have its own dangers. (If I were to implement this as a practical library, I think I’d use flickr’s reflection methods to sort this out, but is there a way to do it that doesn’t require that sort of external oracle?) 3. using method\_missing strikes me as a potential pitfall, but I can re-raise this if/when the flickr API returns its own ‘method not found’ error. Are there any other ‘gotchas’ I should watch out for? Thanks in advance for any feedback. matthew smillie. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 1:06am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/2 "2005-12-06T01:06:56Z")
Matthew S. wrote: > Hello all, > … > In general I’m asking (like the subject suggests) if my metaprogramming > underpants are showing? Have I defied any particular conventions? Does > this seem like a sensible approach, and if so, a sensible > implementation? Have I set myself up for some rather spectacular failures? > > Specifically, though, there are three aspects of the code that I’m > particularly curious whether anyone has any alternate approaches: A suggestion: move the code that munges method\_id and \*params into separate methods. Much easier to test and make sure it does what you want and that it doesn’t barf on weird input. James – [http://www.ruby-doc.org](http://www.ruby-doc.org) - Ruby Help & Documentation [Ruby Code & Style](http://www.artima.com/rubycs/) - Ruby Code & Style: Writers wanted [http://www.rubystuff.com](http://www.rubystuff.com) - The Ruby Store for Ruby Stuff [http://www.jamesbritt.com](http://www.jamesbritt.com) - Playing with Better Toys [http://www.30secondrule.com](http://www.30secondrule.com) - Building Better Tools ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 3:20am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/3 "2005-12-06T03:20:46Z")
On Dec 6, 2005, at 0:03, James B. wrote: > > I’m particularly curious whether anyone has any alternate > > approaches: > > A suggestion: move the code that munges method\_id and \*params into > separate methods. Much easier to test and make sure it does what > you want and that it doesn’t barf on weird input. Done, with a due sense of embarrassment since I as soon as I started thinking about that aspect of the code, I immediately found a bug involving using capitalize/downcase and flickr’s camelCase method names. Whoops & thanks for the reminder (I’ll flatter myself that I would have caught that during refactoring and testing anyway). While I appreciate the input, I’m not particularly worried about weird input and barfing at this stage, rather that my overall approach with the metaprogramming is relatively sane. I’ve barfed over lots of weird input in my time (mixing wine and spirits, for instance), but the metaprogramming is relatively new. thanks again, matthew smillie. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 4:13am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/4 "2005-12-06T04:13:37Z")
The whole class as method thing seems very odd.n I suspect there’s a better way. But I’m not sure what you’re trying to do exactly (the link to the Fliker API didn’t work btw) T. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 5:50am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/5 "2005-12-06T05:50:46Z")
> Here is, hopefully, a fuller explanation Ah, I see. Okay. I’m too tired to go into tonight, but I get back to you in the morning. In the mean time, iyou have some time, you might want to have a look at the Functor class --that may give you some ideas (see [http://rubyforge.org/frs/?group\_id=483](http://rubyforge.org/frs/?group_id=483)) T. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 6, 2005, 4:54am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/6 "2005-12-06T04:54:05Z")
* * * Matthew S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Institute for Communicating and Collaborative Systems University of Edinburgh On Dec 6, 2005, at 2:37, Trans wrote: > The whole class as method thing seems very odd.n I suspect there’s a > better way. But I’m not sure what you’re trying to do exactly (the > link > to the Fliker API didn’t work btw) > > T. Gah. Serves me right for doing this late at night. Here is the fixed link: > **[Flickr Services](https://www.flickr.com/services/api/)** > > The safest and most inclusive global community of photography enthusiasts. The best place for inspiration, connection, and sharing! Do you mind if I ask how you find it odd? Or what you might do otherwise? Here is, hopefully, a fuller explanation: The reasoning was like this. I wanted the Ruby method calls to look just like they’re defined in the api (modulus the parameters), so like this: flickr.test.echo({“api\_key” =\> “…”}) The catch is that in Ruby, that’s a calling ‘echo’ on ‘flickr.test’, and so the ‘echo’ there needs to somehow know what the name of the entire method (flickr.test.echo) is. The point of the exercise for me was to avoid defining each and every API method specifically, so I needed a way encode the entire API method name into that ‘echo’, and the class heirarchy seemed like a reasonable fit: - simpler than tracing callers So when a the above call is made, this happens: - flickr object create a Flickr::Test class, and an instance of it in @test - flickr.test creates a Flickr::Test::Echo class, and an instance of it in @echo, and then calls Flickr::Test::Echo#request - #request extracts the flickr API method name (flickr.test.echo) by downcasing the first letter of each element in the class name (the case-mangling was necessary since Ruby classnames are constants). The first and simplest thing I did was to use method\_missing just to dynamically construct the API method name by concatenating the method\_id’s in #method\_missing and returning self. This worked, but didn’t leave open much flexibility for adding, well, much of anything. Another alternative I considered was instead of using instances of the classes, was just to instead create a class method with analogous behaviour (e.g. Flickr.test would create Flickr::Test, etc), but that meant treating the Flickr class differently from the other classes, since it would need to be instantiated to match the “flickr = Flickr.new” intuition as well as the 'flickr.x.y" requirement. Seemed better to do it all at once. thanks once again, matthew smillie. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 7, 2005, 7:35am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/7 "2005-12-07T07:35:46Z")
Sorry I didn’t get to this until this evening. Hope it’s helpful. -T. require ‘calibre/functor’ module Flickr extend self ``` @@api = {} def method_missing( sym , *args ) @@api[sym] ||= Functor.new { |op, *args| api_call("#{sym}.#{op}", *args ) } end def api_call( method, *args ) puts "call: http://flickr.com/services/rest/?method='#{method}'" puts "with parameters:" p args end ``` end Flickr.test.echo(“api\_key” =\> “something long”, “foo” =\> “bar”) ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 8, 2005, 1:55am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/8 "2005-12-08T01:55:42Z")
> Sorry I didn’t get to this until this evening. Hope it’s helpful. -T. Noone’s in a hurry over here, so no worries. > end > Well, it helps (and that calibre library is quite cool), but: it > fails when there’s more than three terms in the method, e.g.: > Flickr.photos.licenses.getInfo({“something” =\> “blah”}) > undefined method `getInfo’ for nil:NilClass (NoMethodError) The general case (flickr.a.b…n), doesn’t seem possible to me without the object returned from #method\_missing having the same behaviour (i.e. implementation of #method\_missing) as the initial object, but with the previous calls as part of the object’s state. matt. ---
**Author:** ![M.B.Smillie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a88e4f/32.png) [@M.B.Smillie](https://www.ruby-forum.com/u/M.B.Smillie)\ **Post date:** [December 8, 2005, 4:09am UTC](https://www.ruby-forum.com/t/are-my-metaprogramming-underpants-showing/49549/9 "2005-12-08T04:09:20Z")
Hmm… that does make it trickier b/c when will the chain end? The only thing I can think of the top of my head is to use an ‘!’ method to indicate it. Flickr.test.echo! Then you can just return the same Functor-like object collecting the parts along the way until the ‘!’ is hit. T.