I have some modules in an array. All the modules define a method called "process", and I'd like to call each of these process methods in sequence. The code I have looks something like this(assume the modules are all defined inside the Mod class):
modules.each do |mod|
extend Mod.const_get(mod)
process(data)
end
This works fine for the first time, but the method doesn't get overwritten after the first pass of the loop. I've tried adding undef process as the last line inside the each block but that didn't work.
Is there any way I can do this?
modules can only be included into an inheritance chain once.
Also, what are you doing is really weird, you should think about redesigning your system.
is making the 'process' method into a module-method an option (by defining it def self.process)?
If so, sending the method 'process' directly to the returned constant would work:
modules.each do |mod|
Mod.const_get(mod).send(:process, data)
end
EDIT
Come to think of it, why not call the method directly?
Mod.const_get(mod).process(data)
Related
So, I'm currently learning about metaprogramming in Ruby and I want to fully understand what is happening behind the scenes.
I followed a tutorial where I included some of the methods in my own small project, an importer for CSV files and I have difficulties to wrap my hand around one of the methods used.
I know that the define_method method in Ruby exists to create methods "on the fly", which is great. Now, in the tutorial the method initialize to instantiate an object from a class is defined with this method, so basically it looks like this:
class Foo
def self.define_initialize(attributes)
define_method(:initialize) do |*args|
attributes.zip(args) do |attribute, value|
instance_variable_set("##{attribute}", value)
end
end
end
end
Next, in an initializer of the other class first this method is called with Foo.define_initialize(attributes), where attributes are the header row from the CSV file like ["attr_1", "attr_2", ...], so the *args are not provided yet.
Then in the next step a loop loops over the the data:
#foos = data[1..-1].map do |d|
Foo.new(*d)
end
So here the *d get passed as the *args to the initialize method respectively to the block.
So, is it right that when Foo.define_initialize gets called, the method is just "built" for later calls to the class?
So I theoretically get a class which now has this method like:
def initialize(*args)
... do stuff
end
Because otherwise, it had to throw an exception like "missing arguments" or something - so, in other words, it just defines the method like the name implies.
I hope that I made my question clear enough, cause as a Rails developer coming from the "Rails magic" I would really like to understand what is happening behind the scenes in some cases :).
Thanks for any helpful reply!
Short answer, yes, long answer:
First, let's start explaining in a really (REALLY) simple way, how metaprogramming works on Ruby. In Ruby, the definition of anything is never close, that means that you can add, update, or delete the behavior of anything (really, almost anything) at any moment. So, if you want to add a method to Object class, you are allowed, same for delete or update.
In your example, you are doing nothing more than update or create the initialize method of a given class. Note that initialize is not mandatory, because ruby builds a default "blank" one for you if you didn't create one. You may think, "what happens if the initialize method already exist?" and the answer is "nothing". I mean, ruby is going to rewrite the initialize method again, and new Foo.new calls are going to call the new initialize.
Some context
I'm playing with Ruby to deepen my knowledge and have fun while at the same time improving my knowledge of Esperanto with a just starting toy project called Ĝue. Basically, the aim is to use Ruby facilities to implement a DSL that matches Esperanto traits that I think interesting in the context of a programming language.
The actual problem
So a first trait I would like to implement is inflection of verbs, using infinitive in method declaration (ending with -i), and jussive (ending with -u) for call to the method.
A first working basic implementation is like that:
module Ĝue
def method_missing(igo, *args, &block)
case igo
when /u$/
celo = igo.to_s.sub(/u$/, 'i').to_s
send(celo)
else
super
end
end
end
And it works. Now the next step is to make it more resilient, because there is no guaranty that celo will exists when the module try to call it. That is, the module should implement the respond_to? method. Thus the question, how do the module know if the context where module was required include the corresponding infinitive method? Even after adding extend self at the beginning of the module, inside of the module methods.include? :testi still return false when tested with the following code, although the testu call works perfectly:
#!/usr/bin/env ruby
require './teke/ĝue.rb'
include Ĝue
def testi; puts 'testo!' ;end
testu
Note that the test is run directly into the main scope. I don't know if this makes any difference with using a dedicated class scope, I would guess that no, as to the best of my knowledge everything is an object in Ruby.
Found a working solution through In Ruby, how do I check if method "foo=()" is defined?
So in this case, this would be checkable through
eval("defined? #{celo}") == 'method'
I have been thinking about blocks in Ruby.
Please consider this code:
div {
h2 'Hello world!'
drag
}
This calls the method div(), and passes a block to it.
With yield I can evaluate the block.
h2() is also a method, and so is drag().
Now the thing is - h2() is defined in a module, which
is included. drag() on the other hand resides on an
object and also needs some additional information.
I can provide this at run-time, but not at call-time.
In other words, I need to be able to "intercept"
drag(), change it, and then call that method
on another object.
Is there a way to evaluate yield() line by line
or some other way? I don't have to call yield
yet, it would also be possible to get this
code as string, modify drag(), and then
eval() on it (although this sounds ugly, I
just need to have this available anyway
no mater how).
If I'm understanding you correctly, it seems that you're looking for the .tap method. Tap allows you to access intermediate results within a method chain. Of course, this would require you to restructure how this is set up.
You can kind of do this with instance_eval and a proxy object.
The general idea would be something like this:
class DSLProxyObject
def initialize(proxied_object)
#target = proxied_object
end
def drag
# Do some stuff
#target.drag
end
def method_missing(method, *args, &block)
#target.send(method, *args, &block)
end
end
DSLProxyObject.new(target_object).instance_eval(&block)
You could implement each of your DSL's methods, perform whatever modifications you need to when a method is called, and then call what you need to on the underlying object to make the DSL resolve.
It's difficult to answer your question completely without a less general example, but the general idea is that you would create an object context that has the information you need and which wraps the underlying DSL, then evaluate the DSL block in that context, which would let you intercept and modify individual calls on a per-usage basis.
If I have a class where the constructor calls another function, how do I check that it was called and the right number of times?
class MyClass
def initialize(count)
count.times{self.do_something}
end
def do_something
# whatever
end
end
I want to say something like
n = 4
MyClass.new(n).should_receive(:do_something).exactly(n).times
n = 2
MyClass.new(n).should_receive(:do_something).exactly(n).times
but that fails because the call to do_something happens before should_receive gets attached to it (at least I think that's why).
expected: 4 times with any arguments
received: 0 times with any arguments
Or is it just wrong to call stuff like this from the constructor and I should refactor?
Also, this question is very similar to this one:
rspec: How to stub an instance method called by constructor?
but I'm hoping in the last 5 years the answer has gotten better than setting up manual implementations of a stubbed new call.
The new syntax for doing this is as follows:
allow_any_instance_of(ExampleClass).to receive(:example_method).and_return("Example")
expect_any_instance_of(ExampleClass).to receive(:example_method).and_return("Example")
See the docs
Expectations are meant to be placed before the code that will be expected to meet them.
MyClass.any_instance.should_receive(:do_something).exactly(1).time
MyClass.new
That should work.
Another answer is to use rspec spies. The issue here is that you have to create a mock for the methods you want to make requirements on after the fact. Turns out it's not hard, but you still have to enumerate them. The and_call_original is the magic that makes the implementation not change.
MyClass.any_instance.stub(:method_name).and_call_original
Then you can just say:
MyClass.new(4).should have_received(:method_name).exactly(4).times
I'd still love to find a way that doesn't require you to enumerate the methods, though.
Here's the best I came up with, but I don't like it:
Create a new method in Class that runs a block on the allocated, but not yet initialize'd object. I put this in the before block in my spec file, but there's probably a better place for it:
class Class
def rspec_new(*params, &block)
o = allocate
yield(o) if block
o.__send__(:initialize, *params)
return o
end
end
Then call it like this:
MyClass.rspec_new(n){|obj| obj.should_receive(:do_something).exactly(n).times}
it seems to work, but you can't pass a block to your constructor. At least not the way you normally do. That's why I didn't override the regular new method.
I am new to Ruby, and have a gem that I am making to interact with a JSONRPC API and basically all calls and responses are similar enough, that every API call can be handled with one function, like:
Module::api_command('APINamespace.NamespaceMethod')
but I would like to also (for convenience sake) be able to do:
Module::APINamespace.NamespaceMethod
Is there any reason not to do this by using Module.const_missing to return a dummy class that has a method_missing which will allow passing the call from Module::APINamespace.NamespaceMethod to Module::api_command('APINamespace.NamespaceMethod')
Is there a more elegant or civilized way to do this?
Yes, I'm sorry, but to my mind that hack is ridiculous. :)
First of all, i'm assuming that your api_command method is actually invoking methods on the APINamespace module, as implied by this line: Module::api_command('APINamespace.NamespaceMethod')
Given the above, why not just set a constant equal to APINamespace in your module?
MyModule::APINamespace = ::APINamespace
MyModule::APINamespace.NamespaceMethod()
UPDATE:
I'm still not entirely understanding your situation, but perhaps this:
module MyModule
def self.const_missing(c)
Object.const_get(c)
end
end
Now you can invoke any top-level constant as if it was defined on your module; say there was a module called StrangeAPI at top-level, if you use the hack above, you can now invoke its methods as follows:
MyModule::StrangeAPI.Blah()
Is this what you want?