Explain Iterator Syntax on Ruby on Rails

Question

I started learning Ruby on Rails and found myself confounded by the syntax, so I had to read about somet of the Ruby syntax. I learned the syntax from http://www.cs.auckland

Answer 1

method_call do [`|' expr...`|'] expr...end

Is not limited to iteration functions.

In ruby, any method can take a block as an argument. The block can then be called by the method. In the case of an iterator, the method looks something like this:

def iter
  for i in [:x,:y,:z]
    yield i
  end
end

If you call iter with a block, it will loop over [:x, :y, :z] and yield each of them to the block, which can then do whatever you want. e.g. to print them out:

iter { |z| puts z }

You can also use this to hide init and cleanup steps, like opening and closing files. e.g. File.open. File.open, if it were implemented in pure ruby(it's in C for performance) would do something like this.

def File.open filename, opts
  f = File.new filename, opts
  yield f
  f.close
end

Which is why you can use

File.open 'foobar', 'w' do |f|
  f.write 'awesome'
end

respond_to is similar. It works something like this:( check out the real implementation here)

   def respond_to
     responder = Responder.new(self)
     block.call(responder)
     responder.respond
   end

It creates a responder object that has methods like html that take a block and passes it to you. This turns out to be really handy, because it lets you do things like:

def action
  @foo = Foo.new params[:foo]
  respond_to do |format|
    if @foo.save
      format.html { redirect_to foo_path @foo }
      format.xml { render :xml => @foo.to_xml }
    else
      flash[:error] = "Foo could not be saved!"
      format.html { render :new }
      format.xml { render :xml => {:errors => @foo.errors }.to_xml}
    end
  end
end

See how I change the behavior dependent on save inside the block? Doing this would be much more annoying without it.