CODEWITHSUNDEEP
ruby-on-railsrubymetaprogramming
lrleon
lrleon

Reputation: 2648

How to forward a block to a method that generates methods

In my current project, I am having the following repetitive pattern in some classes:

class MyClass

  def method1(pars1, ...)
    preamble

    # implementation method1

    rescue StandardError => e
      recovery
  end

  def method2(pars2, ...)
    preamble

    # implementation method2

    rescue StandardError => e
      recovery
  end

  # more methods with the same pattern
end

So, I have been thinking about how to dry that repetive pattern. My goal is to have something like this:

class MyClass

  define_operation :method1, pars1, ... do
    # implementation method1
  end

  define_operation :method2, pars2, ... do
    # implementation method2
  end

  # more methods with the same pattern but generated with define_wrapper_method
  end

I have tried to implement a kind of metagenerator but I have had problems for forwarding the block that would receive the generator. This is more or less what I have tried:

def define_operation(op_name, *pars, &block)
  define_method(op_name.to_s) do |*pars|
    preamble
    yield # how can I do here for getting the block? <-----
    rescue StandardError => e
      recovery
  end
end

Unfortunately, I cannot find a way for forwarding block to the define_method method. Also, it is very possible that the parameters, whose number is variable, are being passed to define_method in a wrong way.

I would appreciate any clue, help, suggestion.

Upvotes: 2

Views: 296

Answers (2)

engineersmnky
engineersmnky

Reputation: 29318

If I understand correctly you are looking for something like:

class Operation
  def self.op(name,&block)
    define_method(name) do |*args|
      op_wrap(block.arity,*args,&block)
    end
  end

  def op_wrap(arity=0,*args)
    if arity == args.size || (arrity < 0  && args.size >= arity.abs - 1) 
      begin
        preamble 
        yield *args
      rescue StandardError => e
        recovery 
      end  
    else 
      raise ArgumentError, "wrong number of arguments (given #{args.size}, expected #{arity < 0 ? (arity.abs - 1).to_s.+('+') : arity })"
    end
  end

  def preamble
    puts __method__
  end

  def recovery
    puts __method__
  end
end

So your usage would be 

class MyClass < Operation

  op :thing1 do |a,b,c| 
    puts "I got #{a},#{b},#{c}"
  end

  op :thing2 do |a,b|
    raise StandardError
  end

  def thing3
    thing1(1,2,3) 
  end
end

Additionally this offers you both options presented as you could still do 

def thing4(m1,m2,m3) 
   @m1 = m1
   op_wrap(1,'inside_wrapper') do |str| 
     # no need to yield because the m1,m2,m3 are in scope 
     # but you could yield other arguments
     puts "#{str} in #{__method__}" 
   end
end

Allowing you to pre-process arguments and decide what to yield to the block

Examples

m = MyClass.new

m.thing1(4,5,6)
# preamble
# I got 4,5,6
#=> nil
m.thing2('A','B')
# preamble
# recovery
#=> nil
m.thing3  
# preamble
# I got 1,2,3
#=> nil
m.thing1(12)
#=> #<ArgumentError: wrong number of arguments (given 1, expected 3)>

Upvotes: 2

Raj
Raj

Reputation: 22926

You do not need metaprogramming to achieve this. Just add a new method that wraps the common logic like below:

class MyClass

  def method1(param1)
    run_with_recovery(param1) do |param1|
       # implementation method1
    end
  end

  def method2(param1, param2)
    run_with_recovery(param1, param2) do |param1, param2|
       # implementation method2
    end
  end

  private

  def run_with_recovery(*params)
    preamble
    yield(*params)
    rescue StandardError => e
      recovery
  end
end

Test it here: http://rubyfiddle.com/riddles/4b6e2

If you really wanted to do metaprogramming, this will work:

class MyClass

  def self.define_operation(op_name)
    define_method(op_name.to_s) do |*args|
      begin
        puts "preamble"
        yield(args)
      rescue StandardError => e
        puts "recovery"
      end
    end
  end

  define_operation :method1 do |param1|
    puts param1
  end

  define_operation :method2 do |param1, param2|
    puts param1
    puts param2
  end

end

MyClass.new.method1("hi")
MyClass.new.method2("hi", "there")

Test this here: http://rubyfiddle.com/riddles/81b9d/2

Upvotes: 7

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