From: Rob Marshall <lists@...>
Date: 2013-02-07T05:14:28+09:00
Subject: Re: Implementing DRY with a function call

To Robert...yes, I have NOT, yet, grasped the concept of blocks :-)

Let me explain: I'm using FFI to access a library. The way the library 
calls work is that none of them return errors or even raise exceptions. 
The expectation is, that after calling a function, the user will check 
(via a function call) for errors. So what I end up with is:

Module MyModule
  class SubClass
    def initialize(instance_var)
      @instance_var = instance_var
    end

    def my_function_call(arg1,arg2)
      unless @instance_var.nil?
        ret = MyModule.Real_FunctionCall(@instance_var,arg1,arg2)
        if MyModule.CheckLastError != 0
          raise MyException, MyModule.GetLastErrorText
        end
        return ret
      else
        raise ArgumentError, "Instance variable missing"
      end
    end

    def my_other_call(arg1,arg2, arg3)
      ret = MyModule.Real_OtherCall(arg1,arg2,arg3)
      if MyModule.CheckLastError != 0
        raise MyException, MyModule.GetLastErrorText
      end
      return ret
    end
  end
end


The difference is that I have many classes in the module and LOTS of 
functions within each class. As you can see, in some cases I want to 
check an instance variable before making the call (because the the 
function call will need it), and in other cases I don't need to. But in 
ALL cases I need to do the error check. With close to a hundred+ 
functions, that's 300+ repetitious lines of code.

What I think will work is (here's a simplified example):

def check_error(*args)
  if args.length > 0
    args.each_with_index { |v,i| printf "%2d argument value: %s\n", i, v 
}
  end
  ret = yield
  puts "Check error here"
  ret
end

check_error("this","is","a","test"){ return_value("something again") }
 0 argument value: this
 1 argument value: is
 2 argument value: a
 3 argument value: test
Check error here
=> "This is the returning value: something again"

check_error { return_value("something again") }
Check error here
=> "This is the returning value: something again"

The part I didn't understand is how Ruby will allow "you" to put a block 
after the args in parens...Once I figured that out (thanks to: 
http://blog.sidu.in/2007/11/ruby-blocks-gotchas.html#.URK0k-j6bYA) it 
all made sense :-)

Thanks for the help,

Rob

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