From: Michael Judge <mjudge@...>
Date: 2006-12-30T07:34:02+09:00
Subject: Methods validating their arguments: good or bad?


Code Complete* recommends that methods validate the arguments they  
receive.  The author went on to say that in a 1984 study, researchers  
discovered that miscommunication between routines caused 39% of the  
programming errors observed.  The idea is that, if we check our  
arguments before using them, bad data won't flow through undetected,  
causing even vaguer problems later on.  Lucky for us, Ruby checks the  
number of arguments received against the method definition -- finding  
many of these problems at the time they're written.  But checking  
that the arguments make sense are still our responsibility.  Should  
we do it?  Is it too much overhead for too little gain?

 From the Rails source:

def select_tag(name, option_tags = nil, options = {})
   content_tag :select, option_tags, { "name" => name, "id" =>  
name }.update(options.stringify_keys)
end

The arguments to select tag are really easy to screw up.  Adding code  
to validate them would add four or five lines to this terse method.   
Plus it would probably need a few helper methods just to remain  
readable.  But it would also prevent programmers from wasting huge  
amounts of time hunting for why their browser rendered a bad select  
box.  The argument for adding validation is really strong for any non- 
trivial project, but does it ruin Ruby's elegance?

What do you think?  Do you usually validate your arguments?

- Michael Judge

* Code Complete is a book on software construction by Steve McConnell.