From: Ross Bamford <rossrt@...>
Date: 2006-02-02T11:15:46+09:00
Subject: Re: Grammars (mini-scripting languages)

On Thu, 2006-02-02 at 09:57 +0900, Michael Judge wrote:

> Does this seem like a good way to approach it?
>     1. Store each command's matching regular expression and ruby code 
> within the database. (sample fixture below)
>     2. For each line in script:
>          Test line against each command's corresponding regular 
> expression
>          If matched, execute the command's ruby code using an 
> instance_eval.
> 
> My thoughts are:
>     1. Storing executable code in the database is a security problem
>     2. instance_eval is slow
>     3. The alternative (a big if/elsif tree) would span many pages and 
> be unweildy.
> 
> Have a better suggestion?

Not necessarily better, but how about something like:

	class Commands
	  def Q(args)
	    puts args
	  end

	  def X(args)
	    if args =~ /^(\d+) (.*)$/
	      puts "  o #{$2}"
	    end
	  end

	  def M(args)
	    if args =~ /^(\d+) (.*)$/
	      puts "  [ ] #{$2}"
	    end
	  end

	  def dispatch(line)
	    if line =~ /([QXM])-?(.*)/
	      send($1.intern, $2)
	    else
	      raise "Invalid input: #{line}"
	    end
	  end
	end

	s = <<EOS
	Q Just a label
	X-23 Single punched
	M-11 Multi punched
	J-12 Bad input
	EOS

	cmds = Commands.new
	s.each { |c| cmds.dispatch(c) }

(Obviously I guessed a bit with the input format).
Output:

	 Just a label
	  o Single punched
	  [ ] Multi punched
	-:22:in `dispatch': Invalid input: J-12 Bad input (RuntimeError)
	        from -:35
	        from -:35

-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk