From: ara.t.howard@...
Date: 2006-01-07T14:59:38+09:00
Subject: Re: Iterator Fu Failing Me

On Sat, 7 Jan 2006, James Edward Gray II wrote:

> I have a group of classes, all implementing a parse?() class method.  Calling 
> parse?(token) will return the constructed object, if it can be parsed by this 
> class, or false otherwise.
>
> I want to run a bunch of tokens through these classes, grabbing the first 
> fit.  For example:
>
>  elements = tokens.map do |token|
>    ClassA.parse?(token) or
>    ClassB.parse?(token) or
>    ClassC.parse?(token)
>  end
>
> That works.  Now can anyone give me a version of the middle section that 
> doesn't require I call parse?() 50 times?  I want something close to:
>
>  elements = tokens.map do |token|
>    [ClassA, ClassB, ClassC].find { |kind| kind.parse?(token) }
>  end
>
> Except that I want the return result of parse?(), instead of the class that 
> took it.


   harp:~ > cat a.rb

   module Parser
     module ClassMethods
       def parse tokens, klasses
         tokens.map{|token| catch('parse!'){ klasses.each{|klass| klass.parse! token}; nil}}
       end
       def parse! token
         ret = parse?(token) and throw 'parse!', ret
       end
       def parse? token
         return "<#{ token }> parsed by <#{ self }>" if token =~ self::RE
       end
     end
     module InstanceMethods
     end
     def self::included other
       other.module_eval{ extend ClassMethods and include InstanceMethods }
     end
     extend ClassMethods and include InstanceMethods
   end

   class ClassA
     include Parser
     RE = /a/
   end

   class ClassB
     include Parser
     RE = /b/
   end

   class ClassC
     include Parser
     RE = /c/
   end

   elements = Parser::parse %w( a b c d ), [ClassA, ClassB, ClassC]

   p elements


   harp:~ > ruby a.rb
   ["<a> parsed by <ClassA>", "<b> parsed by <ClassB>", "<c> parsed by <ClassC>", nil]


you can probably come up with something shorter - but using catch/throw with
parsers is very useful to break of conditions like these.

cheers.

-a
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