From: Robert Klemme <bob.news@...>
Date: 2006-01-07T22:33:00+09:00
Subject: Re: Iterator Fu Failing Me

James Edward Gray II <james@grayproductions.net> 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.
> 
> Thanks for any tips you can offer.
> 
> James Edward Gray II

elements = tokens.map do |tok|
  parsers.inject(false) {|a,par| a=par.parse(tok) and break a}
end

alternative using a method definition

def parse(token)
  parsers.each {|par| a=par.parse(token) and return a}
  false
end

But the best solution is

elements = tokens.map do |tok|
  parsers.detect {|par| par.parse(tok)}
end

:-)

Kind regards

    robert