From: Logan Capaldo <logancapaldo@...>
Date: 2006-05-17T05:44:18+09:00
Subject: Re: Enumerable#collect and #select best practices


On May 16, 2006, at 2:48 PM, Daniel Schierbeck wrote:

> Joe Bowers wrote:
>> This looks like prime .inject territory
>>   enum.inject([]) do |l,obj|     (l << obj.a_method) if  
>> obj.respond_to? :a_method
>>   end
>
> Wonderful!
>
> Now, what if I wanted to generalize it? That is, a method on  
> Enumerable  that returns an array of the results of calling a a  
> given block with each item in the array, excluding the items that  
> do not respond to that method.
>
> This is what I've got:
>
>   module Enumerable
>     # need a better name...
>     def collect_x
>       collect do |obj|
>         begin
>           yield obj
>         rescue NoMethodError
>           nil
>         end
>       end.compact
>     end
>   end
>
> Though it's still verbose...
>

I don't think you should use #compact. What if #a_method happens to  
return nil as a result? Also rescuing NoMethodError, is risky if you  
have a bug inside #a_method.

I would write it:

module Enumerable
   def selective_collect(msg_name, *args)
       inject([]) do |result_list, value|
         if value.respond_to? msg_name
            result_list << value.send(msg_name, *args)
         end
         result_list
       end
   end
end

x = [ ... ]
x.selective_collect(:a_method)

of x.selective_collect(:another_method, "hello")