From: "David A. Black" <dblack@...>
Date: 2008-10-28T20:50:36+09:00
Subject: Re: Hash.merge_add! extension - how does this code look?

Hi --

On Tue, 28 Oct 2008, Brian Candler wrote:

> Greg Hauptmann wrote:
>> interesting - is it really Ruby approach to let things 'break' during
>> a method so to speak
>
> Generally speaking, yes it is.
>
> Depending on your implementation, you probably only make use of a one or
> two Hash methods - says 'keys' and '[]'
>
> By not testing the class, you allow other objects which also implement
> these methods to work, which typically makes your code more useful.
>
> If you wanted to be really anal, you could do:
>
>  raise "Unsuitable object (doesn't implement 'keys')" unless
>        hash.respond_to?(:keys)
>  raise "Unsuitable object (doesn't implement '[]')" unless
>        hash.respond_to?(:[])
>
> However the error message you would then generate would be almost
> identical to the default Ruby one:
>
>>      "<NoMethodError: undefined method `keys' for 123:Fixnum>"
>
> Now, in this particular case, I'd say an ideal API would only depend on
> 'each', and then it could be used for any enumerable object which yields
> two values. For example:
>
>  class Hash
>    def merge_add!(h)
>      h.each do |k,v|
>        if has_key?(k)
>          self[k] = Array[self[k]] + Array[v]
>        else
>          self[k] = v
>        end
>      end
>    end
>  end

And to fix the name problem (the rogue "!"), you could also do:

   class Hash
     def merge_add(h)
       dup.merge_add!(h)
     end
   end

which would give the ! a reason to be there and follows the semantics
of merge/merge!.


David

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