From: Trans <transfire@...>
Date: 2007-03-15T06:14:27+09:00
Subject: Re: non-destructive merging of hashes in array



On Mar 14, 4:49 pm, "Giles Bowkett" <gil...@gmail.com> wrote:
> The functor seems pretty cool. The sad thing is, I still have the most
> primitive implementation possible in my actual code. The reason is, I
> don't want to pop it in without fully understanding it.
>
> The memo solution looks cleanest, that's really just a gut feeling
> though. Let me just make sure I get it. Here it is:
>
> hashes.inject{|memo, hash| memo.merge(hash) {|k,e,v| e+ v}}
>
> Now partly it turns out my problem is slightly more complicated. It's
> not an array of hashes; it's an array of objects which can return
> hashes. So it basically looks like this:
>
> def enter_output
>   @items = get_the_items
>   @happy_output_hash = {}
>   # non-destructively merge all hashes within @items into one hash
>   @items.each do |item|
>     item.hash_within.each do |key, value|
>       if @happy_output_hash[key]
>         @happy_output_hash[key] += value
>       else
>         @happy_output_hash[key] = value
>       end
>     end
>   end
> end
>
> (Code altered to enhance obviousness.)
>
> Would the correct translation of the memo solution to accomodate this
> be something like this?
>
> items.inject{|happy_output_hash, item|
> happy_output_hash.merge(item.hash_within) {|k,e,v| e + v}}

Should work. Or you could split it into two lines if it's easier to
read:

  hashes = items.collect{ |item| item.hash_within }

> Also, what does the Functor solution actually do? That sounds Lispy,
> which appeals to me, but I want to be sure the code is maintainable by
> lesser mortals, such as, for example, me.

(Depending on what your doing exactly) the inject/merge is probably
just fine.

Functor is pretty straight forward. The basic definition is:

  class Functor
    def initialize(*objs, &block)
      @objs  = objs
      @block = block
    end
    def method_missing(sym,*args)
      @block.call(@sym,*(@objs + args))
    end
  end

T.