From: Andy Lowry <lists@...>
Date: 2013-07-23T02:37:28+09:00
Subject: tap { break } idiom deserves its own Kernel method?

I use this idiom from time to time:

  x = expr.tap{|value| break expr-involving-value)}

It works well, but reading it requires the reader to understand a
relatively obscure part of ruby, namely the behavior of break during a
yield.

Here's my most recent case where the idiom useful: I've got a list of
model object ids, and I want produce an array of the model objects in
the order their ids appear in that list.

Obviously, this works:

   id_list.map{|id| Model.find(id)}

Here's an approach that does a single query:

   unsorted = Model.where(id: id_list)
   objs_map = unsorted.reduce(Hash.new) {|h,o| h[o.id] = o; h}
   sorted = id_list.map{|id| objs_map[id]}

Here's the same thing using tap/break, to make it more obvious that the
only thing I'm really interested in is that final value "sorted":

   sorted = Model.where(id: id_list).tap do |unsorted|
     break unsorted.reduce(Hash.new) {|h,o| h[o.id] = o; h}
   end.tap do |objs_hash|
     break id_list.map{|id| objs_hash[id]}
   end

I find enough uses for this idiom that I'm thinking it'd be worth giving
it a name and its own Kernel method, rather than forcing the use of a
relatively narrowly understood language feature (behavior of break
during yield). "pipe" or "transform" may be a good name for it.
Definition would be almost identical to that of tap.

  Kernel.module_eval do
    def pipe
      yield self
    end
  end

Then the example above turns into:

  sorted = Model.where(id: id_list).pipe do |unsorted|
    unsorted.reduce(Hash.new) {|h,o| h[o.id] = o; h}
  end.pipe do |objs_map|
    id_list.map{|id| objs_hash[id]}
  end

Any thoughts?

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