From: Marc-Andre Lafortune <ruby-core-mailing-list@...>
Date: 2009-10-26T20:03:54+09:00
Subject: [ruby-core:26317] Re: New Enumerable#flat_map

Thanks to both of you for the answers :-)

I've written some basic Rubyspecs for it.

On Sun, Oct 25, 2009 at 6:48 PM, Yukihiro Matsumoto <matz@ruby-lang.org> wrote:
>
> In message "Re: [ruby-core:26282] New Enumerable#flat_map"
> � �on Sun, 25 Oct 2009 05:02:28 +0900, Marc-Andre Lafortune <ruby-core-mailing-list@marc-andre.ca> writes:
> |
> |Hi Matz,
> |
> |I noticed the new Enumerable#flat_map (aka collect_concat) and I was
> |wondering what prompted the creation of this method.
>
> It's taken from flatMap from Scala or concatMap from Haskell.
>
> |In what kind of circumstances is flat_map needed?
>
> The following is the direct translation from the example in the
> "Programming Scala" book:
>
> �class Person
> � �def initialize(name, is_male, *children)
> � � �@name = name
> � � �@is_male = is_male
> � � �@children = children
> � �end
> � �attr_reader :name, :children
> � �def male?
> � � �!! @is_male
> � �end
> �end
>
> �alice = Person.new("Alice", false)
> �bob = Person.new("Bob", true)
> �chris = Person.new("Chris", false, alice, bob)
>
> �p [alice, bob, chris].reject(&:male?)
> � �.flat_map{|p| p.children.map{|c| [p.name,c.name]}}
>
> which prints name pairs of a mother and a child.
>
> |I'm probably missing something, but what is the difference between:
> | � �enum.collect_concat(&block)
> |and
> | � �enum.map(&block).flatten(1)
> |?
>
> They do same thing, besides flat_map is shorter, clearer, and bit more
> efficient, without any magic number. �I should have made the default
> value for #flatten to 1, but it's different story.
>
> � � � � � � � � � � � � � � � � � � � � � � � � � � � �matz.
>
>