From: "Jörg W Mittag" <JoergWMittag+Ruby@...>
Date: 2010-04-18T07:55:05+09:00
Subject: Re: 1.9's New Methods

Intransition wrote:
> First a big shout-out to Marc-Andre Lafortune and his <a href="http://
> github.com/marcandre/backports">backports</a> project. Nice work.
> 
> Now I want to ask if others have noticed all the new methods being add
> to 1.9+? I'm quite happy about vast majority of it, but there was at
> least one method I thought pretty peculiar. This Enumerable method:
> 
>   def flat_map(&block)
>     return to_enum(:flat_map) unless block_given?
>     map(&block).flatten(1)
>   end unless method_defined? :flat_map
>   Backports.alias_method self, :collect_concat, :flat_map
> 
> I am very curious to know how it was decided that a normal map
> followed by a 1-deep flatten is common enough to warrant its own
> method? Two in fact!

#flat_map is monadic bind (also known as flatMap in Scala, SelectMany
in .NET and (>>=) in Haskell). The other monadic operator that is
needed to build a monad, is unit (aka return in Haskell), but in an OO
language, that's just a factory method, IOW it's just .new in Ruby.

So, #flat_map can potentially turn any class that mixes in Enumerable
into a monad, provided that it also obeys the monad laws, of course.

Whether or not that warrants its own method ... well, I have no idea.

jwm