From: "Aleksi Niemelä" <aleksi.niemela@...>
Date: 2000-12-14T15:21:04+09:00
Subject: [ruby-talk:7198] Harmful preselection by yielding?

Dave's answer made me wonder:

>   def getpwent
>     File.foreach("/etc/passwd") do |line|
>       yield line.split(/:/)
>     end
>   end
> 
>   # loop over entries in password file
> 
>   getpwent do |login, pw, uid, gid, name, home, shell|
>     puts "#{login} #{home}"
>   end


If you want to let the caller use the information the routine is producing
do you prefer:

  def foo
    @line.split(/:/)
  end

or

  def foo
    yield @line.split(/:/)
  end

I'd go for the case 1, but that was just prelude.

In Dave's reply, I felt the code is simple and nice, does what it should and
everybody is happy. And then suddenly I realize it leads me to more abstract
thinking. Here we expand the problem to the "second order":

  def foo
    lines.collect { |line| line.split(/:/) }
  end

  foo().each {|parts| bar parts }

or

  def foo
    lines.each {|line| yield line.split(/:/) }
  end

  foo {|parts| barract parts }

So, do you prefer to "fix in" the yielding instead of returning the produced
data structure. And which is the criteria you base your decision on (feeling
goes, but something more tangible would be great)? 

Returning the array of arrays gives more freedom, to do whatever is needed,
yielding restricts the use to be done in one way. OTOH, that one way might
be very convenient, and even the only one people ever would like to use. But
nevertheless, I feel this is kind of design issue for the Ruby code to be
developed.

I've always tried to write my code to return the whole datastructure, and
that has yielded (:-) to easier reusing. However, Ruby world is might be
different in this sense, so maybe I should start to look more carefully when
there's really no need to return "everything" -- when the simple
predetermined iteration does the trick.

Maybe my fear comes from the fact I'm dismissing some potential uses
prematurely. Like in this case, returning enumerable leaves the user to do
whatever he wishes, but yielding "preselects" only Enumerable#each (and as a
side-effect eases up this common usage).

Was I confusing...

	- Aleksi