From: dblack@...
Date: 2002-11-16T06:07:50+09:00
Subject: Re: ruby-dev summary 18613-18710

Hi --

On Sat, 16 Nov 2002, Florian Frank wrote:

> On 2002-11-15 21:37:46 +0900, dblack@candle.superlink.net wrote:
> > No, I mean the actual return value of the method call, which is a
> > separate matter from the block.  In your example, if there were no
> > block, the return value would be different.  In other words, given:
> >
> >   n = [a,b,c].zip
> >   m = [a,b,c].zip { |x,y,z| x*y+z }
> >
> > n and m would not be the same.  That's where I'm scratching my head
> > and wondering if that's the best way.
>
> The first case is equivalent to the second case with the block
> { |*tuple| tuple }. If no block is given, zip just uses the array of
> parameters of this block.

Yes, that's the very behavior that raises the question (to the extent
that it is a question) about the presence/absence of the block
affecting the return value of the method.  Which for some reason seems
anomalous to me....  I'm still not entirely sure why.  I have to do
some digging and refresh my memory as to how blocks with optional
methods usually behave.  (Hmmm, what did we do in scanf? :-)

> In fact that's exacly the way I implemented zip and unzip while
> converting some Haskell functions into Ruby. Haskell has zip, zip3,
> zipWith and zipWith3 and all of this functions can be replaced by a
> single Ruby method which even can do zipWithN:
>
> module Enumerable
>
> 	def zip(&block)
> 		block ||= lambda { |*tuple| tuple }
> 		size = nil
> 		map { |x| x.size }.each do |s|
> 			size ||= s
> 			s == size or raise "zip with unequal length lists"
> 		end
> 		result = []
> 		i = 0
> 		catch(:stop) do

(Don't you like regular Ruby iterators? :-)

> 			tuple = []
> 			each do |l|
> 				l[i] or throw :stop

That test is going to prevent you from zipping arrays that have nil or
false as elements, for example:

  a = [1,2,3]
  b = [4,nil,6]

  p [a,b].zip   # [[1,4]]

I notice that you've made this a method of Enumerable, rather than
Array.  What would the effect be of calling #zip on a non-Array
Enumerable, for example a String?

  "abcde".zip   # => ?

(I gather from the original post that there was some discussion of
Enumerable#zip in the Japanese list... not sure what was said,
though, and I'm not clear on how it would work.)


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav