From: trans <transfire@...>
Date: 2009-06-17T13:49:33+09:00
Subject: Re: each by arity



On Jun 16, 7:43 pm, Robert Dober <robert.do...@gmail.com> wrote:
> On Tue, Jun 16, 2009 at 12:33 PM, trans<transf...@gmail.com> wrote:
>
> > On Jun 16, 2:10 am, Robert Klemme <shortcut...@googlemail.com> wrote:
>
> >> Question is whether it is clever to duplicate all methods with another
> >> one that has "_slice" appended to the name.  I personally favor the
> >> approach with Enumerator, e.g. depending on version
>
> > Not clever... facetious. Of course Enumerator is useful, and with 1.9
> > fairly elegant. But rather, the question is why we must do
>
> >  enum.each_slice(2).map {|a,b| ...}
>
> > when
>
> >  enum.map {|a,b| ...}
>
> > could be made to work. This is especially interesting b/c it seems the
> > underlying implementation stands to be simpler and faster by doing so.
>
> That is intriguing and I am surprised.
>
> However, why should
>
>   enum.map{ |a,b| }
> have the semantics of
>   enum.each_slice( 2 )....
> and not
>    enum.each_cons( 2 ).... ?
>
> Maybe we need to push the abstraction even higher. Hopefully you
> understand what that sentence means, because I do not, but it felt
> just right to write it. Strange, strange.

Simply b/c cons is uncommon. We want to iterate over elements once and
only once, where as cons hits on the same elements multiple times. Of
course, I see no reason cons could not look at arity too.

With this idea, I've been wondering about possibilities for passing
blocks that might have one, two or more block arguments --the arity
unknown to the method processing them, and how that might or might
not  be useful. One obvious issue is that arity > 1 produces Array
elements, whereas arity of 1 would not necessarily.

> Now I am not so fond of enumerator, because it is really crazy.
>
>   x=%w{a b c d}
>   y = x.enum_for( :rand )
>   y.each do | a | p a end
> => 0.680141319345242
>
> Do you agree with me that enum_for should use #public_send instead of #send?

Oh that's nice. I did not think you could use it for anything but
Enumerable methods. I'm not sure it matters if they are public or not.
It still can be crazy.

y = x.enum_for(:to_s)
=> #<Enumerable::Enumerator:0x7f64cd403c68>
>> y.each{ |a| a }
=> "abcd"
>> y.each{ |a| a+'!' }
=> "abcd"
>> y.to_a
=> []

T.