From: Robert Dober <robert.dober@...>
Date: 2008-06-12T00:36:44+09:00
Subject: Re: each with else

On Wed, Jun 11, 2008 at 1:41 PM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> 2008/6/11 Thorsten Müller <thorsten@80beans.com>:
>> Hi all,
>>
>> that's just an idea I came up with and thought worth a discussion.
>>
>> Would it be possible to give each an else part?
>>
>> For Rails programmers it is very common to do something like this:
>>
>> In controller search for records:
>>
>> @orders = Order. find(:all)
>>
>> In view:
>>
>> <% unless @orders.empty? then %>
>>  <%   @orders.each do |order| %>
>>    <%= output order somehow %>
>>  <% end %>
>> <% else %>
>>  output message, that there are no orders
>> <% end %>
>>
>> If each would have it's own else which runs if the
>> used array, enumerable or whatever is empty,
>> that code could look much nicer:
>>
>> <% @orders.each do |order| %>
>>    <%= output order somehow %>
>> <% else %>
>>  output message, that there are no orders
>> <% end %>
>>
>> I guess, that would be true for any other Ruby code, too.
>
> No.  This does not work because your bit translates to
>
> @orders.each do |order|
> # output order somehow expression
> else
> end
>
> This would mean that the "else" branch is executed for each iteration
> because it is *inside* the block.  This is something you clearly do
> not want.
>
>> Please give your opinions to that idea and if you think
>> there would be a chance to get this implemented one day...
>
> I'd go with Robert's suggestion:
>
> <% @orders.each do |order| %>
>   <%= output order somehow %>
>  <% end.empty? and begin %>
>  output message, that there are no orders
> <% end %>
>
> or
>
> <% if @orders.each do |order| %>
>   <%= output order somehow %>
> <% end.empty? %>
>  output message, that there are no orders
> <% end %>
>
> Note that depending on the type of @orders you do not even need to
> define #empty? (Array and Hash have it).  If there are special types
> you could do
>
> module Enumerable
>  def each_checked
>    found = false
>
>    each do |item|
>      found = true
>      yield item
>    end
>
>    found
>  end
> end
>
> and then
>
> <% unless @orders.each_checked do |order| %>
>   <%= output order somehow %>
> <% end then %>
>  output message, that there are no orders
> <% end %>
>
> Note: "then" is optional.
>
> Kind regards
>
> robert
>
> --
> use.inject do |as, often| as.you_can - without end
>
>

This is one of the cases where the elegance of Smalltalk shines, you
would simply write something like

ary eachDo: [ :ele | ... ] orIfEmpty: [ whateverElse ]

makes me wanting Eric's patch for the simplified lambda syntax even
more, than we can write in pure Ruby

module Enumerable
    def each_else each_blk, else_blk=nil
           return else_blk && else_blk.call if empty?
           each &each_blk
    end
end

and we could call it with

ary.each_else do | ele | puts ele end, do puts "empty" end
or
ary.each_else {|ele| puts ele }, {|| puts "empty" }
without having checked for precedence, maybe some "()" are needed ;)

Guess that this is pretty much what OP wants, right? ;)

Cheers
Robert





-- 
http://ruby-smalltalk.blogspot.com/

---
As simple as possible, but not simpler.
Albert Einstein