From: ara.t.howard@...
Date: 2006-02-03T00:35:52+09:00
Subject: Re: ruby-dev summary 28206-28273

On Thu, 2 Feb 2006, Minero Aoki wrote:

> [ruby-dev:28217] ANDCALL operator
>
>  Nobuyoshi Nakada suggested a new operator `&?' (this notation is temporary)
>  which evaluates left-hand-side expression, and if it is true then call
>  right-hand-side method.  For example:
>
>    if a[1] and a[1].strip.empty?
>                ||
>    if a[1] &? strip.empty?
>
>    h["key"] and h["key"].dispatch
>                ||
>    h["key"] &? dispatch
>
>  The motivation of this operator is to avoid duplication of expression.
>
>  Takaaki Tateishi proposed another idea, Object#nil? with block
>  (again, this name is temporary).
>
>    a[1].nil? {|str| str.strip.empty? }
>    h["key"].nil? {|h| h.dispatch }
>
>  This issue is still open.


suggestion:


     harp:~ > cat a.rb
     #
     # predicate methods to avoid double evaluation of expr
     #
       module Kernel
         def iff?(a, &b) a.instance_eval &b if a end
         alias_method "if?", "iff?"
         def iff!(a, &b) a.instance_eval &b unless a end
         alias_method "if!", "iff!"
       end

       a = [ 42 ]
       b = [ false ]

     #
     # if, and only if
     #

       iff?(a.first){ p self }

     #
     # if, and only if not
     #

       iff!(b.first){ p self }



     harp:~ > ruby a.rb
     42
     false


kind regards.

-a

-- 
happiness is not something ready-made.  it comes from your own actions.
- h.h. the 14th dali lama