From: Daniel Berger <Daniel.Berger@...>
Date: 2006-02-03T05:17:03+09:00
Subject: Re: ruby-dev summary 28206-28273

ara.t.howard@noaa.gov wrote:
> 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

(Reposted to ruby-talk)

Nice.  If only "if" were a Kernel method that we could tie it to back to 
"if/else" constructs somehow:

iff a.first
    ...
else
    ...
end

Cue the Smalltalk weenies...

Regards,

Dan