From: Christopher Dicely <cmdicely@...>
Date: 2011-05-29T04:08:23+09:00
Subject: Re: Problem in using %:whatever in enumumerations

On Sat, May 28, 2011 at 7:00 AM, RichardOnRails
<RichardDummyMailbox58407@uscomputergurus.com> wrote:
> On May 28, 2:20 am, Victor Deryagin <vderya...@gmail.com> wrote:
>
> Thanks for your response.
>
>> > greek = %w{Alpha Beta Gamma}
>> > puts greek.all? { |s| s.length == 5 }              #  (correct)  => false
>> > puts greek.all? { |s| (4..5).include? s.length }   #  (correct)  => true
>> > puts greek.all?(&:length)                      #  (programmer error)     => true
>
>> ... If that's not what you want (e.g. you need { |s| s.length == 5 }) -
>> construct block yourself.
>
> I did construct the block myself in the first couple of examples
> (lines 2 & 3) but was searching (in line 4) for another way of
> expressing the requirement.
>
> I finally found a way to use a lambda in the following statements in
> place of line 4:
>
> lam = lambda { |item, len| item.length == 5 }
> puts greek.all? { |s| lam.call(s,5)}                    #  (correct)  => false

Note that the second argument to lambda is never used, as you've
hardcoded 5 for the length to use for the test rather than using the
second argument. If you want something where you can change the length
to test for on different calls, replace the first line with:

lam = lambda { |item, len| item.length == len }

>
> Of course, the verbosity of this pair of statements is unsatisfying,
> so I'm still searching for succinct alternatives.

There isn't anything I can think of built in that's more succinct, but
if this kind of test is common in your code, you can abstract part of
it out to a library method on Enumerable and make the code more
succinct and clear at the point of call.

# elsewhere, extend Enumerable
# (obvious parallels any_have?, one_has?, count_having, etc. could
also be defined)

module Enumerable
  def all_have?(attribute, value)
    all? { |item| item.__send__(attribute) == value }
  end
end

# then, in the point you were working on

greek.all_have? :length, 5