From: jweirich@...
Date: 2000-11-07T10:49:08+09:00
Subject: [ruby-talk:6087] detect:ifNone: in Ruby

I was recently converting a bit of Smalltalk code to Ruby.  The code
was written by Ron Jeffries to demonstrate Test First programming in
the XP mailing list.  (BTW, Dave Thomas was working on it as well and
posted his version at
http://www.egroups.com/message/extremeprogramming/15261?source=1 ...
you can find Ron's Smalltalk version in the book "Extreme Programming
Installed").

One of Ron's methods used detect:ifNone: ...

    matchingSum: aSum
        ^summary
            detect: [:each| each name = aSum name]
            ifNone: [summary add: (Sum
                name: aSum name
                amount: 0)]

I went digging through the Ruby documentation and found "detect", but
nothing that matched detect:ifNone:.  No problem, I wrote the Ruby
version as follows ...

  def matching_sum (aSum)
    result = @summary.detect { |each| each.name == aSum.name }
    if result == nil then
      result = Sum.new (aSum.name, 0)
      @summary << result
    end
    result
  end

Because Ruby only allows a single code block to follow a method call,
it is difficult to write methods that expect two separate code blocks.

After a bit of thought, I realized that Smalltalk's detect:ifNone:
method really combined two separate algorithms ... (1) Search for a
value and (2) provide an alternative value.  If we capture (2) in the
following code ...

  def if_none (value)
    if value then value else yield end
  end

Then we can rewrite matching_sum into ...

  def matching_sum (aSum)
    if_none (@summary.detect {|each| each.name == aSum.name}) do
      result = Sum.new (aSum.name, 0)
      @summary << result
      result
    end
  end

There, that captures some of the flavor of the Smalltalk version.

Is it worthwhile?  Is it clearer?  I'm not sure, but I thought it was
interesting and that this list might enjoy it.

-- 
-- Jim Weirich     jweirich@one.net    http://w3.one.net/~jweirich
---------------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)