From: Reimer Behrends <behrends@...>
Date: 2002-09-06T20:30:30+09:00
Subject: Re: Ruby aesthetics

Gavin Sinclair (gsinclair@soyabean.com.au) wrote:
[...]
>    def sqrt(x)
>      good_enough = lambda { |g1,g2| ((g1 - g2).abs / g2) < .001 }
>      improve = lambda { |guess| average(guess, x/guess) }
>      sqrt_iter = lambda { |g1,g2|
>        (good_enough[g1,g2]) ? g2 : sqrt_iter[g2, improve[g2]]
>      }
>  
>      sqrt_iter[1.0, 2.0]
>    end

If I may suggest something? The natural habitat of a collection of
functions and data in an object-oriented system is the class. E.g.:

class SqrtNewton
  def average(a,b) (a+b)/2.0 end # Just enough to make the example run
  def good_enough(g1,g2) (g1-g2).abs/g2 < .001 end
  def improve(guess) average(guess, @x/guess) end
  def sqrt_iter(g1, g2)
    if good_enough(g1, g2) then
      g2
    else
      sqrt_iter(g2, improve(g2))
    end
  end
  def sqrt(x)
    @x = x
    sqrt_iter(1.0, 2.0)
  end
end

# This also allows us to refine our implementation via
# inheritance:

class SqrtBetter < SqrtNewton
  attr_accessor :epsilon
  def initialize(epsilon=.001)
    @epsilon = epsilon
  end
  def good_enough(g1, g2) (g1-g2).abs/g2 < @epsilon end
end

SQRT_CALCULATOR = SqrtBetter.new(.0001)

def sqrt(x) SQRT_CALCULATOR.sqrt(x) end

# We probably want to stick all this inside a module so as not to
# pollute the global namespace, but it should be sufficient for
# the purposes of this example.

The problem with nested definitions is that you have identified
an abstraction (the nested defs) that cannot possibly be reused
or varied, since they are hidden within that function.

		Reimer Behrends