From: Brian Candler <b.candler@...>
Date: 2008-12-21T01:03:05+09:00
Subject: Re: about Float

You're right, it works if you explicitly promote some of the constants 
to BigDecimal:

  class Numeric
    def bd
      BigDecimal.new(to_s,100)
    end
  end

  def f(x,y)
    (333.75.bd-x**2)* y**6 + x**2 * (11 * x**2 * y**2-121 * y**4 - 2) +
      5.5.bd * y**8 + x/(2*y)
  end

  puts f(77617.bd,33096.bd)

I didn't change any parentheses, just added .bd at certain places.

The trouble seems to be: mixing BigDecimal and Integer works fine 
(promotes to BigDecimal), but mixing BigDecimal with Float downgrades to 
Float.

irb(main):007:0> BigDecimal.new("7",100) + 2
=> #<BigDecimal:b7f07910,'0.9E1',4(12)>
irb(main):008:0> 2 + BigDecimal.new("7",100)
=> #<BigDecimal:b7f02b68,'0.9E1',4(12)>
irb(main):009:0> BigDecimal.new("7",100) + 2.0
=> 9.0
irb(main):010:0> (BigDecimal.new("7",100) + 2.0).class
=> Float

So you have to be careful not to mix any Float constants in.
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