From: Brian Candler <B.Candler@...>
Date: 2007-03-12T17:47:26+09:00
Subject: Re: #define alternative for Ruby

On Mon, Mar 12, 2007 at 05:16:08PM +0900, Sebastian Hungerecker wrote:
> ara.t.howard@noaa.gov wrote:
> > On Mon, 12 Mar 2007, Mike wrote:
> > > I was talking more about macros rather then constants.  But I agree that
> > > constant are convenient way to substitute "#define myconst 15". But can
> > > you substitute "#define myfunc(x) (sin(x)*cos(x)*exp(x))"?  You should
> > > know that "x" could be as much other functions as a constant/variable.
> >
> > perhaps i'm being dense, but i fail to see how the above example is none
> > other than a simple function in ruby:
> 
> Imagine you would call the function with rand() as parameter. With #define 
> rand would be called three times, with def it would be called once and its 
> value would be used three times.

???

irb(main):001:0> def myfunc(x); Math.sin(x)*Math.cos(x)*Math.exp(x); end
=> nil
irb(main):002:0> p myfunc(rand)
0.44720293351854
=> nil
irb(main):003:0> p myfunc(rand)
1.07121692170638
=> nil
irb(main):004:0> p myfunc(rand)
1.01827292445777
=> nil

Now, if you *store* the value of rand within the function, that's a
different case. But Ruby has different idioms for deferred execution: one of
those is

class Foo
  def initialize(&blk)
    @gen = blk
  end
  def value
    @gen.call
  end
end

a = Foo.new { rand }
puts a.value
puts a.value
puts a.value