From: Brian Candler <B.Candler@...>
Date: 2007-03-12T18:21:04+09:00
Subject: Re: #define alternative for Ruby

On Mon, Mar 12, 2007 at 06:05:25PM +0900, Mike wrote:
> > > Imagine you would call the function with rand() as parameter. With #define
> > > 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
> 
> Just try to implement:
> set_default(var, val)
> -- if variable "var" is not defined (not existed) then assign value
> "val"

I think we have a stuck record here :-)

For this particular example, the code you are looking for is:

  var ||= val

with no method call at all. This is a tiny pattern and not worth
abstracting.

Local variables are *static*. Local variables are not intended to be handled
in the way you are asking. The language provides other facilities (i.e. not
local variables) when you want to be more dynamic.

"Doctor, it hurts when I do this."
"Then don't do that!"

Try the following if you don't understand what I mean by local variables
being "static".

  def x
    "This method is never called"
  end

  if false
    x = "Local variable"
  end

  puts x

This code prints "nil". Reflect on why this is the case.

Regards,

Brian.