From: Robert Dober <robert.dober@...>
Date: 2007-03-12T19:44:49+09:00
Subject: Re: #define alternative for Ruby

On 3/12/07, Brian Candler <B.Candler@pobox.com> wrote:
> 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
Please note that
    var ||= val
is not the same than
   var = val unless defined? var

Very often it is preferable of course.
Robert

<snip>