From: Michael Strelnikov <michaelst@...>
Date: 2007-03-12T18:31:02+09:00
Subject: Re: #define alternative for Ruby

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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
>
> 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.
>
>
The "local variables" is exactly what confuses me.  Why can't I use
variables (read/set) inside of function that is at the same hierarchy as
variable is?

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