From: Bertram Scharpf <lists@...>
Date: 2005-01-19T01:58:13+09:00
Subject: Re: Creating an instance from a variable

Hi Robert,

Am Dienstag, 18. Jan 2005, 17:01:12 +0900 schrieb Robert Klemme:
> "Bertram Scharpf" <lists@bertram-scharpf.de> schrieb im Newsbeitrag 
> >
> >I'm porting my Make replacement Bake to Ruby and I do something
> >like
> >
> >   class Real ; ...
> >   class Phony ; ...
> >
> >   b = Builder.create target, name, prereqs, cmd
> >
> >where `target' is either `Real' or `Phony'.
> 
> ???  Why can't you do
> 
> b = target.new name, prereqs, cmd
> 
> I mean, a class *is* a factory for its instances.  If you don't have the 
> requirement that the actual class of created instances might be different 
> or want to keep that flexibility I don't see any added value in introducing 
> a method for this.

Hm, seems to be a long time since I designed this. I
remember having thought very long and thoroughly about it.

I think I didn't tell the whole truth above. My code
actually looks like

  Rule.new   Real,  'prog', 'prog.o', 'gcc ...'
  Rule.new   Phony, 'all',  'prog'
  Suffix.new Real,  '.o',   '.c',     'gcc -c ...'

Multiple kinds of rules can instantiate multiple types of
targets. So, it is not possible to derive one from the
other.

Further, the suffix rule has to determine the target name
before it holds enough information to instantiate the target
itself.

(The whole program will be available soon.)

Bertram


-- 
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-scharpf.de