discrimination of constructors with same number of parameters

[spir]

On Thu, 30 Dec 2010 14:26:21 -0500
"Steven Schveighoffer" <schveiguy at yahoo.com> wrote:

> On Thu, 30 Dec 2010 14:08:49 -0500, spir <denis.spir at gmail.com> wrote:
> 
> > On Thu, 30 Dec 2010 16:33:39 -0200
> > Guilherme Vieira <n2.nitrogen at gmail.com> wrote:
> >
> >> When I create factory methods like those proposed in this thread, I feel
> >> like I'm hijacking a core aspect of the language. Goddamn it, it's the
> >> constructor! IMHO, everybody expects to construct things.. using
> >> constructors (unless there's a restriction like not being allowed to
> >> construct stuff anytime, but only under certain conditions, which was  
> >> the
> >> case with SoundSources).
> >>
> >> Factory methods should, again just in my opinion, be used to exploit
> >> polymorphism or enhance the system design, not to solve problems of the
> >> language dealing with method overloading. It's just.. creepy. Is it  
> >> just me?
> >> I feel like I'm giving too much and barely getting one thirth in  
> >> return...
> >
> > @Steven: This is great answer to the question you asked me (and I did  
> > not answer) about the "clear meaning" of constructors in a PL. (Else,  
> > let's just get rid of them alltogether, and of the notion too, and just  
> > use object factories?)
> 
> A factory method *forwards to* a constructor.  You are not losing the  
> construction aspect (or its special qualities), in fact, it can sit beside  
> a constructor.
> 
> There are many cases where a constructor is not as clear as a factory  
> method.  Take for std.array.Appender.  The constructor that takes an  
> initial array as input:
> 
> int[1024] buf;
> 
> auto app = Appender!(int[])(buf);
> 
> vs.
> 
> auto app = appender(buf);
> 
> The factory method looks clearer to me.
> 
> I think it's really subjective depending on the situation.  All I'm saying  
> is that there is nothing IMO that rules out factory methods as  
> construction means on principal.

Good example, indeed. I must admit I used a load of factory --methods-- plain functions in a parsing lib, just to avoid repetitive "new"'s, imo simply obfuscating the code:
	auto digit = klass("0-9");
	auto natural = oneOrMore(digit);
	auto integer = composition(option(literal("-")), natural);
instead of:
	auto digit = new Klass("0-9");
	auto natural = new OneOrMore(digit);
	auto integer = new Composition(new Option(new Literal("-")), natural);
;-)

Denis
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