From: Wilson Bilkovich <wilsonb@...>
Date: 2006-04-15T02:33:33+09:00
Subject: Re: Default value for any type?

On 4/14/06, Victor Shepelev <vshepelev@imho.com.ua> wrote:
>
>
> > -----Original Message-----
> > From: Matthew Desmarais [mailto:desmarm@gmail.com]
> > Sent: Friday, April 14, 2006 6:58 PM
> > To: ruby-talk ML
> > Subject: Re: Default value for any type?
> >
> > Victor Shepelev wrote:
> > >>> Hello all.
> > >>>
> > >>> I have code:
> > >>>
> > >>> def create_default(type)
> > >>>   type.new
> > >>> end
> > >>>
> > >>> But with, for example, Float, it suddenly said:
> > >>>
> > >>> create_default(Float) #<=== undefined method `new' for Float:Class
> > >>> (NoMethodError)
> > >>>
> > >>> Hmmm? How can I uniformly obtain default value for given class?
> > >>>
> > >> What should the default value for a Float be? What are you trying to
> > >> really do here?
> > >>
> > >> There are a number of classes that you're not going to be able to call
> > >> #new on.
> > >>
> > >
> > > Not so long time ago I've been a C++ guy. There we could do:
> > > float(); //0.0
> > > int(); //0
> > > ...etc...
> > >
> > > Generalized version:
> > > template<typename T>
> > > T create_default()
> > > {
> > >   return T();
> > > }
> > >
> > > //usage:
> > > create_default<float>();       // => 0.0
> > > create_default<std::string>(); // => ""
> > >
> > > and so on. I think, argument-less constructor, creating some "default"
> > > value, is a very useful thing.
> > >
> > > (yes, I can define Float#default and so on for myself ;) But I want to
> > know,
> > > *why*?)
> > >
> > >
> > >
> > >> -austin
> > >> --
> > >> Austin Ziegler * halostatue@gmail.com
> > >>                * Alternate: austin@halostatue.ca
> > >>
> > >
> > > Victor.
> > >
> > Hi Victor,
> >
> > I think that Austin's question about what you are trying to accomplish
> > is probably a good. one.  If you can post some Ruby code that would rely
> > on something like this that would be great.  Maybe the folks on the list
> > can help you find a solution for you.
>
> Hmmm... Maybe I beginning to understand your point. The task was: before
> serializing (or storing in DB) some complex class, ensure that all fields
> aren't nil. I does some metaprogramming, so description of the fields looked
> like:
>
> class Something
>   data :name, String, :default => ''
>   data :price, Float, :default => 0.0
>   data :quan, Fixnum, :default => 0
>   data :type, String, :default => 'unknown'
>   data :coef, Float , :default = > 1.0
>
>   def store_in_db
>     self.validate!
>     ....
>   end
>
>   def validate!
>     #ensure each field isn't nil and has right type;
>     #set it's value to default in other case
>   end
> end
>
> (I hope the code above is quite self-explaining.)
> Later I saw that most of default values seems to like "empty value" of the
> corresponding type (I was C++ guy, remember). So, I want not to write
> :default => ... for :name, :price, :quan
>
> But after this topic (and Austins and yours answers) I think that overall
> construction don't looks very rubyish. Maybe, I must look for alternatives.
> >

This reminds me of the Metakoans quiz, actually:
http://www.rubyquiz.com/quiz67.html

If you haven't seen that, it may give you some ideas.

--Wilson.