From: sera@... (Francis Hwang)
Date: 2004-05-28T22:00:47+09:00
Subject: Re: defining condititions

Guillaume Marcais <guslist@free.fr> wrote in message news:<1085504342.29598.2.camel@comp>...
> Kansas, written by Kirk Haines, has such a conditions system to write
> SQL queries. You may want to check it out:
> 
> http://enigo.com/projects/kansas/index.html
> 
> Guillaume.

Other examples to check out would include:

- Criteria, for querying databases and other data stores:
http://mephle.org/Criteria/
- Lafcadio, an O-R mapping layer for MySQL:
http://lafcadio.rubyforge.org/
- Runt, for defining time-based rulesets for defining calendars
without explicitly choosing dates: http://runt.rubyforge.org/

Probably this sort of thing is easier if you have an object that's
explicitly the receiver of the various methods. So instead of what
you'd originally asked:

my_set.rules = !name("orange") && hasProperty("color") && 
(hasProperty("size") || hasProperty("weight"))

It might be easier to define something that looks like:

my_set.createRules { |set|
  set.name('orange').not & set.hasProperty('color') &
  ( set.hasProperty('size') | set.hasProperty('weight')
}

Inside the block, the variable "set" is actually a special object that
pretends to be a set but really has funny methods defined to create
the various Rule primitives you've already defined. This is how I
implemented it in Lafcadio and it's worked well so far.

One other note: Beyond the fact that you can't override && and ||, you
also cannot override unary negation, so you get no control over what
!name('orange') means. That's why in the example code I wrote it calls
set.name('orange').not instead ... if you make it a method you can do
whatever's necessary.

Francis