From: Thomas <sanobast-2005a@...>
Date: 2005-06-11T02:27:27+09:00
Subject: [ANN] constraint 0.1 -- ensure that object always satisfy a specified set of constraints

Hi,

I would like to announce 0.1 of contraint.rb. This is a first draft.

This library provides a way to ensure that object always satisfy a 
specified set of constraints. An object that can be constrained must be 
of kind Constraint::Shell. Constraints can be added to classes and 
single objects. It is possible to define methods that handle constraint 
violations and can make the object fit the demands.

Project
     http://rubyforge.org/projects/constraint/
Download
     http://rubyforge.org/frs/?group_id=748
Support
     http://rubyforge.org/forum/?group_id=748

In my personal experience, a certain type of error in dynamically typed 
languages is caused by pushing a wrong object to a collection or 
similar. What makes this type of error so awkward is that they often 
result in an exception in a different part of your program, which is why 
it can be unnecessarily difficult to track down what is actually causing 
the problem. The goal of this library is to raise an exception right 
where such a thing happens.

Constrained classes should inherit from Constraint::Shell. The subclass 
should provide a way for defining new instances (by defining 
#new_constrained) and redefine #processconstrainedvalue. The 
@constrained_value instance variable contains the shell's actual values.

Examples:

     class ConstrainedArray < Constraint::Shell
         def new_constrained
             []
         end

         def process_constrained_value
             @constrained_value.collect! {|e| yield e}
         end
     end

     class NumericArray < ConstrainedArray
         or_constraint("Numeric") {|e| e.kind_of?(Numeric)}
     end

     class EvenNumericArray < NumericArray
         and_constraint("Even") {|e| e.modulo(2) == 0}
     end

     class EvenInteger < Constraint::Shell
         and_constraint("Numeric") {|e| e.kind_of?(Integer)}
         and_constraint("Even") {|e| e.modulo(2) == 0}
     end

     enum = EvenInteger.new(2)
     enum.and_constraint("LT10") {|e| e < 10}

or:

     class DuckPalace < ConstrainedArray
         and_constraint('Quack') {|e| e.do_you_quack?}
         and_constraint('Ticket') {|e| e.your_ticket_please == :valid}
     end

     whiteduckpalace = DuckPalace.new
     whiteduckpalace.and_constraint('White') {|e| e.colour == 'white'}
     class << whiteduckpalace
         def handle_constraint_violation(contraint_name, object)
             case contraint_name
             when 'white'
                 return object.paint_me('white') if 
object.respond_to?(:paint_me)
             end
             return super
         end
     end


Regards,
Thomas.

	

	
		
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