From: John Carter <john.carter@...>
Date: 2007-12-06T08:48:47+09:00
Subject: Worth an RCR? static_type_check, polymorphic_type_check, 	quacks_like

Is there another library like this? I would love it if it were just
standard methods on Object.

I find this very useful and 'require' it into all code I write. Just
helps me mop up those problems that static type checking would have
found for me...

Heres a typical usage, create an object and initialize it with
stuff. Only much later when you invoke the methods on the object do
you find you initialized it with the wrong stuff.

Got the parameter order wrong or something. But then the bug isn't on
the backtrace. So put when you initialize your instance variables you
can type check'em like so...

eg.
class MyObject
   def initialize( _name, _drawable, _theme)
     @name     = _name.static_type_check String
     @drawable = _drawable.polymorphic_type_check Drawable
     @theme    = _theme.quacks_like :border_color, :fill_colour
   end

...

end



# Abstract base class for all the type check exceptions
class TypeCheckException  < Exception
end

# This exception is thrown in event of a method being invoked with an
# object of the wrong duck type.
class DuckTypingException < TypeCheckException
end

# This exception is thrown in event of a method being invoked with a
# parameter of of the wrong static type.
class StaticTypeException < TypeCheckException
end

# This exception is thrown in event of a method being invoked with a
# parameter of of the wrong polymorphic type.
class PolymorphicTypeException < TypeCheckException
end

class Object
    # Raise a DuckTypingException unless the object responds to all these symbols.
    def quacks_like( *symbols)
       symbols.each do |symbol|
          raise DuckTypingException, "Duck typing error, expected this object to respond to :#{symbol}, but found class #{self.class}\n\t\t#{symbol.inspect}" unless
             respond_to? symbol
       end
       self
    end

    def static_type_check_boolean
       raise StaticTypeException, "Static type check error, expected object to be a boolean, found '#{self.class}'
\t\t#{self.inspect}" unless
          (self.class == TrueClass) ||
          (self.class == FalseClass) ||
          (self.class == NilClass)
       self
    end


    def static_type_check( klass)
       raise StaticTypeException, "Static type check error, expected object to be exactly class '#{klass}', found '#{self.class}'\n\t\t#{self.inspect}" unless
          self.class == klass
       self
    end

    def static_type_check_each( klass)
       each do |object|
          object.static_type_check klass
       end
       self
    end

    def polymorphic_type_check_each( klass)
       each do |object|
          object.polymorphic_type_check klass
       end
       self
    end

    def polymorphic_type_check( klass)
       raise PolymorphicTypeException, "Polymorphic type check error, expected object to be a kind of '#{klass}', found '#{self.class}'\n\t\t#{self.inspect}" unless
          self.kind_of? klass
       self
    end

end



John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand