From: Joel VanderWerf <vjoel@...>
Date: 2005-10-02T04:24:07+09:00
Subject: Re: Better way to make struct with defaults?

Gavin Kistner wrote:
> I'm using the same ideas as Struct to quickly create a class that:
> a) Has default values for the properties
> b) Uses symbols inside #new to set the values (instead of argument  order)
> 
> I have it working (code below) but whenever I construct a string for 
> use with *_eval, I feel that I'm probably being boneheaded, and  there's
> a better way to do it. What suggestions do people have for  cleaning up
> and/or simplifying the following code?
> 
> def Struct.with_defaults( property_hash )
>     klass = self.new( *property_hash.keys )
>     klass.class_eval <<-ENDCODE
>         def initialize( values={} )
>             #{property_hash.map{ |prop,default|
>                 "self.#{prop} = values[#{prop.inspect}] || #
> {default.inspect}"
>             }.join("\n")}
>         end
>     ENDCODE
>     klass
> end
> 
> Person = Struct.with_defaults :sex=>'male', :age=>0, :name=>nil
> gk = Person.new :name=>'Gavin', :age=>32
> p gk
> #=> #<struct Person sex="male", name="Gavin", age=32>
> 

def Struct.with_defaults( property_hash )
    klass = self.new( *property_hash.keys )

    default_setters = property_hash.map do |prop,default|
        ["#{prop}=".intern, prop, default]
    end

    klass.class_eval do
        define_method :initialize do |values|
            values ||= {}
            default_setters.each do |setter,prop,default|
                value = values.key?(prop) ? values[prop] : default
                send setter, value
            end
        end
    end
    klass
end

Person = Struct.with_defaults :sex=>'male', :age=>0, :name=>nil
gk = Person.new :name=>'Gavin', :age=>32
p gk

__END__

output:

#<struct Person sex="male", age=32, name="Gavin">

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407