gistfile1.rb

module Factor
    def self.add(name, &block)
        Factor::Resolvers.add(name, block)
    end

    # Facts are provided by returning immediate values and lazily
    # evaluating value strategy objects (things that respond to #call, like
    # simple Procs)
    def self.value(name)
        value = Factor::Resolvers[name]
        value = value.call while value.respond_to?(:call)
    end

end

module Factor::Resolvers
    # resolver table
    RESOLVERS = {}
    def self.[](key); RESOLVERS[key] end

    def self.add(name, block)
        RESOLVERS[name] = instance_eval(block)
    end

    private

    def self.exec(command, &block)
        block.call(`#{command}`)
    end

    # Lazy is used here to wrap an (imaginary) Windows Registry class with the necessary #call
    # interface.
    def self.registry(key)
        lazy { Factor::Platform::Windows::RegistryService.new(key).value }
    end

    def self.command(&block)
        block
    end

    def self.extract(regexp, options)
        exec(options[:from]){|output| output[regexp]}
    end

    def self.setcode(str=nil, &block)
        str ? exec(str) : block
    end

    # Provides lazy evaluation of a fact
    def lazy(&block); block end
end


# Example:
Factor.add(:hostname) do

    setcode do
        hostname = nil
        name = Factor::Util::Resolution.exec('hostname') or nil
        if name
            if name =~ /^([\w-]+)\.(.+)$/
                hostname = $1
                # the Domain class uses this
                $domain = $2
            else
                hostname = name
            end
            hostname
        else
            nil
        end
    end

    # becomes

    exec('hostname') do |name|
        name[/^([\w-]+)\.?/]
    end

    # becomes

    extract /^([\w-]+)\.?/, :from => 'hostname'
end