Puppet plugin in the style of extlookup or yamlvar, but with precedence-based merging of results.

dilly.rb

# USAGE: $variable = dilly(key, [order])
# Where order is an optional array of the order of precedence of the YAML files.
# If array $dillyorder exists in the scope, that will work too.
# Passing 'nodefault' in the order array will prevent default.yaml from being considered.

module Puppet::Parser::Functions
    require 'yaml'
    require 'deep_merge'

    newfunction(:dilly, :type => :rvalue) do |args|
        if args[1] and not args[1].is_a?(Array) then
            raise Puppet::ParseError, "Dilly 'order' must be an array"
        end

        key = args[0]
        order = []

        # By default we only consider "default.yaml"; others must be specified somehow.
        if args[1] then
            order = args[1]
        elsif lookupvar('dillyorder').is_a?(Array) then
            order = lookupvar('dillyorder')
        end
        order.push('default') unless order.include?('nodefault')
        order.delete('nodefault')

        # Dilly YAMLs are stored in puppet/dillydata/
        dillydata = File.join(File.dirname(Puppet.settings[:config]), 'dillydata')
        raise Puppet::ParseError, "#{dillydata} is not a directory" unless File.directory?(dillydata)

        result = {}
        # Merge the YAMLs in order of precedence to produce the final result.
        order.each do |x|
            f = "#{dillydata}/#{x}.yaml"
            current = {}
            current = YAML.load_file(f) unless not File.exists?(f)
            if current[key] then
                result = result.deep_merge(key=>current[key])
            end
        end

        raise Puppet::ParseError, "Could not find '#{key}' in dillydata" unless not result.empty?

        # There should only be one key at the first level of the result hash,
        # and that key will be the name of the requested variable, which is not
        # interesting. :P                              
        return result[result.keys[0]]
    end
end

So here's what would happen :

# dev.yaml

---
database:
  host: 'db-dev.company.int'

# default.yaml

---
database:
  host: 'db.company.int'
  port: 3306

# manifest.pp
$dillyorder = ['dev']
$database = dilly('database')
notice($database)

Results in :

notice: Scope(Class[manifest]): {"host"=>"db-dev.company.int", "port"=>3306}