How do I read a Hiera value from a Puppet function?

Question

I have to write a function that will, in order, perform the following to read the value of a variable:

• Check if there is a facter variable defined. If not,
• Read the value of the variable from Hiera. If not,
• Use a default value.

I have managed to do it in my Puppet script using this if condition.

  # foo is read from (in order of preference): facter fact, hiera hash, hard coded value
  if $::foo == undef {
     $foo = hiera('foo', 'default_value')
  } else {
     $foo = $::foo
  }

But I want to avoid repeating this if condition for each variable I wish to parse in this manner, and therefore thought of writing a new Puppet function of the format get_args('foo', 'default_value') which will return me the value from

1. a facter fact if it exists,
2. a hiera variable, or
3. just return default_value.

I know that I can use lookupvar to read a facter fact from a ruby function. How do I read a hiera variable from my Puppet ruby function?

Answer 1

You can call defined functions using the function_ prefix.

You have found the lookupvar function already.

Putting it all together:

module Puppet::Parser::Functions
  newfunction(:get_args, :type => :rvalue) do |args|
    # retrieve variable with the name of the first argument
    variable_value = lookupvar(args[0])
    return variable_value if !variable_value.nil?
    # otherwise, defer to the hiera function
    function_hiera(args)
  end
end