gistfile1.txt

module Puppet::Parser::Functions
  newfunction(:testfunction, :type => :rvalue) do |*a|
    p a
    p a.class
  end
end

$a = inline_template("<%= scope.function_testfunction('a', 'b', 'c') %>")
$b = inline_template("<%= scope.function_testfunction('a') %>")
$c = inline_template("<%= scope.function_testfunction(['a', 'b', 'c']) %>")

["a", "b", "c"]
Array
["a"]
Array
[["a", "b", "c"]]
Array

To make Puppet > 2.6.x ...

$a = inline_template("<%= Puppet::Parser::Functions.autoloader.loadall ; scope.function_testfunction('a', 'b', 'c') %>")
$b = inline_template("<%= Puppet::Parser::Functions.autoloader.loadall ; scope.function_testfunction('a', 'b', 'c') %>")
$c = inline_template("<%= Puppet::Parser::Functions.autoloader.loadall ; scope.function_testfunction(['a', 'b', 'c']) %>")