longest_spec.rb

# <modulepath>/custom_functions/lib/puppet/parser/functions/longest.rb

Puppet::Parser::Functions.newfunction(:longest, :type => :rvalue, :doc => "Longest Function") do |args|
  raise Puppet::Error, "#longest: accepts only one argument, you passed #{args.length}" if args.length != 1
  raise Puppet::Error, "#longest: accepts a single Array, you passed: #{args[0].class}" if args[0].class != Array

  @longest = ''
  args[0].each do |item|
    @longest = item if item.length > @longest.length
  end

  @longest
end
[root@gary modules]# cat longest/spec/unit/puppet/parser/functions/longest_spec.rb
# <modulepath>/longest/spec/unit/puppet/parser/functions/longest_spec.rb

require 'puppet'
require 'rspec-puppet'

describe Puppet::Parser::Functions.function(:longest) do
  let :scope do
    PuppetlabsSpec::PuppetInternals.scope
  end

  it "should return the longest string in an array of strings" do
    scope.function_longest([['foo','bar','bazzle']]).should == 'bazzle'
  end

  it "should raise an error if you pass more or less than a single argument" do
    expect { scope.function_longest(['foo', 'bar']) }.to raise_error Puppet::Error, /#longest: accepts only one argumen/
  end

  it "should raise an error if you don't pass an Array" do
    expect { scope.function_longest(['foo']) }.to raise_error Puppet::Error, /#longest: accepts a single Array/
  end
end