(PUP-638) Add a pick_default() function that always returns a value.

This version of pick() does not error out, instead always returning at least
the last argument, even if that too has no "real" value.

lib/puppet/parser/functions/pick_default.rb

@@ -0,0 +1,35 @@
+module Puppet::Parser::Functions
+ newfunction(:pick_default, :type => :rvalue, :doc => <<-EOS
+
+This function is similar to a coalesce function in SQL in that it will return
+the first value in a list of values that is not undefined or an empty string
+(two things in Puppet that will return a boolean false value). If no value is
+found, it will return the last argument.
+
+Typically, this function is used to check for a value in the Puppet
+Dashboard/Enterprise Console, and failover to a default value like the
+following:
+
+  $real_jenkins_version = pick_default($::jenkins_version, '1.449')
+
+The value of $real_jenkins_version will first look for a top-scope variable
+called 'jenkins_version' (note that parameters set in the Puppet Dashboard/
+Enterprise Console are brought into Puppet as top-scope variables), and,
+failing that, will use a default value of 1.449.
+
+Note that, contrary to the pick() function, the pick_default does not fail if
+all arguments are empty. This allows pick_default to use an empty value as
+default.
+
+EOS
+) do |args|
+   fail "Must receive at least one argument." if args.empty?
+   default = args.last
+   args = args[0..-2].compact
+   args.delete(:undef)
+   args.delete(:undefined)
+   args.delete("")
+   args << default
+   return args[0]
+ end
+end

spec/unit/puppet/parser/functions/pick_default_spec.rb

@@ -0,0 +1,58 @@
+#!/usr/bin/env ruby -S rspec
+require 'spec_helper'
+
+describe "the pick_default function" do
+  let(:scope) { PuppetlabsSpec::PuppetInternals.scope }
+
+  it "should exist" do
+    Puppet::Parser::Functions.function("pick_default").should == "function_pick_default"
+  end
+
+  it 'should return the correct value' do
+    scope.function_pick_default(['first', 'second']).should == 'first'
+  end
+
+  it 'should return the correct value if the first value is empty' do
+    scope.function_pick_default(['', 'second']).should == 'second'
+  end
+
+  it 'should skip empty string values' do
+    scope.function_pick_default(['', 'first']).should == 'first'
+  end
+
+  it 'should skip :undef values' do
+    scope.function_pick_default([:undef, 'first']).should == 'first'
+  end
+
+  it 'should skip :undefined values' do
+    scope.function_pick_default([:undefined, 'first']).should == 'first'
+  end
+
+  it 'should return the empty string if it is the last possibility' do
+    scope.function_pick_default([:undef, :undefined, '']).should == ''
+  end
+
+  it 'should return :undef if it is the last possibility' do
+    scope.function_pick_default(['', :undefined, :undef]).should == :undef
+  end
+
+  it 'should return :undefined if it is the last possibility' do
+    scope.function_pick_default([:undef, '', :undefined]).should == :undefined
+  end
+
+  it 'should return the empty string if it is the only possibility' do
+    scope.function_pick_default(['']).should == ''
+  end
+
+  it 'should return :undef if it is the only possibility' do
+    scope.function_pick_default([:undef]).should == :undef
+  end
+
+  it 'should return :undefined if it is the only possibility' do
+    scope.function_pick_default([:undefined]).should == :undefined
+  end
+
+  it 'should error if no values are passed' do
+    expect { scope.function_pick_default([]) }.to raise_error(Puppet::Error, /Must receive at least one argument./)
+  end
+end