diff --git a/lib/puppet/functions/archive/go_md5.rb b/lib/puppet/functions/archive/go_md5.rb
new file mode 100644
index 0000000..017e421
--- /dev/null
+++ b/lib/puppet/functions/archive/go_md5.rb
@@ -0,0 +1,36 @@
+require_relative '../../../puppet_x/bodeco/util.rb'
+
+# @summary
+#   Retrieves and returns specific file's md5 from GoCD server md5 checksum file
+# @api private
+# @see http://www.thoughtworks.com/products/docs/go/12.4/help/Artifacts_API.html
+Puppet::Functions.create_function(:'archive::go_md5') do
+  # @param username
+  #   GoCD username
+  # @param password
+  #   GoCD password
+  # @param file
+  #   GoCD filename
+  # @param url
+  #   The GoCD MD5 checkum URL
+  # @return [String]
+  #   The MD5 string
+  dispatch :default_impl do
+    param 'String', :username
+    param 'String', :password
+    param 'String[1]', :file
+    param 'Stdlib::HTTPUrl', :url
+    return_type 'String'
+  end
+
+  def default_impl(username, password, file, url)
+    uri = URI(url)
+    response = PuppetX::Bodeco::Util.content(uri, username: username, password: password)
+
+    checksums = response.split("\n")
+    line = checksums.find { |x| x =~ %r{#{file}=} }
+    md5 = line.match(%r{\b[0-9a-f]{5,40}\b}) unless line.nil?
+    raise("Could not parse md5 from url #{url} response: #{response}") unless md5
+    md5[0]
+  end
+end
diff --git a/lib/puppet/parser/functions/go_md5.rb b/lib/puppet/parser/functions/go_md5.rb
deleted file mode 100644
index 668cf2d..0000000
--- a/lib/puppet/parser/functions/go_md5.rb
+++ /dev/null
@@ -1,28 +0,0 @@
-require 'puppet_x/bodeco/util'
-
-module Puppet::Parser::Functions
-  # Public: go file md5 checksum
-  #
-  # args[0] - username
-  # args[1] - password
-  # args[2] - file_name
-  # args[3] - go md5 checksum url
-  #
-  # http://www.thoughtworks.com/products/docs/go/12.4/help/Artifacts_API.html
-  #
-  # Returns specific file's md5 from go server md5 checksum file
-  newfunction(:go_md5, type: :rvalue) do |args|
-    raise(ArgumentError, "Invalid go md5 info url #{args}") unless args.size == 4
-
-    username, password, file, url = args
-
-    uri = URI(url)
-    response = PuppetX::Bodeco::Util.content(uri, username: username, password: password)
-
-    checksums = response.split("\n")
-    line = checksums.find { |x| x =~ %r{#{file}=} }
-    md5 = line.match(%r{\b[0-9a-f]{5,40}\b})
-    raise("Could not parse md5 from url#{url} response: #{response.body}") unless md5
-    md5[0]
-  end
-end
diff --git a/manifests/go.pp b/manifests/go.pp
index 8fde0a7..f9a9285 100644
--- a/manifests/go.pp
+++ b/manifests/go.pp
@@ -1,61 +1,61 @@
 # download from go
 define archive::go (
   String                    $server,
   Integer                   $port,
   String                    $url_path,
   String                    $md5_url_path,
   String                    $username,
   String                    $password,
   Enum['present', 'absent'] $ensure       = present,
   String                    $path         = $name,
   Optional[String]          $owner        = undef,
   Optional[String]          $group        = undef,
   Optional[String]          $mode         = undef,
   Optional[Boolean]         $extract      = undef,
   Optional[String]          $extract_path = undef,
   Optional[String]          $creates      = undef,
   Optional[Boolean]         $cleanup      = undef,
   Optional[Stdlib::Compat::Absolute_path] $archive_path = undef,
 ) {
 
   include archive::params
 
   if $archive_path {
     $file_path = "${archive_path}/${name}"
   } else {
     $file_path = $path
   }
 
   if $file_path !~ Stdlib::Compat::Absolute_path {
     fail("archive::go[${name}]: \$name or \$archive_path must be an absolute path!") # lint:ignore:trailing_comma
   }
 
   $go_url = "http://${server}:${port}"
   $file_url = "${go_url}/${url_path}"
   $md5_url = "${go_url}/${md5_url_path}"
 
   archive { $file_path:
     ensure        => $ensure,
     path          => $file_path,
     extract       => $extract,
     extract_path  => $extract_path,
     source        => $file_url,
-    checksum      => go_md5($username, $password, $name, $md5_url),
+    checksum      => archive::go_md5($username, $password, $name, $md5_url),
     checksum_type => 'md5',
     creates       => $creates,
     cleanup       => $cleanup,
     username      => $username,
     password      => $password,
   }
 
   $file_owner = pick($owner, $archive::params::owner)
   $file_group = pick($group, $archive::params::group)
   $file_mode  = pick($mode, $archive::params::mode)
 
   file { $file_path:
     owner   => $file_owner,
     group   => $file_group,
     mode    => $file_mode,
     require => Archive[$file_path],
   }
 }
diff --git a/spec/defines/go_spec.rb b/spec/defines/go_spec.rb
index 2889fe8..7d34799 100644
--- a/spec/defines/go_spec.rb
+++ b/spec/defines/go_spec.rb
@@ -1,79 +1,82 @@
 require 'spec_helper'
 
 describe 'archive::go' do
   let(:facts) { { os: { family: 'RedHat' }, puppetversion: '4.4.0' } }
 
-  before do
-    MockFunction.new('go_md5') do |f|
-      f.stub.returns('0d4f4b4b039c10917cfc49f6f6be71e4')
-    end
+  # Mock Puppet V4 API ruby function with a puppet language function equivalent
+  let(:pre_condition) do
+    <<-PUPPET
+    function archive::go_md5(String $username, String $password, String $file, Stdlib::HTTPUrl $url) {
+      return '0d4f4b4b039c10917cfc49f6f6be71e4'
+    }
+    PUPPET
   end
 
   context 'go archive with defaults' do
     let(:title) { '/opt/app/example.zip' }
     let(:params) do
       {
         server: 'home.lan',
         port: 8081,
         url_path: 'go/example.zip',
         md5_url_path: 'go/example.zip/checksum',
         username: 'username',
         password: 'password'
       }
     end
 
     it do
       is_expected.to contain_archive('/opt/app/example.zip').with(
         path: '/opt/app/example.zip',
         source: 'http://home.lan:8081/go/example.zip',
         checksum: '0d4f4b4b039c10917cfc49f6f6be71e4',
         checksum_type: 'md5'
       )
     end
 
     it do
       is_expected.to contain_file('/opt/app/example.zip').with(
         owner: '0',
         group: '0',
         mode: '0640',
         require: 'Archive[/opt/app/example.zip]'
       )
     end
   end
 
   context 'go archive with path' do
     let(:title) { 'example.zip' }
     let(:params) do
       {
         archive_path: '/opt/app',
         server: 'home.lan',
         port: 8081,
         url_path: 'go/example.zip',
         md5_url_path: 'go/example.zip/checksum',
         username: 'username',
         password: 'password',
         owner: 'app',
         group: 'app',
         mode: '0400'
       }
     end
 
     it do
       is_expected.to contain_archive('/opt/app/example.zip').with(
         path: '/opt/app/example.zip',
         source: 'http://home.lan:8081/go/example.zip',
         checksum: '0d4f4b4b039c10917cfc49f6f6be71e4',
         checksum_type: 'md5'
       )
     end
 
     it do
       is_expected.to contain_file('/opt/app/example.zip').with(
         owner: 'app',
         group: 'app',
         mode: '0400',
         require: 'Archive[/opt/app/example.zip]'
       )
     end
   end
 end
diff --git a/spec/functions/archive_go_md5_spec.rb b/spec/functions/archive_go_md5_spec.rb
new file mode 100644
index 0000000..d880bdb
--- /dev/null
+++ b/spec/functions/archive_go_md5_spec.rb
@@ -0,0 +1,21 @@
+require 'spec_helper'
+
+describe 'archive::go_md5' do
+  let(:example_md5) { File.read(fixtures('checksum', 'gocd.md5')) }
+  let(:url) { 'https://gocd.lan/path/file.md5' }
+  let(:uri) { URI(url) }
+
+  it { is_expected.not_to eq(nil) }
+
+  it 'retreives file md5' do
+    PuppetX::Bodeco::Util.stubs(:content).with(uri, username: 'user', password: 'pass').returns(example_md5)
+    is_expected.to run.with_params('user', 'pass', 'filea', url).and_return('283158c7da8c0ada74502794fa8745eb')
+  end
+
+  context 'when file doesn\'t exist' do
+    it 'raises error' do
+      PuppetX::Bodeco::Util.stubs(:content).with(uri, username: 'user', password: 'pass').returns(example_md5)
+      is_expected.to run.with_params('user', 'pass', 'non-existent-file', url).and_raise_error(RuntimeError, "Could not parse md5 from url https://gocd\.lan/path/file\.md5 response: #{example_md5}")
+    end
+  end
+end
diff --git a/spec/unit/puppet/parser/functions/go_md5_spec.rb b/spec/unit/puppet/parser/functions/go_md5_spec.rb
deleted file mode 100644
index 7dad352..0000000
--- a/spec/unit/puppet/parser/functions/go_md5_spec.rb
+++ /dev/null
@@ -1,18 +0,0 @@
-require 'spec_helper'
-
-describe :go_md5 do # rubocop:disable RSpec/DescribeSymbol
-  before :all do # rubocop:disable RSpec/BeforeAfterAll
-    Puppet::Parser::Functions.autoloader.loadall
-  end
-
-  let(:scope) { PuppetlabsSpec::PuppetInternals.scope }
-
-  example_md5 = File.read(fixtures('checksum', 'gocd.md5'))
-
-  it 'retreives file md5' do
-    url = 'https://gocd.lan/path/file.md5'
-    uri = URI(url)
-    PuppetX::Bodeco::Util.stubs(:content).with(uri, username: 'user', password: 'pass').returns(example_md5)
-    expect(scope.function_go_md5(['user', 'pass', 'filea', url])).to eq '283158c7da8c0ada74502794fa8745eb'
-  end
-end