diff --git a/lib/puppet/functions/assert_type.rb b/lib/puppet/functions/assert_type.rb
new file mode 100644
index 000000000..9b2ce2d15
--- /dev/null
+++ b/lib/puppet/functions/assert_type.rb
@@ -0,0 +1,16 @@
+# Returns the give value if it is an instance of the given type, and raises an error otherwise.
+#
+Puppet::Functions.create_function(:assert_type) do
+  dispatch :assert_type do
+    param type_type(), 'type'
+    param optional(object()), 'value'
+  end
+
+  def assert_type(type, value)
+    unless Puppet::Pops::Types::TypeCalculator.instance?(type,value)
+      inferred_type = Puppet::Pops::Types::TypeCalculator.infer(value)
+      raise ArgumentError, "assert_type(): Expected type #{type} does not match actual: #{inferred_type}"
+    end
+    value
+  end
+end
diff --git a/spec/unit/functions/assert_type_spec.rb b/spec/unit/functions/assert_type_spec.rb
new file mode 100644
index 000000000..76d3222a6
--- /dev/null
+++ b/spec/unit/functions/assert_type_spec.rb
@@ -0,0 +1,48 @@
+require 'spec_helper'
+require 'puppet/pops'
+require 'puppet/loaders'
+
+describe 'the assert_type function' do
+
+  after(:all) { Puppet::Pops::Loaders.clear }
+  let(:func) do
+    loaders = Puppet::Pops::Loaders.new()
+    loaders.puppet_system_loader.load(:function, 'assert_type')
+  end
+  it 'asserts compliant type by returning the value' do
+#    loaders = Puppet::Pops::Loaders.new()
+#    func = loaders.puppet_system_loader.load(:function, 'assert_type')
+    expect(func.call({}, type(String), 'hello world')).to eql('hello world')
+  end
+
+  it 'asserts non compliant type by raising an error' do
+    expect do
+      func.call({}, type(Integer), 'hello world')
+    end.to raise_error(ArgumentError, /does not match actual/)
+  end
+
+  it 'checks that first argument is a type' do
+    expect do
+      func.call({}, 10, 10)
+    end.to raise_error(ArgumentError, Regexp.new(Regexp.escape(
+"function 'assert_type' called with mis-matched arguments
+expected:
+  assert_type(Type type, Optional[Object] value) - arg count {2}
+actual:
+  assert_type(Integer, Integer) - arg count {2}")))
+  end
+
+  it 'allows the second arg to be undef/nil)' do
+    expect do
+      func.call({}, optional(String), nil)
+    end.to_not raise_error(ArgumentError)
+  end
+
+  def optional(type_ref)
+    Puppet::Pops::Types::TypeFactory.optional(type(type_ref))
+  end
+
+  def type(type_ref)
+    Puppet::Pops::Types::TypeFactory.type_of(type_ref)
+  end
+end