(MODULES-2516) Adds an is_a() function

The data type system is very hard to understand. Many people don't
understand why

    type_of([1,2,3]) == Array

will fail, but

    type_of([1,2,3]) <= Array

passes. This does a simpler validation that doesn't rely on explicit
data types. Instead, use

    $foo = [1,2,3]
    if $foo.is_a(Array) {
      notify { 'This is an array': }
    }

This is based on code by Ben Ford <ben.ford@puppetlabs.com>.

  * Added acceptance tests
  * Added dispatch
  * Improved unit tests
  * Added docs to README

1 files changed, 32 insertions, 0 deletions

diff --git a/lib/puppet/functions/is_a.rb b/lib/puppet/functions/is_a.rb
new file mode 100644
index 0000000..da98b03
--- /dev/null
+++ b/lib/puppet/functions/is_a.rb
@@ -0,0 +1,32 @@
+# Boolean check to determine whether a variable is of a given data type. This is equivalent to the `=~` type checks.
+#
+# @example how to check a data type
+#   # check a data type
+#       foo = 3
+#       $bar = [1,2,3]
+#       $baz = 'A string!'
+#
+#       if $foo.is_a(Integer) {
+#         notify  { 'foo!': }
+#       }
+#       if $bar.is_a(Array) {
+#         notify { 'bar!': }
+#       }
+#       if $baz.is_a(String) {
+#         notify { 'baz!': }
+#       }
+#
+# See the documentation for "The Puppet Type System" for more information about types.
+# See the `assert_type()` function for flexible ways to assert the type of a value.
+#
+Puppet::Functions.create_function(:is_a) do
+  dispatch :is_a do
+    param 'Any', :value
+    param 'Type', :type
+  end
+
+  def is_a(value, type)
+    # See puppet's lib/puppet/pops/evaluator/evaluator_impl.rb eval_MatchExpression
+    Puppet::Pops::Types::TypeCalculator.instance?(type, value)
+  end
+end