POC deferrable epp function

This function will detect when it's passed a deferred function and will
effectively defer itself rather than rendering a template directly. It
does this by either returning a deferred inline_epp function with the
template source and variables compiled into the catalot, or by rendering
directly with epp and just returning that output.

Author: binford2k

functions/deferrable_epp.pp

@@ -0,0 +1,19 @@
+# This function returns either a rendered template or a deferred function to render at runtime.
+# If any of the values in the variables hash are deferred, then the template will be deferred.
+#
+# Note: this function requires all parameters to be explicitly passed in. It cannot expect to
+# use facts, class variables, and other variables in scope. This is because when deferred, we
+# have to explicitly pass the entire scope to the client.
+#
+function stdlib::deferrable_epp(String $template, Hash $variables) >> Variant[String, Deferred] {
+  if $variables.any |$key, $value| { $value.is_a(Deferred) } {
+    Deferred(
+      'inline_epp',
+      [find_template($template).file, $variables],
+    )
+  }
+  else {
+    epp($template, $variables)
+  }
+}
+