Defining a Saxon integrated extension function in Clojure

has-class.clj

; Define a Saxon integrated extension function that checks whether
; the given element has the given DITA class.
;
; For example, given:
;
;   <topic id="topic" class="- topic/topic "> ... </topic>
;
; This XPath function, when defined and aliased under the "dita"
; namespace prefix, will return true for that element:
;
;   dita:has-class(., 'topic/topic')
(defextfn has-class
  "http://eerohele.github.io"
  [[:one :any-node] [:one :string]]
  [:one :boolean]
  [el class-name]
  ; predicates/has-class? is an imaginary Clojure function that does the actual work.
  (XdmAtomicValue. (predicates/has-class? class-name el)))

; Register the extension function with Saxon, using the Saxon library for
; Clojure (https://github.com/eerohele/saxon):
(.registerExtensionFunction saxon/proc has-class)