Space Station Lambda

(asdf:oos 'asdf:load-op 'cl-who)
 
(defmacro demand-image-attrs (img want-attrs have-attrs)
  "Check an image's accessiblity information for completeness."
  (let ((missing-attrs (gensym)))
    `(let* ((,missing-attrs (loop for attr in ,want-attrs
                                 when (not (find attr ,have-attrs))
                                 collect attr)))
       (unless (null ,missing-attrs)
         (warn "image ~S is missing the following attributes: ~{~A~^, ~}" ,img ,missing-attrs))))
 
(defmacro render-image (src &rest attrs)
  "Produce HTML code from an image location."
  (declare (type string src))
  `(demand-image-attrs src (list :alt :title) attrs)
  `(cl-who:with-html-output (*standard-output*)
     (:img :src ,src ,@attrs)))
 
CL-USER> (render-image "cat.png" :title "Tom")
WARNING: image "cat.png" is missing the following attributes: ALT
"<img src='cat.png' title='Tom' />"