Implement multi-select with tests

Allow multiple selected options inside <select>s with the
`multiple` attribute.  Instead of checking equality using
`=`, a `selected?` function is implemented that checks
whether the current value being rendered is found inside
`selected`, which can be a sequence, a function, or a single
value.

src/hiccup/form.clj

@@ -74,6 +74,19 @@
              :value value
              :checked checked?}]))
 
+;; According to the HTML spec, the value inside the boolean `selected` attribute
+;; can only be "" or "selected" (case insensitive).  Hence, the coercion to
+;; boolean is required (otherwise, the attribute value will be `x` itself).
+;; We also need the `(not (sequential? selected))` requirement in the
+;; second `and` because if `selected` is a vector where `x` is not an element,
+;; the first `and` will fail, and the second will try to run something
+;; like `([a b] c)`, and can easily lead to java.lang.IndexOutOfBoundsException.
+(defn- selected? [x selected]
+  (or
+    (and (sequential? selected) (boolean ((set selected) x)))
+    (and (not (sequential? selected)) (ifn? selected) (selected x))
+    (= x selected)))
+
 (defelem select-options
   "Creates a seq of option tags from a collection."
   ([coll] (select-options coll nil))
@@ -83,8 +96,8 @@
         (let [[text val] x]
           (if (sequential? val)
             [:optgroup {:label text} (select-options val selected)]
-            [:option {:value val :selected (= val selected)} text]))
-        [:option {:selected (= x selected)} x]))))
+            [:option {:value val :selected (selected? val selected)} text]))
+        [:option {:selected (selected? x selected)} x]))))
 
 (defelem drop-down
   "Creates a drop-down box using the `<select>` tag."

test/hiccup/form_test.clj

@@ -71,9 +71,23 @@
            "<option value=\"2\">baz</option></optgroup>")
     (select-options [["Foo" [1 2]]] 2)
       (str "<optgroup label=\"Foo\"><option>1</option>"
-           "<option selected=\"selected\">2</option></optgroup>")))
-
-
+           "<option selected=\"selected\">2</option></optgroup>")
+    (select-options ["foo" "bar" "baz"] ["foo" "bar"])
+      (str "<option selected=\"selected\">foo</option>"
+           "<option selected=\"selected\">bar</option>"
+           "<option>baz</option>")
+    (select-options ["foo" "bar" "baz"] '("foo" "bar"))
+      (str "<option selected=\"selected\">foo</option>"
+           "<option selected=\"selected\">bar</option>"
+           "<option>baz</option>")
+    (select-options [["Foo" 1] ["Bar" 2] ["Baz" 3]] [1 3])
+      (str "<option selected=\"selected\" value=\"1\">Foo</option>"
+           "<option value=\"2\">Bar</option>"
+           "<option selected=\"selected\" value=\"3\">Baz</option>")
+    (select-options ["foo" "bar" "baz"] #(= \b (nth % 0)))
+      (str "<option>foo</option>"
+           "<option selected=\"selected\">bar</option>"
+           "<option selected=\"selected\">baz</option>")))
 
 (deftest test-drop-down
   (let [options ["op1" "op2"]