Allow head-hint to be dynamic

Instead of only a string like before, the head-hint can be anything that
evaluates to a string.

Example:

(defhydra hydra-test (:columns 2)
  "Test"
  ("j" next-line (format-time-string "%H:%M:%S" (current-time)))
  ("k" previous-line (format-time-string "%H:%M:%S" (current-time)))
  ("h" backward-char (format-time-string "%H:%M:%S" (current-time)))
  ("l" forward-char (format-time-string "%H:%M:%S" (current-time))))

Pressing "hjkl" will refresh the hint, and thus update the current time.
Note that the hint needs to evaluate to a string at both compile-time
and run-time. The column formatting depends on the compile-time result.

Fixes #160

hydra.el

@@ -452,6 +452,11 @@ Return DEFAULT if PROP is not in H."
   (format (format "%%%ds: %%%ds" key-width (- -1 doc-width))
           key doc))
 
+(defun hydra--to-string (x)
+  (if (stringp x)
+      x
+    (eval x)))
+
 (defun hydra--hint (body heads)
   "Generate a hint for the echo area.
 BODY, and HEADS are parameters to `defhydra'."
@@ -467,41 +472,48 @@ BODY, and HEADS are parameters to `defhydra'."
              (cons (cadr h)
                    (cons pstr (cl-caddr h)))
              alist)))))
-
     (let ((keys (nreverse (mapcar #'cdr alist)))
-          (n-cols (plist-get (cddr body) :columns)))
+          (n-cols (plist-get (cddr body) :columns))
-      (if n-cols
+          res)
-          (let ((n-rows (1+ (/ (length keys) n-cols)))
+      (setq res
-                (max-key-len (apply #'max (mapcar (lambda (x) (length (car x))) keys)))
+            (if n-cols
-                (max-doc-len (apply #'max (mapcar (lambda (x) (length (cdr x))) keys))))
+                (let ((n-rows (1+ (/ (length keys) n-cols)))
-            (concat
+                      (max-key-len (apply #'max (mapcar (lambda (x) (length (car x))) keys)))
-             "\n"
+                      (max-doc-len (apply #'max (mapcar (lambda (x)
-             (mapconcat #'identity
+                                                          (length (hydra--to-string (cdr x)))) keys))))
-                        (mapcar
+                  `(concat
-                         (lambda (x)
+                    "\n"
-                           (mapconcat
+                    (mapconcat #'identity
-                            (lambda (y)
+                               (mapcar
-                              (and y
+                                (lambda (x)
-                                   (funcall hydra-key-doc-function
+                                  (mapconcat
-                                            (car y)
+                                   (lambda (y)
-                                            max-key-len
+                                     (and y
-                                            (cdr y)
+                                          (funcall hydra-key-doc-function
-                                            max-doc-len))) x ""))
+                                                   (car y)
-                         (hydra--matrix keys n-cols n-rows))
+                                                   ,max-key-len
-                        "\n")))
+                                                   (hydra--to-string (cdr y))
-
+                                                   ,max-doc-len))) x ""))
-
+                                ',(hydra--matrix keys n-cols n-rows))
-        (concat
+                               "\n")))
-         (mapconcat
+
-          (lambda (x)
+
-            (format
+              `(concat
-             (if (> (length (cdr x)) 0)
+                (mapconcat
-                 (concat hydra-head-format (cdr x))
+                 (lambda (x)
-               "%s")
+                   (let ((str (hydra--to-string (cdr x))))
-             (car x)))
+                     (format
-          keys
+                      (if (> (length str) 0)
-          ", ")
+                          (concat hydra-head-format str)
-         (if keys "." ""))))))
+                        "%s")
+                      (car x))))
+                 ',keys
+                 ", ")
+                ,(if keys "." ""))))
+      (if (cl-every #'stringp
+                    (mapcar 'cddr alist))
+          (eval res)
+        res))))
 
 (defvar hydra-fontify-head-function nil
   "Possible replacement for `hydra-fontify-head-default'.")
@@ -612,11 +624,14 @@ The expressions can be auto-expanded according to NAME."
     (if (eq ?\n (aref docstring 0))
         `(concat (format ,(substring docstring 1) ,@(nreverse varlist))
                  ,rest)
-      `(format ,(replace-regexp-in-string
+      (let ((r `(replace-regexp-in-string
                  " +$" ""
-                 (concat docstring ": "
+                 (concat ,docstring ": "
                          (replace-regexp-in-string
-                          "\\(%\\)" "\\1\\1" rest)))))))
+                          "\\(%\\)" "\\1\\1" ,rest)))))
+        (if (stringp rest)
+            `(format ,(eval r))
+          `(format ,r))))))
 
 (defun hydra--complain (format-string &rest args)
   "Forward to (`message' FORMAT-STRING ARGS) unless `hydra-verbose' is nil."
@@ -964,7 +979,8 @@ result of `defhydra'."
                   (t
                    (let ((hint (cl-caddr h)))
                      (unless (or (null hint)
-                                 (stringp hint))
+                                 (stringp hint)
+                                 (stringp (eval hint)))
                        (setcdr (cdr h) (cons
                                         (hydra-plist-get-default body-plist :hint "")
                                         (cddr h)))))