Prepare groundwork for solution to #558 and others

* yasnippet.el (yas-good-grace): Overhaul docstring.
(yas--eval-for-string): Renamed from `yas--eval-lisp'. Redesigned.
(yas--handle-error): Removed.
(yas--eval-for-effect): Renamed from `yas--eval-lisp-no-saves'. Redesigned.
(yas-define-snippets): Update docstring.
(yas-throw): Don't throw, just signal.
(yas--snippet): Add `exit-hook' field.
(yas-next-field, yas--save-backquotes, yas--apply-transform): Use `yas-eval-for-string'.
(yas--safely-run-hook): Renamed from `yas--safely-run-hooks'. Redesigned.
(yas--check-commit-snippet): Collect snippet hooks and run them.
(yas-expand-snippet): Use `yas--eval-for-effect'.
(yas--snippet-create): Stamp the exit-hook variable in the snippet-local hook.

yasnippet.el

@@ -350,9 +350,16 @@ can be overridden on a per-snippet basis."
   :group 'yasnippet)
 
 (defcustom yas-good-grace t
-  "If non-nil, don't raise errors in inline elisp evaluation.
+  "If non-nil, don't raise errors in elisp evaluation 
 
-An error string \"[yas] error\" is returned instead."
+This affects both the inline elisp in snippets and the hook
+variables such as `yas-after-exit-snippet-hook'.
+
+If this variable's value is `inline', an error string \"[yas]
+error\" is returned instead of raising the error. If this
+variable's value is `hooks', a message is output to according to
+`yas-verbosity-level'. If this variable's value is t, both are
+active."
   :type 'boolean
   :group 'yasnippet)
 
@@ -1293,33 +1300,27 @@ yasnippet keeps a list of modes like this to help the judgment."
   (or (fboundp mode)
       (find mode yas--known-modes)))
 
-(defun yas--handle-error (err)
+(defun yas--eval-for-string (form)
-  "Handle error depending on value of `yas-good-grace'."
-  (let ((msg (yas--format "elisp error: %s" (error-message-string err))))
-    (if yas-good-grace msg
-      (error "%s" msg))))
-
-(defun yas--eval-lisp (form)
   "Evaluate FORM and convert the result to string."
-  (let ((retval (catch 'yas--exception
+  (let ((eval-saving-stuff
-                  (condition-case err
+         (lambda (form)
            (save-excursion
              (save-restriction
                (save-match-data
                  (widen)
                  (let ((result (eval form)))
                    (when result
-                                (format "%s" result))))))
+                     (format "%s" result)))))))))
-                    (error (yas--handle-error err))))))
+    (if (memq yas-good-grace '(t inline))
-    (when (and (consp retval)
+        (condition-case oops
-               (eq 'yas--exception (car retval)))
+            (funcall eval-saving-stuff form)
-      (error (cdr retval)))
+          (yas--exception (signal 'yas-exception (cdr oops)))
-    retval))
+          (error (cdr oops)))
+      (funcall eval-saving-stuff form))))
 
-(defun yas--eval-lisp-no-saves (form)
+(defun yas--eval-for-effect (form)
-  (condition-case err
+  ;; FIXME: simulating lexical-binding.
-      (eval form)
+  (yas--safely-run-hook `(lambda () ,form)))
-    (error (message "%s" (yas--handle-error err)))))
 
 (defun yas--read-lisp (string &optional nil-on-error)
   "Read STRING as a elisp expression and return it.
@@ -1673,7 +1674,7 @@ this is a snippet or a snippet-command.
 
 CONDITION, EXPAND-ENV and KEYBINDING are Lisp forms, they have
 been `yas--read-lisp'-ed and will eventually be
-`yas--eval-lisp'-ed.
+`yas--eval-for-string'-ed.
 
 The remaining elements are strings.
 
@@ -2835,7 +2836,8 @@ The last element of POSSIBILITIES may be a list of strings."
 
 (defun yas-throw (text)
   "Throw a yas--exception with TEXT as the reason."
-  (throw 'yas--exception (cons 'yas--exception text)))
+  (signal 'yas--exception text))
+(put 'yas--exception 'error-conditions '(error yas--exception))
 
 (defun yas-verify-value (possibilities)
   "Verify that the current field value is in POSSIBILITIES.
@@ -2920,7 +2922,8 @@ Use this in primary and mirror transformations to tget."
   ;; stacked expansion: the `previous-active-field' slot saves the
   ;; active field where the child expansion took place
   previous-active-field
-  force-exit)
+  force-exit
+  exit-hook)
 
 (defstruct (yas--field (:constructor yas--make-field (number start end parent-field)))
   "A field.
@@ -2979,7 +2982,7 @@ string iff EMPTY-ON-NIL-P is true."
          (transformed (and transform
                            (save-excursion
                              (goto-char start-point)
-                             (let ((ret (yas--eval-lisp transform)))
+                             (let ((ret (yas--eval-for-string transform)))
                                (or ret (and empty-on-nil-p "")))))))
     transformed))
 
@@ -3095,7 +3098,7 @@ If there's none, exit the snippet."
              (yas-text (yas--field-text-for-display active-field))
              (yas-modified-p (yas--field-modified-p active-field)))
         ;; primary field transform: exit call to field-transform
-        (yas--eval-lisp (yas--field-transform active-field))))
+        (yas--eval-for-string (yas--field-transform active-field))))
     ;; Now actually move...
     (cond ((and target-pos (>= target-pos (length live-fields)))
            (yas-exit-snippet snippet))
@@ -3221,11 +3224,16 @@ This renders the snippet as ordinary text."
 
   (yas--message 3 "Snippet %s exited." (yas--snippet-id snippet)))
 
-(defun yas--safely-run-hooks (hook-var)
+(defun yas--safely-run-hook (hook)
+  (let ((run-the-hook (lambda (hook) (funcall hook))))
+    (if (memq yas-good-grace '(t hooks))
+        (funcall run-the-hook hook)
       (condition-case error
-      (run-hooks hook-var)
+          (funcall run-the-hook hook)
         (error
-     (yas--message 3 "%s error: %s" hook-var (error-message-string error)))))
+         (yas--message 3 "Error running %s: %s"
+                       (if (symbolp hook) hook "a hook")
+                       (error-message-string error)))))))
 
 
 (defun yas--check-commit-snippet ()
@@ -3234,10 +3242,11 @@ This renders the snippet as ordinary text."
 If so cleans up the whole snippet up."
   (let* ((snippets (yas--snippets-at-point 'all-snippets))
          (snippets-left snippets)
-         (snippet-exit-transform))
+         (snippet-exit-transform)
+         (exit-hooks))
     (dolist (snippet snippets)
-      (let ((active-field (yas--snippet-active-field snippet)))
       (setq snippet-exit-transform (yas--snippet-force-exit snippet))
+      (let ((active-field (yas--snippet-active-field snippet)))
         (cond ((or snippet-exit-transform
                    (not (and active-field (yas--field-contains-point-p active-field))))
                (setq snippets-left (delete snippet snippets-left))
@@ -3255,11 +3264,14 @@ If so cleans up the whole snippet up."
                  (yas--move-to-field snippet active-field)
                  (yas--update-mirrors snippet)))
               (t
-               nil))))
+               nil)))
+      (let ((exit-hook (yas--snippet-exit-hook snippet)))
+        (when exit-hook
+          (push exit-hook exit-hooks))))
     (unless (or (null snippets) snippets-left)
       (if snippet-exit-transform
-          (yas--eval-lisp-no-saves snippet-exit-transform))
+          (yas--eval-for-effect snippet-exit-transform)))
-      (yas--safely-run-hooks 'yas-after-exit-snippet-hook))))
+    (mapcar #'yas--safely-run-hook exit-hooks)))
 
 ;; Apropos markers-to-points:
 ;;
@@ -3546,7 +3558,7 @@ considered when expanding the snippet."
     (cond ((listp content)
            ;; x) This is a snippet-command
            ;;
-           (yas--eval-lisp-no-saves content))
+           (yas--eval-for-effect content))
           (t
            ;; x) This is a snippet-snippet :-)
            ;;
@@ -3668,6 +3680,12 @@ Returns the newly created snippet."
       ;; Move to end
       (goto-char (point-max))
 
+      ;; The snippet's exit hook is set to the current value of the
+      ;; exit hook variable, unless it already holds something.
+      ;;
+      (unless (yas--snippet-exit-hook snippet)
+        (setf (yas--snippet-exit-hook snippet)
+              yas-after-exit-snippet-hook))
       snippet)))
 
 
@@ -3994,7 +4012,7 @@ with their evaluated value into `yas--backquote-markers-and-strings'."
     (let ((current-string (match-string-no-properties 1)) transformed)
       (save-restriction (widen)
                         (delete-region (match-beginning 0) (match-end 0)))
-      (setq transformed (yas--eval-lisp (yas--read-lisp (yas--restore-escapes current-string '(?`)))))
+      (setq transformed (yas--eval-for-string (yas--read-lisp (yas--restore-escapes current-string '(?`)))))
       (goto-char (match-beginning 0))
       (when transformed
         (let ((marker (make-marker)))