Fix handling of -am-ready-mbox values

0ee97e9 (Explicitly specify --patch-format in git-am calls,
2020-08-09) made it possible for a piem-am-ready-mbox-functions member
to specify the format of the mbox by returning (FUNCTION FORMAT).  If
FUNCTION is returned, then mboxrd is supposed to be taken as the
default format.  The handling is broken, though, because
piem-am-ready-mbox tries to detect the (FUNCTION FORMAT) form with
listp, but that of course also returns true when the return value is
simply a function.

Instead, check to see whether the element matches a valid format
value.  Switch from (FUNCTION FORMAT) to (FUNCTION . FORMAT) to make
it more convenient to pull out FORMAT with cdr-safe.

Message-Id: <877du5c1nz.fsf@kyleam.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/piem.el b/piem.el
index e70c259..ece30bd 100644
--- a/piem.el
+++ b/piem.el
@@ -99,7 +99,7 @@ mbox's contents (in mboxrd format) in the current buffer."
 The functions are called with no arguments.  If a function knows
 how to create an mbox, it should return a function that takes no
 arguments and inserts the mbox's contents in the current buffer.
-The return value can also be (FUNCTION FORMAT), where FORMAT is
+The return value can also be (FUNCTION . FORMAT), where FORMAT is
 either \"mbox\" or \"mboxrd\" and maps to the --patch-format
 value passed to `git am'.  If unspecified, \"mboxrd\" is used."
   :type 'hook)
@@ -337,13 +337,16 @@ intended to be used by libraries implementing a function for
 
 (defun piem-am-ready-mbox ()
   "Generate a buffer containing an am-ready mbox.
-The return value is (BUFFER FORMAT), where FORMAT is either
+The return value is (BUFFER . FORMAT), where FORMAT is either
 \"mbox\" or \"mboxrd\".  Callers are responsible for killing the
 buffer."
   (when-let ((res (run-hook-with-args-until-success
                    'piem-am-ready-mbox-functions)))
     (pcase-let ((buffer (generate-new-buffer " *piem am-ready mbox*"))
-                (`(,fn ,format) (if (listp res) res (list res "mboxrd")))
+                (`(,fn . ,format)
+                 (if (member (cdr-safe res) '("mbox" "mboxrd"))
+                     res
+                   (cons res "mboxrd")))
                 (mid (and piem-add-message-id-header (piem-mid)))
                 (has-content nil))
       (with-current-buffer buffer
@@ -353,7 +356,7 @@ buffer."
           (goto-char (point-min))
           (piem--insert-message-id-header mid)))
       (if has-content
-          (list buffer format)
+          (cons buffer format)
         (kill-buffer buffer)
         nil))))
 
@@ -546,7 +549,7 @@ for a list of possible properties).
 If CODEREPO is given, switch to this directory before calling
 `git am'."
   (interactive
-   (pcase-let ((`(,mbox ,format)
+   (pcase-let ((`(,mbox . ,format)
                 (or (piem-am-ready-mbox)
                     (user-error
                      "Could not find am-ready mbox for current buffer"))))