Improve the error API

Define new functions to manipulate error descriptors and
add support for `signal` to *re*signal a previous error.

 * src/eval.c (Fsignal): Make the second arg optional and document
the possibility of passing a whole error descriptor to re-signal it.
(signal_or_quit): Fix a few corner case issues when
DATA is `nil` and ERROR_SYMBOL is an error descriptor.

* lisp/subr.el (error-type-p, error--p, error-type, error-data)
(error-has-type-p, error-slot-value): New function.

* doc/lispref/control.texi (Handling Errors): Prefer "error descriptor"
to "error description".  Use the new single-arg call to `signal` to
re-throw an error.
Document `error-type`, `error-data` and `error-slot-value`.
(Error Symbols): Document the new functions `error-type-p` and
`error-has-type-p`.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/eval.c b/src/eval.c
index 7ca9d761a7e..0b451b2c891 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1871,10 +1871,15 @@ probably_quit (void)
   unbind_to (gc_count, Qnil);
 }
 
-DEFUN ("signal", Fsignal, Ssignal, 2, 2, 0,
+DEFUN ("signal", Fsignal, Ssignal, 1, 2, 0,
        doc: /* Signal an error.  Args are ERROR-SYMBOL and associated DATA.
 This function does not return.
 
+When signaling a new error, the DATA argument is mandatory.
+When re-signaling an error to propagate it to further handlers,
+DATA has to be omitted and the first argument has to be the whole
+error descriptor.
+
 When `noninteractive' is non-nil (in particular, in batch mode), an
 unhandled error calls `kill-emacs', which terminates the Emacs
 session with a non-zero exit code.
@@ -1942,13 +1947,14 @@ signal_or_quit (Lisp_Object error_symbol, Lisp_Object data, bool continuable)
       /* FIXME: 'handler-bind' makes `signal-hook-function' obsolete?  */
       /* FIXME: Here we still "split" the error object
          into its error-symbol and its error-data?  */
-      calln (Vsignal_hook_function, error_symbol, data);
+      calln (Vsignal_hook_function, real_error_symbol,
+             NILP (data) && CONSP (error) ? XCDR (error) : data);
       unbind_to (count, Qnil);
     }
 
   conditions = Fget (real_error_symbol, Qerror_conditions);
   if (NILP (conditions))
-    signal_error ("Invalid error symbol", error_symbol);
+    signal_error ("Invalid error symbol", real_error_symbol);
 
   /* Remember from where signal was called.  Skip over the frame for
      `signal' itself.  If a frame for `error' follows, skip that,