New way of handling exceptions without unwinding.

* eval.c (handler_bind_s): New symbol variable.
(op_handler_bind): New static function.
(do_expand): Traverse handler-bind forms.
(eval_init): Initialize handler_bind_s variable and register handler-bind
operator.

* lisplib.c (except_set_entries, except_instantiate): New functions.
(lisplib_init): Register new functions in dl_table.

* parser.c (intr_s): New symbol variable.
(repl_intr): Throw exception of type intr, rather than error.
This way we can interrupt accidental exception handling loops involving
exceptions derived from error.
(parse_init): Initialize intr_s.

* share/txr/stdlib/except.tl: New file, defines handle macro.

* unwind.c (uw_push_handler): New function.
(invoke_handler): New static function.
(uw_throw): Search loop looks for and processes handlers in addition to
catches.

* unwind.h (uw_frtype_t): New enum member, UW_HANDLE.
(struct uw_catch): Move member visible so it
is in the same position as in struct uw_handler.
(struct uw_handler): New struct type.
(union uw_frame): New member ha of type struct uw_handler.
(uw_push_handler): Function declared.

* txr.1: Added introductory paragraphs to Exception Handling section.
Documented handler-bind and handle. Some minor errors corrected.

1 files changed, 45 insertions, 0 deletions

diff --git a/share/txr/stdlib/except.tl b/share/txr/stdlib/except.tl
new file mode 100644
index 00000000..e710cacd
--- /dev/null
+++ b/share/txr/stdlib/except.tl
@@ -0,0 +1,45 @@
+;; Copyright 2015
+;; Kaz Kylheku <kaz@kylheku.com>
+;; Vancouver, Canada
+;; All rights reserved.
+;;
+;; Redistribution of this software in source and binary forms, with or without
+;; modification, is permitted provided that the following two conditions are met.
+;;
+;; Use of this software in any manner constitutes agreement with the disclaimer
+;; which follows the two conditions.
+;;
+;; 1. Redistributions of source code must retain the above copyright
+;;    notice, this list of conditions and the following disclaimer.
+;; 2. Redistributions in binary form must reproduce the above copyright
+;;    notice, this list of conditions and the following disclaimer in
+;;    the documentation and/or other materials provided with the
+;;    distribution.
+;;
+;; THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
+;; WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
+;; MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO EVENT SHALL THE
+;; COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DAMAGES, HOWEVER CAUSED,
+;; AND UNDER ANY THEORY OF LIABILITY, ARISING IN ANY WAY OUT OF THE USE OF THIS
+;; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+(defun sys:handle-bad-syntax (item)
+  (throwf 'eval-error "~s: bad clause syntax: ~s" 'handle item))
+
+(defmacro handle (:whole form try-form . handle-clauses)
+  (let* ((exc-sym (gensym))
+         (exc-args (gensym))
+         (syms-fragments (collect-each ((hc handle-clauses))
+                          (tree-case hc
+                            ((name arglist . body)
+                             (unless (symbolp name)
+                               (sys:handle-bad-syntax hc))
+                             (list name ^(apply (lambda ,arglist ,*body)
+                                                ,exc-sym ,exc-args)))
+                            (else (sys:handle-bad-syntax hc))))))
+    ^(handler-bind (lambda (,exc-sym . ,exc-args)
+                     (cond
+                       ,*(mapcar (aret ^((exception-subtype-p ,exc-sym ',@1) ,@2))
+                                 syms-fragments)))
+                   ,[mapcar car syms-fragments]
+                   ,try-form)))