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, 3 insertions, 1 deletions

diff --git a/parser.c b/parser.c
index ccff6a3f..9cdb0a85 100644
--- a/parser.c
+++ b/parser.c
@@ -57,6 +57,7 @@
 
 val parser_s, unique_s;
 val listener_hist_len_s, listener_multi_line_p_s;
+val intr_s;
 
 static val stream_parser_hash;
 
@@ -587,7 +588,7 @@ static char *provide_atom(lino_t *l, const char *str, int n, void *ctx)
 
 static val repl_intr(val signo, val async_p)
 {
-  uw_throw(error_s, lit("intr"));
+  uw_throw(intr_s, lit("intr"));
 }
 
 static val read_eval_ret_last(val env, val in_stream, val out_stream)
@@ -793,6 +794,7 @@ val parser_eof(val parser)
 void parse_init(void)
 {
   parser_s = intern(lit("parser"), user_package);
+  intr_s = intern(lit("intr"), user_package);
   listener_hist_len_s = intern(lit("*listener-hist-len*"), user_package);
   listener_multi_line_p_s = intern(lit("*listener-multi-line-p*"), user_package);
   unique_s = gensym(nil);