txr: support @(if)/@(elif)/@(else) in @(output).

This turns out to be way easier than I thought.

* match.c (do_output_if): New static function.
(do_output): Handle if via do_output_if.

* parser.y (out_if_clause, out_elif_clauses_opt,
out_else_clause_opt): New nonterminal symbols and grammar
rules.
(out_clause): Now produces out_if_clause.
(not_a_clause): Remove ELIF and ELSE; these entries here cause
conflicts now. Here, continue to recognize the Lisp if, which
is distinguished by having at least two arguments.
out_if_clause matches only a one-argument if, and a
no-argumeent one that is diagnosed as erroneous.

* txr.1: Documented.

1 files changed, 14 insertions, 0 deletions

diff --git a/match.c b/match.c
index 69bb374f..13c47a54 100644
--- a/match.c
+++ b/match.c
@@ -2234,6 +2234,18 @@ static void do_repeat(val bindings, val repeat_syntax, val filter, val out)
   }
 }
 
+static void do_output_if(val bindings, val if_syntax, val filter, val out)
+{
+  val args = cdr(if_syntax);
+
+  for (; args; args = cdr(args)) {
+    cons_bind (expr, specs, car(args));
+    if (tleval(args, expr, bindings)) {
+      do_output(bindings, specs, filter, out);
+      return;
+    }
+  }
+}
 
 void do_output(val bindings, val specs, val filter, val out)
 {
@@ -2252,6 +2264,8 @@ void do_output(val bindings, val specs, val filter, val out)
         continue;
       }
 
+      if (sym == if_s) {
+        do_output_if(bindings, first_elem, filter, out);
         continue;
       }
     }