@let: allow trailing comma.HEADmaster

We allow forms like @let(x,) and @let(x, y=1,) with the
trailing comma. This is helpful for code generators and macros,
which can then treat temporary variables as comma-terminated
items.

* awkgram.y (let_var_list_opt): Add a production which matches a
trailing comma after let_var_list.

* test/let1.awk, test/let1.ok: Test cases.

* doc/gawkt.texi, doc/gawktexi.in:  Trailing comma documented.

* awkgram.c: Regenerated with Bison 3.8.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/let1.awk b/test/let1.awk
index 413b6e1e..268222df 100644
--- a/test/let1.awk
+++ b/test/let1.awk
@@ -157,3 +157,17 @@ BEGIN {
   @let (x = 1, y = 2) { print "b6", x, y }
   @let (x = 1, y = 2, z = 3) { print "b6", x, y, z }
 }
+
+function f6()
+{
+  @let (x = 1) { print "f6", x }
+  @let (x = 1, y = 2,) { print "f6", x, y }
+  @let (x = 1, y = 2, z = 3,) { print "f6", x, y, z }
+}
+
+BEGIN {
+  f6()
+  @let (x = 1) { print "b7", x }
+  @let (x = 1, y = 2,) { print "b7", x, y }
+  @let (x = 1, y = 2, z = 3,) { print "b7", x, y, z }
+}