@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.

2 files changed, 12 insertions, 2 deletions

diff --git a/doc/gawk.texi b/doc/gawk.texi
index dea485c6..2535e45a 100644
--- a/doc/gawk.texi
+++ b/doc/gawk.texi
@@ -21658,7 +21658,7 @@ statement.
 
 The @code{@@let} statement is introduced by the @code{@@} symbol
 followed by the special keyword @code{let}. These tokens are
-then followed by a comma-separated list of variable declarators,
+then followed by a comma-separated or comma-terminated list of variable declarators,
 enclosed in parentheses. After the parentheses comes a required statement,
 The list of variables may be empty.
 
@@ -21809,6 +21809,11 @@ The @code{print} statement is then executed in the scope of the
 rightmost @code{x}.  The initializing expressions @code{x + 1}
 have the previous @code{x} still in scope.
 
+The @code{@@let} statement allows a trailing comma after the last
+variable declarator. Effectively, the declarators can be regarded
+as either comma-separated or comma-terminated. Comma termination is
+easier to deal with in macro preprocessing and code generation.
+
 @node Pass By Value/Reference
 @subsubsection Passing Function Arguments by Value Or by Reference
 
diff --git a/doc/gawktexi.in b/doc/gawktexi.in
index 651bd8d2..0bb4f4dc 100644
--- a/doc/gawktexi.in
+++ b/doc/gawktexi.in
@@ -20570,7 +20570,7 @@ statement.
 
 The @code{@@let} statement is introduced by the @code{@@} symbol
 followed by the special keyword @code{let}. These tokens are
-then followed by a comma-separated list of variable declarators,
+then followed by a comma-separated or comma-terminated list of variable declarators,
 enclosed in parentheses. After the parentheses comes a required statement,
 The list of variables may be empty.
 
@@ -20721,6 +20721,11 @@ The @code{print} statement is then executed in the scope of the
 rightmost @code{x}.  The initializing expressions @code{x + 1}
 have the previous @code{x} still in scope.
 
+The @code{@@let} statement allows a trailing comma after the last
+variable declarator. Effectively, the declarators can be regarded
+as either comma-separated or comma-terminated. Comma termination is
+easier to deal with in macro preprocessing and code generation.
+
 @node Pass By Value/Reference
 @subsubsection Passing Function Arguments by Value Or by Reference