From 144cabede201ace9507473a9c9a9200f475eec1a Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 29 Feb 2016 06:05:51 -0800
Subject: Implement @(next nil).

* match.c (v_next): If argument to @(next) is nil, then
evaluate remaining query in context with no list of files,
and no data.

* txr.1: Documented @(next nil).
---
 match.c | 16 ++++++++++------
 txr.1   | 12 ++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/match.c b/match.c
index 187f7ec9..b3684180 100644
--- a/match.c
+++ b/match.c
@@ -2310,9 +2310,7 @@ static val v_next(match_files_ctx *c)
                       if3(c->data, cons(c->data, c->data_lineno), t));
         return nil;
       }
-    }
-
-    if (source == env_k) {
+    } else if (source == env_k) {
       if (rest(args)) {
         sem_error(specline, lit("(next :env) takes no additional arguments"), nao);
       } else {
@@ -2324,12 +2322,18 @@ static val v_next(match_files_ctx *c)
                       if3(c->data, cons(c->data, c->data_lineno), t));
         return nil;
       }
-    }
-
-    if (consp(source)) {
+    } else if (consp(source)) {
       val sym = car(source);
       if (sym == var_s || sym == expr_s)
         meta = t;
+    } else if (!source) {
+      cons_bind (new_bindings, success,
+                 match_files(mf_all(c->spec, nil, c->bindings, nil, lit("empty"))));
+
+      if (success)
+        return cons(new_bindings,
+                    if3(c->data, cons(c->data, c->data_lineno), t));
+      return nil;
     }
 
     if (opt_compat && opt_compat <= 124)
diff --git a/txr.1 b/txr.1
index 3ece5efb..3b1bf55b 100644
--- a/txr.1
+++ b/txr.1
@@ -3163,6 +3163,7 @@ and takes various arguments, according to these possibilities:
 .mets @(next :env)
 .mets @(next :list << expr )
 .mets @(next :string << expr )
+.mets @(next nil)
 .cble
 
 The lone
@@ -3306,6 +3307,17 @@ the list
 which is not an empty input stream, but a stream consisting of
 one empty line.
 
+The
+.code @(next nil)
+variant indicates that the following subquery is applied to empty data,
+and the list of data sources from the command line is considered empty.
+This directive is useful in front of \*(TX code which doesn't process data
+sources from the command line, but takes command line arguments.
+The
+.code @(next nil)
+incantation absolutely prevents \*(TX from trying to open the
+first command line argument as a data source.
+
 Note that the
 .code @(next)
 directive only redirect the source of input over the scope of subquery in which
-- 
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