Default the length argument of truncate-stream.

* stream.c (truncate_stream): If the len argument is missing,
default to the current position, obtained by using the seek
operation.
(stream_init): Fix up registration of truncate-stream for one
optional argument.

* txr.1: Documentation of truncate-stream updated.

2 files changed, 19 insertions, 2 deletions

diff --git a/stream.c b/stream.c
index 9f92dd97..2af67db2 100644
--- a/stream.c
+++ b/stream.c
@@ -3600,6 +3600,8 @@ val seek_stream(val stream, val offset, val whence)
 val truncate_stream(val stream, val len)
 {
   struct strm_ops *ops = coerce(struct strm_ops *, cobj_ops(stream, stream_s));
+  if (missingp(len))
+    len = ops->seek(stream, zero, strm_cur);
   return ops->truncate(stream, len);
 }
 
@@ -4503,7 +4505,7 @@ void stream_init(void)
   reg_fun(intern(lit("fill-buf"), user_package), func_n3o(fill_buf, 1));
   reg_fun(intern(lit("flush-stream"), user_package), func_n1o(flush_stream, 0));
   reg_fun(intern(lit("seek-stream"), user_package), func_n3(seek_stream));
-  reg_fun(intern(lit("truncate-stream"), user_package), func_n2(truncate_stream));
+  reg_fun(intern(lit("truncate-stream"), user_package), func_n2o(truncate_stream, 1));
   reg_fun(intern(lit("streamp"), user_package), func_n1(streamp));
   reg_fun(intern(lit("real-time-stream-p"), user_package), func_n1(real_time_stream_p));
   reg_fun(intern(lit("stream-set-prop"), user_package), func_n3(stream_set_prop));
diff --git a/txr.1 b/txr.1
index b228334f..911143dd 100644
--- a/txr.1
+++ b/txr.1
@@ -42537,7 +42537,7 @@ On failure, it throws an exception of type
 
 .coNP Function @ truncate-stream
 .synb
-.mets (truncate-stream < stream << length )
+.mets (truncate-stream < stream <> [ length ])
 .syne
 .desc
 The
@@ -42550,6 +42550,21 @@ bytes.
 
 The stream must be a file stream, and must be open for writing.
 
+If
+.meta length
+is omitted, then it defaults to the current position, retrieved
+as if by invoking the
+.code seek-stream
+with an
+.meta offset
+argument of zero and
+.meta whence
+argument of
+.codn :from-current .
+Hence, after the
+.code truncate-stream
+operation, that position is one byte past the end of the file.
+
 .coNP Functions @ stream-get-prop and @ stream-set-prop
 .synb
 .mets (stream-get-prop < stream << indicator )