Support for pushing back bytes and characters into streams.

* stream.c (null_ops, stdio_ops, tail_ops, pipe_ops,
string_in_ops, byte_in_ops, string_out_ops, strlist_out_ops,
dir_ops, cat_stream_ops): Structure definition updated
with new initializers for two new virtuals.
(stdio_handle): New member, unget_c.
(snarf_line, stdio_get_char): Handle pushed-back character in
h->unget_c.
(stdio_unget_char, stdio_unget_byte, string_in_unget_char,
byte_in_unget_byte): New static functions.
(make_stdio_stream_common): Initialize unget_c member.
(unget_char, unget_byte): New functions.

* stream.h (struct strm_ops): New virtuals: unget_char and unget_byte.
(unget_char, unget_byte): New functions declared.

* syslog.c (syslog_strm_ops): Two new initializers.

* eval.c (eval_init): Registered unget_char and unget_byte as
intrinsics.

* txr.1: Documented.

1 files changed, 23 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 8f87e428..a6760076 100644
--- a/txr.1
+++ b/txr.1
@@ -10465,6 +10465,29 @@ the two operations will interfere with the UTF-8 decoding.
 These functions return nil when the end of data is reached.  Errors are
 represented as exceptions.
 
+.SS Functions unget-char and unget-byte
+
+.TP
+Syntax:
+
+  (unget-char <char> [<stream>])
+  (unget-byte <byte> [<stream>])
+
+.TP
+Description:
+
+These character put back, into a stream, a character or byte which was
+previously read.  The character or byte must match the one which was most
+recently read. If the <stream> parameter is omitted, then the *stdin*
+stream is used.
+
+If the operation succeeds, the byte or character value is returned.
+A nil return indicates that the operation is unsupported.
+
+Some streams do not support these operations; some support
+only one of them.   In general, if a stream supports get-char,
+it supports unget-char, and likewise for get-byte and unget-byte.
+
 .SS Functions put-string, put-line, put-char and put-byte
 
 .TP