From bb0476aa11a6f4f24c16ddebcaf3917764655f45 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 13 Mar 2017 06:58:26 -0700
Subject: Print method can return : to decline printing.

* struct.c (struct_inst_print): Check return value of print
method call. If it is the colon keyword, then do not return
but rather continue to the regular struct printing code.

* txr.1: Documented.
---
 struct.c |  5 ++---
 txr.1    | 18 +++++++++++++++++-
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/struct.c b/struct.c
index 91e6807c..af2bdaaf 100644
--- a/struct.c
+++ b/struct.c
@@ -1316,9 +1316,8 @@ static void struct_inst_print(val obj, val out, val pretty,
     if (!nullocp(ptr)) {
       if (compat)
         funcall2(deref(ptr), obj, out);
-      else
-        funcall3(deref(ptr), obj, out, pretty);
-      return;
+      else if (funcall3(deref(ptr), obj, out, pretty) != colon_k)
+        return;
     }
   }
 
diff --git a/txr.1 b/txr.1
index 2cb25577..3ec80d72 100644
--- a/txr.1
+++ b/txr.1
@@ -24112,7 +24112,23 @@ is used.
 
 The value returned by the
 .code print
-method is ignored.
+method is significant. If the special keyword symbol
+.code :
+(colon) is returned, then the system will print the object
+in the default way, as if no
+.code print
+method existed: it is understood that the method declined
+the responsibility for printing the object.
+
+If any other value is returned, then it is understood
+that the method
+.code print
+method accepted the responsibility for printing the object,
+and the system consequently will generate into
+.meta stream
+any output output pertaining to
+.metn object 's
+representation.
 
 .coNP Method @ lambda
 .synb
-- 
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