From 9274c3dd4058b3f63ba97570fc2f1598debbc03d Mon Sep 17 00:00:00 2001
From: Andy Wingo <wingo@pobox.com>
Date: Tue, 29 Dec 2009 21:15:08 +0100
Subject: [PATCH] ~:@y allows for truncation to an absolute maximum width

* module/ice-9/format.scm (format): Allow ~:@y to interpret the width as
  the maximum width, inclusive of whatever else has already been output.
* doc/ref/misc-modules.texi (Formatted Output): Document this.
---
 doc/ref/misc-modules.texi |  5 +++++
 module/ice-9/format.scm   | 16 +++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/doc/ref/misc-modules.texi b/doc/ref/misc-modules.texi
index 1449292cd..a6bb6c3cb 100644
--- a/doc/ref/misc-modules.texi
+++ b/doc/ref/misc-modules.texi
@@ -648,6 +648,11 @@ necessary.
 within @var{width} columns (79 by default), on a single line. The
 output will be truncated if necessary.
 
+@nicode{~:@@y} is like @nicode{~@@y}, except the @var{width} parameter
+is interpreted to be the maximum column to which to output. That is to
+say, if you are at column 10, and @nicode{~60:@@y} is seen, the datum
+will be truncated to 50 columns.
+
 @item @nicode{~?}
 @itemx @nicode{~k}
 Sub-format.  No parameters.
diff --git a/module/ice-9/format.scm b/module/ice-9/format.scm
index 2d12dbf04..133f8e4ef 100644
--- a/module/ice-9/format.scm
+++ b/module/ice-9/format.scm
@@ -487,13 +487,23 @@
                                         (car params)
                                         79)))
                          (case modifier
-                           ((colon colon-at)
-                            (format:error "illegal modifier in ~~?"))
                            ((at)
                             (format:out-str
                              (with-output-to-string 
                                (lambda ()
-                                 (truncated-print (next-arg) #:width width)))))
+                                 (truncated-print (next-arg)
+                                                  #:width width)))))
+                           ((colon-at)
+                            (format:out-str
+                             (with-output-to-string 
+                               (lambda ()
+                                 (truncated-print (next-arg)
+                                                  #:width
+                                                  (max (- width
+                                                          format:output-col)
+                                                       1))))))
+                           ((colon)
+                            (format:error "illegal modifier in ~~?"))
                            (else
                             (pretty-print (next-arg) format:port
                                           #:width width)
-- 
2.20.1