*enclose-printer-errors*

This variable controls how to handle errors which occur during printing.

Normal printing follows all of the rules of condition-system handling; and if any handled or unhandled condition results in a call to the debugger, a new break level is established, and the user is expected to deal with the low-level printer error (which usually results from a bad print-method or a trashed object).

But normally, a user doesn't want to debug a bad object or print method, because it is usually part of a larger problem which is either more important or is the cause of the bad printing. So, when this variable is true, those errors result in a printing of an enclosed object that describes the problem. Formerly, the object looked like #<Printer Error>, but now, the object is described including a printing of the condition, for example,

user(1): (format nil "~{abc~:}")
"#<Printer Error, obj=#x30000bc5: Insufficient format args>"
user(2): 

*enclose-printer-errors* should always be bound to true whenever objects are being printed, such as in terminal and window output functions like inspectors, debuggers, etc.

*enclose-printer-errors* has a default binding of t. This is a controversial decision, because it could be interpreted as a non-conformance to the ANSI condition handling rules. However, it is set to true by default to protect against unwanted break-level processing in user code, especially when non Allegro CL printing tools are used. If this default is not desired, it can be set to nil.

Copyright (c) 1998-2001, Franz Inc. Berkeley, CA., USA. All rights reserved.
Documentation for Allegro CL version 6.1 update # 1. This page was not revised.
Created 2001.12.15.