% -*- Mode: TeX -*-
% Destructive Operations

\beginsubsection{Transfer of Control during a Destructive Operation}

%%% This text used to just apply to SORT.
%%% KMP modified it to generalize from errors to arbitrary transfer 
%%%   of control, since not just errors can cause this. Consider:
%%%   (prog foo ((a (list 1 2 3 4 5 6 7 8 9 10)))
%%%     (sort a #'(lambda (x y) 
%%%                 (if (zerop (random 5)) (return-from foo a) (> x y)))))
%%% Barrett suggested further generalizing it for other the sake of 
%%%   application to other destructive operations.
%%% KMP agreed since it's true that things are ill-defined whether we say 
%%% it or not, so we might as well say it explicitly.
%%%
%%% Original text from SORT:
%%%  %% 14.4.0 8
%%%  Should execution of the \param{key} or the \param{predicate} 
%%%  cause an error,
%%%  the state of the \param{sequence} being sorted is
%%%  undefined.  However, if the error is corrected, the sort will
%%%  proceed correctly. 

Should a transfer of control out of a \term{destructive} operation occur
(\eg due to an error) the state of the \param{object} being modified is
\term{implementation-dependent}.

\beginsubsubsection{Examples of Transfer of Control during a Destructive Operation}

The following examples illustrate some of the many ways in which the
\term{implementation-dependent} nature of the modification can manifest
itself.

\code
 (let ((a (list 2 1 4 3 7 6 'five)))
   (ignore-errors (sort a #'<))
   a)
\EV (1 2 3 4 6 7 FIVE)
\OV (2 1 4 3 7 6 FIVE)
\OV (2)

 (prog foo ((a (list 1 2 3 4 5 6 7 8 9 10)))
   (sort a #'(lambda (x y) (if (zerop (random 5)) (return-from foo a) (> x y)))))
\EV (1 2 3 4 5 6 7 8 9 10)
\OV (3 4 5 6 2 7 8 9 10 1)
\OV (1 2 4 3)
\endcode

\endsubsubsection%{Examples of Transfer of Control during a Destructive Operation}

\endsubsection%{Transfer of Control during a Destructive Operation}