dpans/concept-destruction.tex

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

\issue{DESTRUCTIVE-OPERATIONS:SPECIFY}
\beginsubsection{Modification of Literal Objects}

The consequences are undefined if \term{literal} \term{objects} 
are destructively modified.  For this purpose, the following operations 
are considered \term{destructive}:

\beginlist

\itemitem{\typeref{random-state}}

% Using it as an argument to the function RANDOM.

Using it as an \term{argument} to \thefunction{random}.

\itemitem{\typeref{cons}}

% Altering or destructively modifying the CAR or CDR of the cons.

Changing the \term{car}\meaning{1} or \term{cdr}\meaning{1} of the \term{cons},
or performing a \term{destructive} operation on an \term{object} which is either
the \term{car}\meaning{2} or the \term{cdr}\meaning{2} of the \term{cons}.

\itemitem{\typeref{array}}

% Altering or destructively modifying any array element;
% changing the fill pointer, dimensions, or displacement of
% the array (regardless of whether the array is actually adjustable);
% or altering the contents of any array that is displaced to this array
% or that otherwise shares its contents with this array.

Storing a new value into some element of the \term{array},
or performing a \term{destructive} operation 
on an \term{object} that is already such an \term{element}.

Changing the \term{fill pointer}, \term{dimensions}, or displacement of
the \term{array} (regardless of whether the \term{array} is \term{actually adjustable}).

Performing a \term{destructive} operation on another \term{array} 
that is displaced to the \term{array} or that otherwise shares its contents
with the \term{array}.

\itemitem{\typeref{hash-table}}

% Altering or destructively modifying any key or its corresponding
% value, or adding or removing entries from the hash table.

Performing a \term{destructive} operation on any \term{key}.

Storing a new \term{value}\meaning{4} for any \term{key},
or performing a \term{destructive} operation 
on any \term{object} that is such a \term{value}.

Adding or removing entries from the \term{hash table}.

\itemitem{\typeref{structure-object}}

% Altering or destructively modifying the contents of any slot.

Storing a new value into any slot,
or performing a \term{destructive} operation on an \term{object} 
that is the value of some slot.

\itemitem{\typeref{standard-object}}

% Altering or destructively modifying the contents of any slot, or
% changing the class of the object.

Storing a new value into any slot,
or performing a \term{destructive} operation on an \term{object} 
that is the value of some slot.

Changing the class of the \term{object} (\eg using \thefunction{change-class}).

\itemitem{\typeref{readtable}}

% Altering the readtable-case; altering the syntax type of any
% character in this readtable; altering the reader macro function
% associated with any character in this readtable; or altering the
% reader macro functions associated with characters defined as 
% dispatching macro characters in this readtable.

Altering the \term{readtable case}.

Altering the syntax type of any character in this readtable.

Altering the \term{reader macro function} associated with any \term{character}
in the \term{readtable}, or altering the \term{reader macro functions}
associated with \term{characters} defined as \term{dispatching macro characters}
in the \term{readtable}.

\itemitem{\typeref{stream}}

% Performing I/O operations on the stream, or closing the stream.

Performing I/O operations on the \term{stream},
or \term{closing} the \term{stream}.

\itemitem{All other standardized types}

 [This category includes, for example, \typeref{character},
                                      \typeref{condition},
                                      \typeref{function},
                                      \typeref{method-combination},
                                      \typeref{method},
                                      \typeref{number},
                                      \typeref{package},
                                      \typeref{pathname},
                                      \typeref{restart},
                                  and \typeref{symbol}.]

% (including number, character, symbol, package, pathname, function,
%  method, method-combination, condition, restart)
% There are no destructive operations defined on these data types.

There are no \term{standardized} \term{destructive} operations
defined on \term{objects} of these \term{types}.

\endlist

\endsubsection%{Modification of Literal Objects}
\endissue{DESTRUCTIVE-OPERATIONS:SPECIFY}

\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}