dpans/concept-exits.tex

% -*- Mode: TeX -*-
%% Exit Extents

% This text was originally duplicated in each of GO, RETURN-FROM,
% and THROW.  I thought it would be less redundant to centralize it here.
% --sjl 7 Mar 92


\issue{EXIT-EXTENT:MINIMAL}
When a transfer of control is initiated by \specref{go}, 
\specref{return-from}, or \specref{throw}
the following events occur in order to accomplish the transfer of control.
Note that for \specref{go}, 
the \term{exit point} is the \term{form} within the \specref{tagbody}
that is being executed at the time the \specref{go} is performed;
for \specref{return-from},
the \term{exit point} is the corresponding 
\specref{block} \term{form};
and for \specref{throw},
the \term{exit point} is the corresponding 
\specref{catch} \term{form}. 

\beginlist
\itemitem{1.} 
 Intervening \term{exit points} are ``abandoned''
 (\ie their \term{extent} ends 
      and it is no longer valid to attempt to transfer control through them).

\itemitem{2.} 
 The cleanup clauses of any intervening \specref{unwind-protect} clauses
 are evaluated.
 
\itemitem{3.} 
% added condition handlers and restarts  -- sjl 7 Mar 92
 Intervening dynamic \term{bindings} of \declref{special} variables,
 \term{catch tags}, \term{condition handlers}, and \term{restarts}
 are undone.
 
\itemitem{4.} 
 The \term{extent} of the \term{exit point} being invoked ends,
 and control is passed to the target.
\endlist 
 
The extent of an exit being ``abandoned'' because it is being passed over
ends as soon as the transfer of control is initiated. That is,
event 1 occurs at the beginning of the initiation of the transfer of
control. 
The consequences are undefined if an attempt is made to transfer control 
to an \term{exit point} whose \term{dynamic extent} has ended.
 
%Moon had me add the part about "interleaved" -kmp 13-Feb-92
Events 2 and 3 are actually performed interleaved, in the order
corresponding to the reverse order in which they were established.
The effect of this is that the cleanup clauses of an \specref{unwind-protect}
see the same dynamic \term{bindings} 
of variables and \term{catch tags} as were
visible when the \specref{unwind-protect} was entered.
 
Event 4 occurs at the end of the transfer of control.
 
\endissue{EXIT-EXTENT:MINIMAL}