Function: trap
Section: programming/specific
C-Name: trap0
Prototype: DrDEDE
Help: trap({e}, {rec}, seq): try to execute seq, trapping runtime error e (all
 of them if e omitted); sequence rec is executed if the error occurs and
 is the result of the command. THIS FUNCTION IS OBSOLETE: use "IFERR"
Wrapper: (,_,_)
Description:
 (?str,?closure,?closure):gen trap0($1, $2, $3)
Doc: THIS FUNCTION IS OBSOLETE: use \tet{iferr}, which has a nicer and much
 more powerful interface. For compatibility's sake we now describe the
 \emph{obsolete} function \tet{trap}.

 This function tries to
 evaluate \var{seq}, trapping runtime error $e$, that is effectively preventing
 it from aborting computations in the usual way; the recovery sequence
 \var{rec} is executed if the error occurs and the evaluation of \var{rec}
 becomes the result of the command. If $e$ is omitted, all exceptions are
 trapped. See \secref{se:errorrec} for an introduction to error recovery
 under \kbd{gp}.

 \bprog
 ? \\@com trap division by 0
 ? inv(x) = trap (e_INV, INFINITY, 1/x)
 ? inv(2)
 %1 = 1/2
 ? inv(0)
 %2 = INFINITY
 @eprog\noindent
 Note that \var{seq} is effectively evaluated up to the point that produced
 the error, and the recovery sequence is evaluated starting from that same
 context, it does not "undo" whatever happened in the other branch (restore
 the evaluation context):
 \bprog
 ? x = 1; trap (, /* recover: */ x, /* try: */ x = 0; 1/x)
 %1 = 0
 @eprog

 \misctitle{Note} The interface is currently not adequate for trapping
 individual exceptions. In the current version \vers, the following keywords
 are recognized, but the name list will be expanded and changed in the
 future (all library mode errors can be trapped: it's a matter of defining
 the keywords to \kbd{gp}):

 \kbd{e\_ALARM}: alarm time-out

 \kbd{e\_ARCH}: not available on this architecture or operating system

 \kbd{e\_STACK}: the PARI stack overflows

 \kbd{e\_INV}: impossible inverse

 \kbd{e\_IMPL}: not yet implemented

 \kbd{e\_OVERFLOW}: all forms of arithmetic overflow, including length
 or exponent overflow (when a larger value is supplied than the
 implementation can handle).

 \kbd{e\_SYNTAX}: syntax error

 \kbd{e\_MISC}: miscellaneous error

 \kbd{e\_TYPE}: wrong type

 \kbd{e\_USER}: user error (from the \kbd{error} function)