'\"
'\" Copyright (c) 2008 Donal K. Fellows
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
.TH try n 8.6 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
try \- Trap and process errors and exceptions
.SH SYNOPSIS
\fBtry\fI body\fR ?\fIhandler...\fR? ?\fBfinally\fI script\fR?
.BE
.SH DESCRIPTION
.PP
This command executes the script \fIbody\fR and, depending on what the outcome
of that script is (normal exit, error, or some other exceptional result), runs
a handler script to deal with the case. Once that has all happened, if the
\fBfinally\fR clause is present, the \fIscript\fR it includes will be run and
the result of the handler (or the \fIbody\fR if no handler matched) is allowed
to continue to propagate. Note that the \fBfinally\fR clause is processed even
if an error occurs and irrespective of which, if any, \fIhandler\fR is used.
.PP
The \fIhandler\fR clauses are each expressed as several words, and must have
one of the following forms:
.TP
\fBon \fIcode variableList script\fR
.
This clause matches if the evaluation of \fIbody\fR completed with the
exception code \fIcode\fR. The \fIcode\fR may be expressed as an integer or
one of the following literal words: \fBok\fR, \fBerror\fR, \fBreturn\fR,
\fBbreak\fR, or \fBcontinue\fR. Those literals correspond to the integers 0
through 4 respectively.
.TP
\fBtrap \fIpattern variableList script\fR
.
This clause matches if the evaluation of \fIbody\fR resulted in an error and
the prefix of the \fB\-errorcode\fR from the interpreter's status dictionary
is equal to the \fIpattern\fR. The number of prefix words taken from the
\fB\-errorcode\fR is equal to the list-length of \fIpattern\fR, and inter-word
spaces are normalized in both the \fB\-errorcode\fR and \fIpattern\fR before
comparison.
.PP
The \fIvariableList\fR word in each \fIhandler\fR is always interpreted as a
list of variable names. If the first word of the list is present and
non-empty, it names a variable into which the result of the evaluation of
\fIbody\fR (from the main \fBtry\fR) will be placed; this will contain the
human-readable form of any errors. If the second word of the list is present
and non-empty, it names a variable into which the options dictionary of the
interpreter at the moment of completion of execution of \fIbody\fR
will be placed.
.PP
The \fIscript\fR word of each \fIhandler\fR is also always interpreted the
same: as a Tcl script to evaluate if the clause is matched. If \fIscript\fR is
a literal
.QW \-
and the \fIhandler\fR is not the last one, the \fIscript\fR of the following
\fIhandler\fR is invoked instead (just like with the \fBswitch\fR command).
.PP
Note that \fIhandler\fR clauses are matched against in order, and that the
first matching one is always selected. At most one \fIhandler\fR clause will
selected. As a consequence, an \fBon error\fR will mask any subsequent
\fBtrap\fR in the \fBtry\fR. Also note that \fBon error\fR is equivalent to
\fBtrap {}\fR.
.PP
If an exception (i.e. any non-\fBok\fR result) occurs during the evaluation of
either the \fIhandler\fR or the \fBfinally\fR clause, the original exception's
status dictionary will be added to the new exception's status dictionary under
the \fB\-during\fR key.
.SH EXAMPLES
.PP
Ensure that a file is closed no matter what:
.PP
.CS
set f [open /some/file/name a]
\fBtry\fR {
    puts $f "some message"
    # ...
} \fBfinally\fR {
    close $f
}
.CE
.PP
Handle different reasons for a file to not be openable for reading:
.PP
.CS
\fBtry\fR {
    set f [open /some/file/name w]
} \fBtrap\fR {POSIX EISDIR} {} {
    puts "failed to open /some/file/name: it's a directory"
} \fBtrap\fR {POSIX ENOENT} {} {
    puts "failed to open /some/file/name: it doesn't exist"
}
.CE
.SH "SEE ALSO"
catch(n), error(n), return(n), throw(n)
.SH "KEYWORDS"
cleanup, error, exception, final, resource management
'\" Local Variables:
'\" mode: nroff
'\" End:
