path: root/tcllib/idoc/man/files/modules/try/tcllib_try.n

'\"
'\" Generated from file 'tcllib_try\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2008 Donal K\&. Fellows, BSD licensed
'\"
.TH "try" n 1 tcllib "Forward compatibility implementation of [try]"
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See the \\*(So manual entry for details on the standard options.
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Command-Line Name:	\\fB\\$1\\fR
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.SH NAME
try \- try - Trap and process errors and exceptions
.SH SYNOPSIS
package require \fBTcl  8\&.5\fR
.sp
package require \fBtry  ?1?\fR
.sp
\fB::try\fR \fIbody\fR ?\fIhandler\&.\&.\&.\fR? ?\fBfinally\fR \fIscript\fR?
.sp
.BE
.SH DESCRIPTION
.PP
This package provides a forward-compatibility implementation of Tcl
8\&.6's try/finally command (TIP 329), for Tcl 8\&.5\&. The code was
directly pulled from Tcl 8\&.6 revision ?, when try/finally was
implemented as Tcl procedure instead of in C\&.
.TP
\fB::try\fR \fIbody\fR ?\fIhandler\&.\&.\&.\fR? ?\fBfinally\fR \fIscript\fR?
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\&.
.sp
The \fIhandler\fR clauses are each expressed as several words,
and must have one of the following forms:
.RS
.TP
\fBon\fR \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\fR \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\&.
.sp
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\&.
.sp
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 \fB-\fR 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)\&.
.sp
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\&.
.sp
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\&.
.RE
.PP
.SH EXAMPLES
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]
} \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 "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fItry\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.SH "SEE ALSO"
catch(n), error(n), return(n), throw(n)
.SH KEYWORDS
cleanup, error, exception, final, resource management
.SH CATEGORY
Utility
.SH COPYRIGHT
.nf
Copyright (c) 2008 Donal K\&. Fellows, BSD licensed

.fi