Ubuntu Manpage: try - try - Trap and process errors and exceptions

NAME

       try - try - Trap and process errors and exceptions

SYNOPSIS

       package require Tcl  8.5

       package require try  ?1?

       ::try body ?handler...? ?finally script?

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DESCRIPTION

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.

::try body ?handler...? ?finally script?
This command executes the script body 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 finally clause is
present, the script it includes will be run and the result of the handler (or the
body if no handler matched) is allowed to continue to propagate. Note that the
finally clause is processed even if an error occurs and irrespective of which, if
any, handler is used.

The handler clauses are each expressed as several words, and must have one of the
following forms:

on code variableList script
This clause matches if the evaluation of body completed with the exception
code code. The code may be expressed as an integer or one of the following
literal words: ok, error, return, break, or continue. Those literals
correspond to the integers 0 through 4 respectively.

trap pattern variableList script
This clause matches if the evaluation of body resulted in an error and the
prefix of the -errorcode from the interpreter's status dictionary is equal
to the pattern. The number of prefix words taken from the -errorcode is
equal to the list-length of pattern, and inter-word spaces are normalized in
both the -errorcode and pattern before comparison.

The variableList word in each handler 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 body (from the
main try) 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 body will be placed.

The script word of each handler is also always interpreted the same: as a
Tcl script to evaluate if the clause is matched. If script is a literal -
and the handler is not the last one, the script of the following handler is
invoked instead (just like with the switch command).

Note that handler clauses are matched against in order, and that the first
matching one is always selected. At most one handler clause will selected.
As a consequence, an on error will mask any subsequent trap in the try. Also
note that on error is equivalent to trap {}.

If an exception (i.e. any non-ok result) occurs during the evaluation of
either the handler or the finally clause, the original exception's status
dictionary will be added to the new exception's status dictionary under the
-during key.

EXAMPLES

       Ensure that a file is closed no matter what:

              set f [open /some/file/name a]
              try {
                  puts \$f "some message"
                  # ...
              } finally {
                  close \$f
              }

       Handle different reasons for a file to not be openable for reading:

              try {
                  set f [open /some/file/name]
              } trap {POSIX EISDIR} {} {
                  puts "failed to open /some/file/name: it's a directory"
              } trap {POSIX ENOENT} {} {
                  puts "failed to open /some/file/name: it doesn't exist"
              }

BUGS, IDEAS, FEEDBACK

       This document, and the package it describes,  will  undoubtedly  contain  bugs  and  other
       problems.    Please   report   such   in   the   category   try  of  the  Tcllib  Trackers
       [http://core.tcl.tk/tcllib/reportlist].  Please also report any ideas for enhancements you
       may have for either package and/or documentation.

KEYWORDS

       cleanup, error, exception, final, resource management

COPYRIGHT

       Copyright (c) 2008 Donal K. Fellows, BSD licensed