bionic (3) tcllib_try.3tcl.gz

NAME

       try - try - Trap and process errors and exceptions

SYNOPSIS

       package require Tcl  8.5

       package require try  ?1?

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

________________________________________________________________________________________________________________

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.

       When proposing code changes, please provide unified diffs, i.e the output of diff -u.

       Note further that attachments are strongly preferred over inlined patches. Attachments  can  be  made  by
       going  to the Edit form of the ticket immediately after its creation, and then using the left-most button
       in the secondary navigation bar.

SEE ALSO

       catch(3tcl), error(3tcl), return(3tcl), throw(3tcl)

KEYWORDS

       cleanup, error, exception, final, resource management

CATEGORY

       Utility

COPYRIGHT

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