try(3tcl) Forward compatibility implementation of [try] try(3tcl)
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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 ex-
ceptional 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 corre-
spond 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 inter-
preter'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 com-
parison.
The variableList word in each handler is always inter-
preted 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 inter-
preted the same: as a Tcl script to evaluate if the
clause is matched. If script is a literal - and the han-
dler 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 out-
put 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
tcllib 1 try(3tcl)