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.
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