Tcl8.6.14/Tk8.6.14 Documentation > Tcl Commands, version 8.6.14 > try

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try — Trap and process errors and exceptions


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.


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 r]
} 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"


catch, error, return, throw


cleanup, error, exception, final, resource management
Copyright © 2008 Donal K. Fellows