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