DOKK / manpages / debian 11 / tcl8.6-doc / try.3tcl.en
try(3tcl) Tcl Built-In Commands try(3tcl) 







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:



  

  
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.

  

  
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 w]
} 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(3tcl), error(3tcl), return(3tcl), throw(3tcl)








cleanup, error, exception, final, resource management







  

    8.6
    Tcl