Tcl8.6.6/Tk8.6.6 Documentation > Tcl Commands, version 8.6.6 > throw

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throw — Generate a machine-readable error


throw type message


This command causes the current evaluation to be unwound with an error. The error created is described by the type and message arguments: type must contain a list of words describing the error in a form that is machine-readable (and which will form the error-code part of the result dictionary), and message should contain text that is intended for display to a human being.

The stack will be unwound until the error is trapped by a suitable catch or try command. If it reaches the event loop without being trapped, it will be reported through the bgerror mechanism. If it reaches the top level of script evaluation in tclsh, it will be printed on the console before, in the non-interactive case, causing an exit (the behavior in other programs will depend on the details of how Tcl is embedded and used).

By convention, the words in the type argument should go from most general to most specific.


The following produces an error that is identical to that produced by expr when trying to divide a value by zero.

throw {ARITH DIVZERO {divide by zero}} {divide by zero}


catch, error, errorCode, errorInfo, return, try


error, exception
Copyright © 2008 Donal K. Fellows