Syntax: throw type message

Throws an error with error code type and the given error message. The error may be caught using the catch command.

The error code is usually defined as a TCL list of symbols, e.g., ARITH DIVZERO. Most standard TCL error codes begin with ARITH (for arithmetic errors) or TCL.

proc myproc {x} {
    if {$x < 0} {
        throw NEGNUM "input must be non-negative"
    }
    ...
}

Note that the error command is equivalent to throw NONE; also, the return command can also throw an error with an error code. The three following commands are semantically identical:

error "My error message"

throw NONE "My error message"

return -code error -level 0 -errorcode NONE "My error message"