Tornado

Tornado 2.0
User’s Guide
For example, in Tornado tools, events are represented in Tcl as structured strings.
The first element of the string is the name of the event. Tcl scripts that handle
events can search for the appropriate procedure to handle a particular event by
mapping the event name to a procedure name, and calling that procedure if it
exists. The following Tcl script demonstrates this approach:
proc shEventDispatch {event} {
set handlerProc "[lindex $event 0]_Handler"
}
if {[info procs $handlerProc] != ""} {
$handlerProc $event
} {
#event has no handler--do nothing.
}
B.2.8 Control Structures
Tcl provides all the popular control structures: conditionals (if), loops (while, for,
and foreach), case statements (switch), and explicit variable-scope control (global,
upvar, and uplevel variable declarations). By using these facilities, you can even
define your own control structures. While there is nothing mysterious about these
facilities, more detailed descriptions are beyond the scope of this summary. For
detailed information, see the books cited in the introduction to B.2 A Taste of Tcl,
p.318.
B.2.9 Tcl Error Handling
Every Tcl procedure, whether built-in or script, normally returns a string. Tcl
procedures may signal an error instead: in a defined procedure, this is done with
the error command. This starts a process called unwinding. When a procedure
signals an error, it passes to its caller a string containing information about the
error. Control is passed to the calling procedure. If that procedure did not provide
for this possibility by using the Tcl catch command, control is passed to its caller in
turn. This recursive unwinding continues until the top level, the Tcl interpreter, is
reached.
As control is passed along, any procedure can catch the error and take one of two
actions: signal another error and provide error information, or work around the
error and return as usual, ending the unwinding process.
At each unwinding step, the Tcl interpreter adds a description of the current
execution context to the Tcl variable errorInfo. After unwinding ends, you can
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