Purpose: to discuss the issues of when and how to write exception handling
code.

   * What is exception handling?
   * When should one write exception handling code?
   * When should one ''NOT'' write exception handling code?
   * What tools exist to aid in writing exception handling code? 
   * How exception handling works

----

[hv] 2011-07-15: Here is a short snippet showing how to use the [return] command with '''-code error''' to throw an exception

======
proc double x {
    if {![string is integer $x]} {
        return -code error "double expects an int, not '$x'"
    }

    return [expr {$x * 2}]
}

set x 5
puts "x = $x"
puts "2x = [double $x]"

set x Hello
puts "x = $x"
if {[catch {puts "2x = [double $x]"} errmsg]} {
    puts "Error: $errmsg"
}
======

Output:

======none
x = 5
2x = 10
x = Hello
Error: double expects an int, not 'Hello'
======



PYK 2016-07-11:  For some time I was under the misapprehension that `[return]
-code error message` was the "standard" way to throw an error.  It's not.
`[error]` is usually preferable.  For one thing, `error` reports the line
number where it was called.  More importantly, the purpose of `[return]` is to
communicate information back to the caller, which is a more general function.
`[return]` can be considered a sort of restricted `[eval]`, providing more
power for expressing various runtime conditions, while `[error]` and `[throw]`
are the most direct methods of signalling an error.


** See Also **

   [try ... finally ...]:   

   [throw]:   

   [catch]:   


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