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Catching exceptions

Here the stack is empty, so . triggers stack underflow.

clearstack . 
:21: Address alignment exception
clearstack >>>.<<<
Backtrace:
$104EB8728 dup 
$104EB9E50 s>d

Catch the exception.

clearstack '. catch  ok

Throwing errors and exceptions

A common idiom to THROW a specific error if a flag is true is this: ( flag ) 0<> errno and throw (Gforth Manual 5.8.6 Exception Handling)

If a flag is false:

( flag ) 0= errno and throw

To provide a message:

( flag ) 0= s" msg" exception and throw

5 2 / 2 = 0= s" not equal" exception and throw  ok
5 2 / 1 = 0= s" not equal" exception and throw 
:3: not equal
5 2 / 1 = 0= s" not equal" exception and >>>throw<<<
Backtrace: