Forth Exception Handling

The following diagram illustrates Word foo successfully trying bar, i.e. bar does not raise an exception.

Foo Successfully tries bar.

In the next diagram, foo again tries bar, but this time, bar raises an exception called x-y-z:

Foo tries bar, with exception.

Raising an exception outside of a try block results restoring the state to whatever the ExcStackFramePtr variable points to. To avoid unexpected behavior, the top-level REPL (i.e. the quit loop) is placed within a try-block.

Caveat

A caveat to keep in mind: A raised exception returns the Data Stack to its state before the try call. Consequently, code within a try-block that might throw an exception should not manipulate data stack items outside of the try-block.

For example:

: x-y-z ." x-y-z exception raised." cr;

: double-it ( n -- n')
  2*
  ['] x-y-z ?raise
;

: foo ( -- n )
    3
    [: dup double-it ;] try ( n exception-xt )
    drop ( n )
;

: bar ( -- n )
    3 dup
    [: double-it ;] try ( n exception-xt )
    drop ( n )
;

foo . cr
bar . cr

Foo . cr will print the value 3. However, bar . cr will print the value 6 because in bar's case, double-it reaches outside of the data stack frame restored by ?raise.