Forth: Portable indefinite loop with multiple exit points

Question

I need to implement an idefinite loop with multiple exit points. Unfortunately the most obvious solution - REPEAT - UNTIL with multiple WHILE doesn't work neither in Gforth nor in swapforth (Of course the loop in the example below may be implemented with DO - LOOP. However, that code is just a demonstration. The real problem is related to hardware control in an embedded system, so the loop indeed must be indefinite):

: test1 ( step -- step count )
    0
    begin
      over +
      dup .
      dup 20 < while
      dup 13 = while
    repeat
;

3 test1 

In "Thinking Forth" there is Moore's statement quoted:

Many times conditionals are used to get out of loops. That particular use can be avoided by having loops with multiple exit points. This is a live topic, because of the multiple WHILE construct which is in poly- Forth but hasn’t percolated up to Forth ’83. It’s a simple way of defining multiple WHILEs in the same REPEAT. Also Dean Sanderson [of Forth, Inc.] has invented a new construct that introduces two exit points to a DO LOOP. Given that construction you’ll have fewer tests.

Unfortunately I failed to find the Dean's solution. Is there any portable way to implement multiple exit points in the indefinite loop in Forth?

Answer 1

EXIT of course provides multiple exits to a definition. You can make the body of the loop one and the same with a definition either by having a separate word or, more neatly, by using quotations:

: test ( start step -- count step )
  swap [: begin
    over + dup .
    dup 20 > if ." >20" exit then
    dup 13 = if ." =13" exit then
    dup 17 = if ." =17" exit then
  again ;] execute
  ( EXITs from loop continue here ) ;