BREAK/CONTINUE-Statements for Standard Forth

BREAK/CONTINUE-Statements for Standard Forth

Ulrich Hoffmann <>

Version 1.0.0 - 2017-07-25

This package provides an implementation of C like BREAK and CONTINUE statements on top of Forth-94 (ANS Forth).

BREAK allows to exit the innermost textual enclosing loop. It works for BEGIN-loops as well as for DO-loops. CONTINUE stops executing the current loop cycle and restarts the loop in its next cycle.

If no surrounding loop is present then both BREAK and CONTINUE exit the current definition.


Display whether or not a given number is a prime number.

: .prime ( n -- )
    dup  1 = 
    IF false swap
    ELSE dup 2 = 
    IF true swap
    ELSE dup  2 mod
    IF true swap 3 
      BEGIN ( f n u )
        2dup dup * >=
      WHILE ( f n u )
        2dup mod 0= IF >r >r drop false r> r> BREAK THEN
        2 +
      REPEAT ( f n u )
      false swap
    . ." is " 0= IF ." not " THEN ." a prime number."

1 .prime 1 is not a prime number. ok
2 .prime 2 is a prime number. ok
3 .prime 3 is a prime number. ok
4 .prime 4 is not a prime number. ok
5 .prime 5 is a prime number. ok
6 .prime 6 is not a prime number. ok
7 .prime 7 is a prime number. ok
8 .prime 8 is not a prime number. ok
9 .prime 9 is not a prime number. ok


Some people ask for definitions to provide such a BREAK feature for Forth:

Newsgroups: comp.lang.forth
Date: Wed, 19 Jul 2017 10:22:39 -0700 (PDT)
User-Agent: G2/1.0
Subject: Re: A Start With Forth 2017 – new eBook
From: Raimond Dragomir
Injection-Date: Wed, 19 Jul 2017 17:22:40 +0000
Content-Type: text/plain; charset="UTF-8"

But I can propose onemyself: make a BREAK word in Swiftforth, VFX and gforth. a BREAK that's working for EVERY/ANY loop (including the DO LOOPs). It can be a primitive written in assembler or C or whatever. No need to be written in ANS forth (chicken or egg problem? no, make it a primitive). How hard is it? Really hard isn't it? Too hard to really DO IT ?.

So here it is - based on Forth-94 (ANS Forth).
You can use this implementation as a guideline to incorporate BREAK and CONTINUE into specific systems of your choice if your have "carnal knowledge" about their inner working.
This implementation only assumes what is ensured by a standard system. See section Implementation below for details.

Others are not convinced that BREAK (and even less CONTINUE) is necessary and favor a style that leaves loops in definitions by exiting the definition (via EXIT or UNLOOP EXIT).

For example the above definition of .prime could have been (separating calculation and output) facorized as:

: divides? ( n u -- f )  mod 0= ;
: even? ( n -- f ) 2 divides? ;
: square ( n -- n^2 ) dup * ;

: prime? ( n -- f )
    dup 1 =   IF drop false EXIT THEN
    dup 2 =   IF drop true  EXIT THEN
    dup even? IF drop false EXIT THEN
    3 BEGIN ( n u )
       2dup square >=
    WHILE ( n u )
       2dup divides? IF 2drop false EXIT THEN
       2 +
    REPEAT ( n u )
    2drop true ;

  : .prime ( n -- ) dup . ." is " prime? 0= IF ." not " THEN ." a prime number" cr ;

If you dare - consult the discussion on comp.lang.forth (Subject: BREAK proposal) of July 2017...

Your milage may vary. Choose wisely.


To use the BREAK or CONTINUE, just S" break.fs" INCLUDED on any standard system. This makes the new words available and redefines the existing control structures accordingly. After loading you don't have a standard system anymore as e.g. the compilation semantics of IF is no longer the standard effect on the control flow stack:

Compilation: ( C: -- orig )

but is now:

Compilation: ( C: orig_1 ... orig_u|dest -- orig orig_1 ... orig_u|dest )  ( u v loopID --  u v loopID )

to handle branches implied by BREAK and CONTINUE and to supply information about the surrounding loop.

The package redefines the standard CORE words BEGIN UNTIL AGAIN WHILE REPEAT DO IF ELSE THEN : ; DOES> in a non standard way. Compiler extensions that assume their standard behavior are likely to fail. Their use in ordinary programs should however remain uneffected.

The package also redefines the standard CORE EXT words :NONAME ?DO CASE OF ENDOF ENDCASE and AHEAD from the TOOLS EXT word set if they have already been defined before loading the package.


See the file for a description of the defined words.

Tests and more example usage

The file break-tests.fs contains test definitions based on John Hayes test framework tester.fs. They show additional ways to use BREAK and CONTINUE and their expected results.

To run the test suite issue S" break-tests.fs" INCLUDED. The expected output should look like this

S" break-tests.fs" INCLUDED  

some messages warning about redefinitions of standard words

break control structure
break tests done 

Tests have been run successfully on

which all use the data stack as control flow stack and also on

that has a separate control flow stack.


The following pitfalls have been identified:


When using CONTINUE inside DO ... +LOOP be aware that control flow is transfered directly from CONTINUE to +LOOP in order to start the next loop iteration. Any calculation for the loop increment value n (see 6.1.0140 +LOOP in the Forth-94 standards document) in front of +LOOP is skipped. An appropriate calculation has to be supplied in front of CONTINUE already.


: continue-+loop ( -- ) 
       10 0 DO  I 8 = IF BREAK THEN  
                I 4 = IF 2 CONTINUE THEN
            2 +LOOP 99 ;
t{ continue-+loop -> 0 2 6 99 }t


The core idea of this implementation is to hold appropriate branch origins for BREAKs and CONTINUEs as well as the branch destination of the beginning of the loop (along with the already existing items there) on top of the control flow stack.

Additional information about the loop is put on top of the data stack. This information is

( u v loopID ) where

  • u specifies the number of items additionally put on the control flow stack,

  • v specifies the number of used OF-ENDOF pairs when using a CASE structure, and

  • loopID is a loop identifier distinguishing between

    • BEGIN-loops (loopID=1),
    • DO-loops (loopID=2), and
    • no loop (loopID=0).

Both BREAK and CONTINUE compile different code, depending on the kind of loop they appear in.


  • inside a BEGIN-loop BREAK compiles an unconditionals forward branch past the end of the loop.

  • inside a DO-loop BREAK compiles a LEAVE statement (which will unravel the loop parameters and branch past the particular LOOP or +LOOP).

  • outside of any loop BREAK compiles EXIT leaving the current definition.


  • inside a BEGIN-loop CONTINUE compiles an unconditional branch to the beginning of the loop.

  • inside a DO-loop CONTINUE compiles an unconditional branch just before the corresponding LOOP or +LOOP.

  • outside of any loop CONTINUE compiles EXIT leaving the current definition.

The package assumes that branches are handled by the underlying standard system:

  • unconditional forward branches by AHEAD (resolved by THEN)
  • unconditional backward branches by AGAIN (to location of BEGIN)

As both BREAK and CONTINUE can occur nested inside (nested) IF or CASE structures the information about the surrounding loop must be moved to the top of control flow and data stack even inside these structures.

So in essence the package

  • modifies the behavior of : :noname DOES> BEGIN DO and ?DO to put appropriate loop information on the data stack,

  • modifies the behavior of ; UNTIL AGAIN LOOP +LOOP to resolve BREAK and CONTINUE branches and to clean up loop information,

  • modifies the behavior of IF ELSE THEN WHILE REPEAT AHEAD and redefines CASE OF ENDOF ENDCASE so they can handle the additional items on the control flow and data stack,

  • defines BREAK and CONTINUE with the compilation semantics described above.

During compilation of control structures a standard system uses the control flow stack. It might use the data stack as control flow stack or a separate stack for this purpose.

Thus a standard program (such as this package) must assume the control flow stack could be either. It must clear the data stack of any items it puts there by itself before using control flow stack dependent words such as the control structures themselves or the control flow stack manipulation words CS-ROLL and CS-PICK.

Note that CS-ROLL or CS-PICK can only work on dest or orig items on the control flow stack but DO-loops also put do-sys items there that CS-ROLL and CS-PICK cannot handle. This effectively also inhibits access to items below do-sys. The same holds for CASE/OFand case-sys/of-sys items. This restricts the manipulation that you can do on the control flow stack in presence of DO or CASE.

Please consult the source code break.fs for exact details.

Bug Reports

Please send bug reports, improvements and suggestions to Ulrich Hoffmann <>


This program conforms to Forth-94 and Forth-2012

Standard conformant labeling

This is an ANS Forth Program with the environmental dependency of using lower case for standard definition names,

  • Requiring NIP 0<> .( \ from the Core Extensions word set.
  • Requiring [THEN] [IF] CS-ROLL CS-PICK AHEAD from the Programming-Tools Extensions word set.
  • Redefining OF ENDOF ENDCASE CASE AGAIN ?DO :NONAME from the Core Extensions word set if available.

  • After loading this program, a Standard System does no longer exist: The program redefines BEGIN UNTIL AGAIN WHILE REPEAT DO LOOP +LOOP IF ELSE THEN : ; DOES> and possibly :NONAME AHEAD ?DO CASE OF ENDOF ENDCASE in a non standard way adding additional information to the data stack and the control flow stack during compilation to reflect the uses of BREAK and CONTINUE. Programs that explicitly access the control flow stack or use these words in compiler extensions assuming their standard compilation semantics will most likely fail. Programs that just use the control flow structures will remain uneffected.

May the Forth be with you!