[ColorForth] How to escape from loops

[Richard Collins <rscollins@xxxxxxxxxxxxxxx>]

Alex Thiel wrote:
> Hello,
>
> I can't seem to figure out how to escape from a FOR loop properly if some 
> exceptional condition occurs. I want to write something like this:
>
> : loop for cond? if default next ; then exception ;
>
> After "exception" has been executed, the loop should terminate. I guess I have 
> to shuffle the return stack a bit for this to work, but I don't know how. 
>
> Can anyone give me a hint?
>
> Cheers,
>
> 	Alex
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Your problem is that when both 'if' and 'for' compile they put the 
address to jump back to onto the stack. When 'next' and 'then' compile, 
they take the address to jump back to off the stack and compile a jump 
back to that address. So you've got to get your nesting right.

In your example

: loop for cond? if default next ; then exception ;

They overlap so the 'next' will be jumping back to the 'if' and the then 
will loop back to the 'for' which is not what you are trying to achieve.

This will work better.

: loop for cond? not if exception ; then exception next ;

As no doubt you know it still won't work since 'i' the counter is stored 
on the return stack.  You need to get rid of that counter before you do 
the return. One way to do it is like this.

: loop for cond? not if exception pop drop ; then exception next ;

If you have to use a 'not' don't use the '-' as this doesn't update the 
flags. You'll have to figure out some craftyness. Adding zero to 
anything doesn't affect the value but updates the flags, though I'm sure 
there is a better way.

Richard Collins
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