21.3.8 BEGIN..WHILE..REPEAT. 4tH 4tH Forth compiler

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CHAPTER 21. PORTING GUIDE 387

21.3.8 BEGIN..WHILE..REPEAT

4tH allows you to use multiple WHILE’s in a BEGIN..WHILE..REPEAT construct. ANS-

Forth allows that too, but requires an extra ’THEN’ for each additional ’WHILE’. In short, this is the 4tH version:

0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat

And this is the ANS-Forth version:

0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat then

To make this work, we have to resort to conditional compilation:

0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat

[undefined] 4th# [if] then [then]

It’s not beautiful, but it works. The same applies when ’UNTIL’ is used instead of ’RE-

PEAT’. BEGIN..WHILE..AGAIN constructs are not supported by ANS-Forth, so be careful when considering ’AGAIN’ too much of an alias of ’REPEAT’. Also bear in mind that

DONE..DONE clauses can be defined in ANS-Forth, but still require conditional compilation if you resort to multiple ’WHILE’s.

21.3.9 CASE..OF..ENDOF..ENDCASE

Many users wonder why this ”essential” construct is missing in 4tH. The explanation is pretty simple: it is horrible! First, this construct puts a heavy burden on the controlstack, which is pretty shallow in 4tH. Why? Because it is essentially a nested IF..ELSE..THEN

construct as you will see later on. Second, these kinds of problems are better handled by a lookup table. 4tH has excellent support for lookup tables, much better than other Forths.

Of course, sometimes you just want to convert a program and don’t feel like redesigning it. Fortunately, converting CASE..OF..ENDOF..ENDCASE constructs is pretty straightforward. It only requires four simple steps. Let’s examine this simple example:

: .WEEKDAY ( daynum --- )

CASE

1 OF ." Sunday" ENDOF

2 OF ." Monday" ENDOF

3 OF ." Tuesday" ENDOF

4 OF ." Wednesday" ENDOF

5 OF ." Thursday" ENDOF

6 OF ." Friday" ENDOF

7 OF ." Saturday" ENDOF

ENDCASE ;

Step 1: Replace ENDOF with ELSE and eliminate CASE

: .WEEKDAY ( daynum --- )