Difference between Rebol 2 and Rebol 3 when mixing SOME syntax with CHANGE

Question

Imagine a simplified example of a block of blocks containing words:

samples: [ [aaca] [aacb] [baca] [cacb] [cccc] ] 

Each block must be [cccc] . Therefore, if the value is 'a , it changes to 'b . If the value is 'b , it changes to 'c . If the value is 'c , we type “C” and move on:

 repeat i length? samples [ prin ["^/Sample" i "- "] parse samples/:i [ some [ s: 'a (change s 'b) :s | s: 'b (change s 'c) :s | 'c (prin "C") ] ] ] 

In Rebol 2, this works as expected:

 Sample 1 - CCCC Sample 2 - CCCC Sample 3 - CCCC Sample 4 - CCCC Sample 5 - CCCC 

But Rebol 3 has a problem (bug?):

 Sample 1 - Sample 2 - Sample 3 - Sample 4 - C Sample 5 - CCCC 

I don't know if this is related, but Rebol Wikibook, which contains a list of parsing changes between Rebol 2 and Rebol 3 , talks about it

SOME subrule - to prevent unwanted endless loops in R3, this rule also stops when the subrule matches the input but does not advance it

(Note: This simplified example provided by @rgchris in the StackOverflow chat is repeated here to better preserve "institutional knowledge" and allow updates.)

Answer 1

If it doesn't really matter if you use ANY (0..n) or SOME (1..n), as is the case in your example, you can use WHILE in R3. WHILE basically corresponds to R2 ANY:

 >> blk: [aaca] >> parse blk [while [s: 'a (change s 'b) :s | s: 'b (change s 'c) :s | 'c]] == true >> blk == [cccc] 

Otherwise, if this is not enough because you really need SOME semantics, you can rewrite SOME using simpler primitives . Instead of rule: [some subrule] you can use rule: [subrule opt rule] :

  >> blk: [aaca] >> subrule: [s: 'a (change s 'b) :s | s: 'b (change s 'c) :s | 'c] >> parse blk rule: [subrule opt rule] == true >> blk == [cccc] 

However, this may force you to press some PARSE restrictions that you will not beat with the original SOME (especially in R2).