CODEWITHSUNDEEP
parsingrebolrebol3
HostileFork says dont trust SE
HostileFork says dont trust SE

Reputation: 33637

Difference between Rebol 2 and Rebol 3 when mixing SOME in parse with CHANGE

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

samples: [
    [a a c a]
    [a a c b]
    [b a c a]
    [c a c b]
    [c c c c]
]

Each block needs to be [c c c c]. So if a value is 'a, it is changed to 'b. If a value is 'b, it is changed to 'c. If a value is 'c, we print "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 seems to have a problem (bug?):

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

I don't know if it's related, but a Rebol Wikibook containing a list of changes to parse between Rebol 2 and Rebol 3 says this:

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

(Note: This simplified example provided by @rgchris in StackOverflow chat, repeated here to better preserve "institutional knowledge" and permit updating.)

Upvotes: 4

Views: 293

Answers (1)

earl
earl

Reputation: 41895

If it does not actually matter whether you use ANY (0..n) or SOME (1..n), as is the case in your example, you can use WHILE in R3. WHILE basically matches R2's ANY:

>> blk: [a a c a]

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

>> blk
== [c c c c]

Alternatively, if that doesn't suffice because you really need SOME semantics, you could rewrite SOME using more basic primitives. Instead of rule: [some subrule] you can use rule: [subrule opt rule]:

 >> blk: [a a c a]

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

 >> blk
 == [c c c c]

However, that might make you hit some PARSE limits which you won't hit with the original SOME (especially in R2).

Upvotes: 4

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