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

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  1. PODifference between Rebol 2 and Rebol 3 when mixing SOME in parse with CHANGE
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    <p>Imagine a simplified example of a block of blocks containing words:</p> <pre><code>samples: [ [a a c a] [a a c b] [b a c a] [c a c b] [c c c c] ] </code></pre> <p>Each block needs to be <code>[c c c c]</code>. So if a value is <code>'a</code>, it is changed to <code>'b</code>. If a value is <code>'b</code>, it is changed to <code>'c</code>. If a value is <code>'c</code>, we print "C" and move on:</p> <pre><code>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") ] ] ] </code></pre> <p>In Rebol 2, this works as expected: </p> <pre><code>Sample 1 - CCCC Sample 2 - CCCC Sample 3 - CCCC Sample 4 - CCCC Sample 5 - CCCC </code></pre> <p>But Rebol 3 seems to have a problem (bug?):</p> <pre><code>Sample 1 - Sample 2 - Sample 3 - Sample 4 - C Sample 5 - CCCC </code></pre> <p>I don't know if it's related, but a Rebol Wikibook containing a list of <a href="http://en.wikibooks.org/w/index.php?title=REBOL_Programming/Language_Features/Parse/Parse_expressions&amp;stable=0#Differences_between_R2_and_R3_parsing" rel="nofollow">changes to parse between Rebol 2 and Rebol 3</a> says this:</p> <blockquote> <p>SOME subrule - to prevent unwanted infinite loops in R3 this rule stops also when the subrule matches the input but does not advance it</p> </blockquote> <p><em>(Note: This simplified example <a href="http://chat.stackoverflow.com/transcript/message/7199761#7199761">provided by @rgchris in StackOverflow chat</a>, repeated here to better preserve "institutional knowledge" and permit updating.)</em></p>
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