maard has asked for the wisdom of the Perl Monks concerning the following question:

Hello, wise monks!

I've been trying to construct parser for Clarion language and decided to start with simple things. After reading Parse::RecDescent's documentation, I took Parse::RecDescent Tutorial as an example to study and modify. The article has a code example which parses different code pieces, among which there's a code with assignment and printing of value: $parser->startrule("a = 5 ; print a"); That works fine. Until there're other instructions after print_instruction, in which case it fails to parse anything after first print: 

$parser->startrule("a = 5 ; print a ; print a"); # this fails
$parser->startrule("a = 5 ; print a ; b = 1"); # this fails either

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Investigating parser trace I've found that the only text which is consumed for the entire input is [a = 5 ; print].

Can wise monks help to understand, why the argument to print isn't consumed? And how to rewrite those rules for print_instruction to work correctly?

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Re: Parse::RecDescent problem (wrong example in perl.com article)
by kaif (Friar) on Jun 06, 2005 at 00:12 UTC

    Talk about a bug that took me an hour to fix. Changing 

    { return $main::VARIABLE{$item{VARIABLE}} }

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    to 
    { $return = $main::VARIABLE{$item{VARIABLE}} }

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    does the trick. For completeness, I would change the other two return FOO to $return = FOO also.

    I think you should write to the author of that tutorial and give him a heads-up. Also, this is off-topic, but the line /\w[a-z0-9_]*/i isn't quite right. Indeed, /\w/ is /[a-z0-9_]/i, so what is probably intended is /[a-z]\w*/i. In this case, it makes little difference (the former allows variables beginning with underscore), but if some code is rearranged then errors might pop up.

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