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Re^3: next in continue doesn't DWIM

by Eily (Parson)
on Nov 20, 2013 at 18:23 UTC ( #1063570=note: print w/replies, xml ) Need Help??

in reply to Re^2: next in continue doesn't DWIM
in thread next in continue doesn't DWIM

For reason of retro compatibility, and the Principle Of Least Astonishment (well, talk about DWIM ...) when people are used to this behaviour, do not expect this to change. And there might be CPAN modules that rely on this feature, which would go broken if it changed. That's actually the first thing I was told here, because I asked about a possible change of regex syntax (oh foolish me).

Now as for a legitimate use of it ... I never saw one. But I guess it could for exemple be used to separate what should be done on all data samples and what should be only done on "valid data" like:

$data = init(); while(still_alive) { process($data); } continue { $data = getSample(); print $logfile "Input contained $data"; next if invalid($data); }
If process is what is important, and invalid samples are rare enough that you don't want to visually polute the main process, but you still have to deal with them this could a way to do it.

Here is another example:

STATE: until ($state->name eq 'Ending') { $state->action(); redo STATE if nothingHasChanged(); } continue { $state = $state->nextState(context); next STATE unless $blackList->contains($state); #here we want to go +to the next state without processing the current one }

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Re^4: next in continue doesn't DWIM
by QM (Parson) on Nov 21, 2013 at 09:33 UTC
    I agree with you about changing entrenched behaviors, and POLA. Oh, well.

    I applaud your examples of almost justifiable code. However, I think that these should be refactored to meet POLA as well. IMNSHO, the point of the continue block is to have a landing place for cleanup before restarting the loop, where, for convenience, any number of nexts can expect to go to get the house in order before the next loop. (last and redo don't skip any continue block.)

    In the first example, if that code really needs to be in continue for some reason (I realized that process($data); might just be a placeholder for something bigger), then I would rewrite the continue block like so:

    continue { do { $data = getSample(); print $logfile "Input contained $data"; } until (not invalid($data)); }

    But I would also argue, for serious code, that even this doesn't belong here, and the main loop body needs to be refactored. I guess I'm thinking of a new aphorism, something like:

    Don't do anything significant in a continue.

    I would approach the second example much the same way, so I leave the details to OMAR.

    Anyway, thanks for the debate. Short of getting anything changed, at least I've got it out of my system, even if I end up needing a Ka D'argo-like beating to do so =)

    Quantum Mechanics: The dreams stuff is made of

      In the end I still do agree with you, I expect the examples I gave not to be first shots, but modification of already complex code where it was easier to add one line in the continue block than one if in the loop body and finding where to put the braces. And when you have to explain how risky changes are to a colleague or customer that doesn't understand much about coding, showing that you added one line in a tiny block instead of a few characters in several places of a long mess might be easier. My point is that this feature might be used all over the place.

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