Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Perl code that is syntactically correct only on Fridays

github.com

Post reply on HN
Print view View original

161–170 of 233 posts

Re: Perl code that is syntactically correct only on Fridays

#161

Post by amelius »

knodi123 wrote: post #51
johnnyo wrote: post #10

I had an intern once who wrote similar code. They confused 12 hour and 24 hour clocks, so the code would break after lunch. Unfortunately, the intern only worked the mornings, so it took several days of back and forth before the bug could be finally put to rest.

Ha. I had an intern who wrote code that blew up if the timestamp of our DB server was ever >= 1 second after the timestamp of our app server. It couldn't fail at his desk, because he ran both servers locally while developing. And it wouldn't fail in production in the morning, since we had a cron job that synced the clocks at midnight. It took until after 5pm for the clocks to drift enough, meaning I was on call and t…

To be honest, I wrote code that blew up after the time server updated the time from the network, a few seconds after booting.

Re: Perl code that is syntactically correct only on Fridays

#162

Post by mypalmike »

inquist wrote: post #64
knodi123 wrote: post #51

Earlier quoted context omitted.

Ha. I had an intern who wrote code that blew up if the timestamp of our DB server was ever >= 1 second after the timestamp of our app server. It couldn't fail at his desk, because he ran both servers locally while developing. And it wouldn't fail in production in the morning, since we had a cron job that synced the clocks at midnight. It took until after 5pm for the clocks to drift enough, meaning I was on call and t…

> an intern who wrote code .... his code had been in production You get what you pay for, eh?

Depends. FAANG interns can make (annualized) income at or near 6 figures.

Re: Perl code that is syntactically correct only on Fridays

#163

Post by dane-pgp »

DocTomoe wrote: post #16
nicoburns wrote: post #14

Earlier quoted context omitted.

> I have written code that only works in winter because of daylight saving times and some unfortunate date conversions Not sure where you are located, but this can be particularly bad in the UK where winter time happens to coincide with UTC.

Let's be pedantic about this, because this is one of the very few areas where pedantry matters: GMT ("British Winter Time") is NOT UTC, and it can differ from UTC up to 0.9 seconds. GMT is a time zone (corresponding to UT1). UTC is a time standard.

I appreciate your pedantry, and I wonder if you know of a widely used computer system which genuinely implements the GMT time zone as something different from UTC+0.

There may be some niche astronomical or legal(?) applications which have to keep track of the potential 0.9 seconds skew, but my impression is that any app which claims to support a "GMT" (or Europe/London) option is actually only supporting "GMT-ish".

Re: Perl code that is syntactically correct only on Fridays

#164

Post by hnu0847 »

G3rn0ti wrote: post #40

This is how it works: BEGIN { # an execution block that runs when the module is compiled *f = # assigns to the symbol "f" inside the symbol table ... (localtime->wdayname eq 'Fri') # is it Friday? ? sub() {} # ... on friday an anonymous subroutine taking no arguments : sub {}; # ... on every other day a subrouting taking arguments } # closes the execution block # The following line executes the subroutine "f" and div…

Why does the following expect arguments:

    : sub {};
Don't all perl subroutines allow any number of arguments (including 0) to be passed?

Does the fact that the above is an anonymous subroutine affect the answer to my question?

Re: Perl code that is syntactically correct only on Fridays

#165

Post by RandalSchwartz »

anned20 wrote: post #12
johnnyo wrote: post #10

I had an intern once who wrote similar code. They confused 12 hour and 24 hour clocks, so the code would break after lunch. Unfortunately, the intern only worked the mornings, so it took several days of back and forth before the bug could be finally put to rest.

If you tried, you couldn't even come up with this.

I could. :)

Re: Perl code that is syntactically correct only on Fridays

#166

Post by RandalSchwartz »

lloeki wrote: post #43
JohnDeHope wrote: post #19

Earlier quoted context omitted.

Right, the beauty of this is that it's a syntax error he's produced. Sure any language can write code to produce a runtime error under any condition at all. And a lot of languages have features that allow them to run code at compile time, and similarly produce errors then too. But those languages don't allow you to ruin the syntax of the language. I guess it's something that requires macros. Would it be safe to say t…

It is formally proven that Perl can't be parsed! https://www.perlmonks.org/index.pl?node_id=663393

It has been said that "the Perl compiler is based on yacc, lex, smoke, and mirrors."

Re: Perl code that is syntactically correct only on Fridays

#167

Post by RandalSchwartz »

cestith wrote: post #67
saba2008 wrote: post #11

Earlier quoted context omitted.

Most other languages would require `eval` and having incorrect code as syntactically correct string literal. But because Perl is parsed as it is executed, incorrect code raises syntax error only when it is reached by execution.

Perl is not parsed as it is executed. It is compiled to opcodes then run. The BEGIN block, however, explicitly runs code at compile time. It is literally compiling different code on different days, then attempting to run it.

Technically not opcodes, but internal Data Structures. A serializer was written for that, to permit ".pmc files".

Re: Perl code that is syntactically correct only on Fridays

#168

Post by ipsi »

Nextgrid wrote: post #120
monkpit wrote: post #69

Earlier quoted context omitted.

I feel like this is relatively common (unfortunately)

It is. I'm in London which means that our time is +/- 1 hour of UTC depending on daylight saving time. There's been a couple projects where if I pushed a commit between 11pm and 1am some tests would break. I'm pretty sure the issue is inconsistent use of UTC vs local time which during that period will differ by one day.

One of the perils of working in the UK is _assuming_ that you've been using UTC timestamps this whole time, and then summer rolls around and suddenly a bunch of tests start failing, and all your production logs are an hour out because you were wrong, and actually used local timestamps everywhere. Oops.

Re: Perl code that is syntactically correct only on Fridays

#169

Post by RandalSchwartz »

bastardoperator wrote: post #118

I worked with Randal L. Schwartz for several years in the Los Angeles area. I was already doing Perl but getting an opportunity to see his code and ask him questions was something that changed my career for the better. Thanks Merlyn!

Have you seen my "parse JSON in a single Perl Regex"? https://www.perlmonks.org/?node_id=995856

Re: Perl code that is syntactically correct only on Fridays

#170

Post by jwilk »

hnu0847 wrote: post #164
G3rn0ti wrote: post #40

This is how it works: BEGIN { # an execution block that runs when the module is compiled *f = # assigns to the symbol "f" inside the symbol table ... (localtime->wdayname eq 'Fri') # is it Friday? ? sub() {} # ... on friday an anonymous subroutine taking no arguments : sub {}; # ... on every other day a subrouting taking arguments } # closes the execution block # The following line executes the subroutine "f" and div…

Why does the following expect arguments: : sub {}; Don't all perl subroutines allow any number of arguments (including 0) to be passed? Does the fact that the above is an anonymous subroutine affect the answer to my question?

A sub without prototype (anonymous or not) indeed allows any number of arguments. That's sufficient to trigger the syntax error.
Post reply on HN