Re: "or die" versus "|| die"

It's too easy to make precedence mistakes if you use the || variety for this kind of flow control. Witness later in that same faq: http://perldoc.perl.org/perlfaq5.html#How-do-I-randomly-update-a-binary-file%3f the error in the line:

  read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $r
+ecno: $!";
[download]

Rule of thumb: use || when you are actually using the result of the operation, otherwise use or.

Comment on Re: "or die" versus "\|\| die" Download Code

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Re^2: "or die" versus "\|\| die" by brian_d_foy (Abbot) on Jun 24, 2005 at 15:59 UTC
Which error? The numeric comparison operator has a higher precedence. If the record size that perl read is the expected record size, the LHS evaluates to one and the short circuit terminates. The better rule of thumb is to use `or` when you want everything else to happen first. If you know about problems in the perlfaq, however, you can send them to perlfaq-workers @ perl.org and we'll fix them up. -- brian d foy <brian@stonehenge.com>	[reply] [d/l]
Re^3: "or die" versus "\|\| die" by halley (Prior) on Jun 24, 2005 at 16:33 UTC
In terms of your "rule of thumb," I like to say that `\|\|` goes between expressions, and `or` goes between statements. It's kinda like how `,` goes between elements, but `;` goes between statements. I'm not sure if my phrasing is any more clear to the uninitiated than talking in greek, though. -- `[ e d @ h a l l e y . c c ]`	[reply] [d/l] [select]
Re^4: "or die" versus "\|\| die" by brian_d_foy (Abbot) on Jun 24, 2005 at 16:39 UTC
That's not a good rule though because it doesn't actually provide useful guidance and it makes people think about the operators all wrong. It has nothing to do with expressions or statements. It's all about operators and in which order perl evaluates them. That's the information you need to pass on in your rule so that people can figure out what to do. -- brian d foy <brian@stonehenge.com>	[reply]
Re^3: "or die" versus "\|\| die" by ysth (Canon) on Jun 24, 2005 at 19:11 UTC
Gah, you are right, and I should know better.	[reply]


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