Hi

I can't remember where I read it, but I have read somewhere that you should use 'or' over '||' in some cases because || has a higher precedence than or and might give unexpected results? Anyone else know of anything like this?

I remember reading that when doing functions like

open FILEHANDLE
|| die ...

You should instead use

open FILEHANDLE
or die

Because || has a great presence than open. Does this make sense?

- wil

I don't know poop, but I do know that || is Perl 4 and 'or' was introduced in Perl 5. I'd like to know more about what you find out.

OK. I remember I read this in Programming Perl, 3rd edition. I'll find the reference for you and post.

Cheers

- wil

You should use 'or' for things like:

open HANDLE, "foo" or die $!;

and || for stuff like:

my $var = 'something' || 'somethingelse';

Yes, but did you see anything about different levels? || has a higher presencde than or which I found interesting.

- wil

Hi,

I'm sure the example was from programming perl book:

open FH, $file || die "oops: $!";

translates to:

open FH, ( $file || die "oops: $!" );

so basically it won't die if the file does not exist, or can't be opened.

or has a lower precedense so:

open FH, $file or die "oops: $!";

translates to:

open (FH, $file) or die "oops: $!";

which is what you want.

Cheers,

Alex


--
Gossamer Threads Inc.

Bingo! Thanks for the explanation.

- wil

Hi guys,

I wouldn't have thought it possible that this innocent word can have so many different spellings. I am even more surprised that these different spellings were all used by native speakers....

Wil: precedence, i.e. correct first time, congratulations, but then presence ...
Wil again: presencde, seems to be slightly confused.
Alex: explains perl-well, but ... precedense


Ivan
-----
Iyengar Yoga Resources / GT Plugins

:-)

- wil

'or' has a lower precedence than || - so low, in fact, that it is below Perl's binding of variables to function call arguments, assignments, ',' and '=>', etc. In fact, there is nothing lower than or/xor on the precedence table ('and' is just above, 'not' just above 'and').

When you run this code:


perl will split it up on the or's:


The variable '$var' will be passed as the only argument to foo.

On the other hand, when assigning:


the precedence of '=' is lower than ||, so the || gets evaluated first. The same with function calls:


If you get into the habit of always using (brackets) around every combination, it makes absolutely no difference which one you use. These are identical:


And these are the same too:


So, really, it isn't any "better" or "worse" to use '||' versus 'or' - so long as you know that 'or' binds very loosely. I just happen to think 'or' looks better - and it saves on ugly () in the code.

---

Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com