On 4/27/05, John W. Krahn <[EMAIL PROTECTED]> wrote:
> Jay Savage wrote:
> >
> > 4> open (V4, "samcmd v4 2>\&1 |" ) || die "unable to open pipe...
> > Broken?$!";
> >
> > Don't do this. the precedence of || is too high. your code attempts
> > to open a pipe, and if it can't, then it attempts to open "die..." and
> > starts throwing exceptions.
>
> No, that is NOT what happens, it will NEVER attempt to open "die..." with or
> without the parentheses. The ONLY time it will attempt to open "die..." is if
> there are no parentheses and the expression on the left hand side of the ||
> operator evaluates to false.
>
> open V4, '0' || die $!;
>
> But even then it will NOT attempt to open "die..." because die() exits the
> program!
>
>
> John
Is stand corrected. There is no exception; I guess any time I've run
into it, I've relied on whatever was opened, and died anyway. But I
don't know what else to call opens behavior, except attempting to open
die. Except in the case of parenthesis, as you noted, the behavior of
open || die sure looks like this to me: open (X, badfile || die).
The only difference between the two expressions below is the
precedence of the operator.
[EMAIL PROTECTED]:~> perl -e 'open FH, "< BAdFiLe" || die "$!"'
[EMAIL PROTECTED]:~> perl -e 'open FH, "< BAdFiLe" or die "$!"'
No such file or directory at -e line 1.
Open may not technically be trying to open an expression and failing,
I don't know. To be honest, I've never taken apart the source to see.
But the appearance is certainly that that's what happens, and the
result is so similar as to not matter. Especially consider the
following:
perl -e 'open FH, "< BAdFiLe" || die or die "$!"; print "didnt die\n" '
No such file or directory at -e line 1.
Where did the first die go if || didn't attempt to pass it to open?
if the reason for the failed open were the attempt to open "BAdFiLe",
the first die would execute and the program would exit bore it got to
the second. But clearly that's not what's happening. The first die
is getting slurped up by ||, which is presumably trying to pass it on
to open. When that fails, the second die executes, exiting with $!.
at least that's what it looks like to me.
So what's really happeneing here?
--jay
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