Question

Can someone please explain what’s going on here, please?

I’ve simplified as much as I can.

( 1 || 2 == 1 )     is TRUE (as expected)
( 1 || 2 == 2 )     is TRUE (as expected)

I would also expect the following to both be true (but this may suggest a lack of understanding…)

( 1 == ( 1 || 2 ) ) is TRUE  
( 2 == ( 1 || 2 ) ) is FALSE <--- !!! I don't understand this..

Now it starts getting a bit odd…

( 2 == 1 || 2 )     is TRUE  
( 3 == 1 || 2 )     is TRUE  <--- !!! I don't understand this..

After some more playing about, I find the following:

( 1 == ( 1 || 2 ) ) is FALSE
( 2 == ( 1 || 2 ) ) is TRUE

( 1 == ( 1 && 2 ) ) is TRUE
( 2 == ( 1 && 2 ) ) is FALSE

You might have guess that I’m trying to do something along the lines of:

sub foo {
    # ... blah blah ...
    return 0 if $status == ( 2 || 4 || 7 || 9);
    # ... do other stuff ...
    return 1;
}

my $result = foo();
if ($result) { 
    # then do something based on a result
    } else {
    # log failure or whatever
}

I know I can use this idiom, but the extra “$status”(es) seem superfluous:

return 0 if ( $status == 1 || $status == 4 || $status == 7 ); # and so on...

I also know I could use a regex with alternation or to check if the value is in an array of the allowable values.

But I’d like to understand why this is not as I expect.

Answer 1

$status == ( 2 || 4 || 7 || 9)

will not do what you expect. (And if it did do what you expect, what do you think $status == (2 && 4 && 7 && 9) should mean?) That statement is equivalent to

$status == 2

|| and && are binary operators that always return one of their scalar operands. It may help to think of them as equivalent to these binary functions:

sub OR {
    my ($a,$b) = @_;
    if ($a) {
        return $a;
    } else {
        return $b;
    }
}

sub AND {
    my ($a,$b) = @_;
    if ($a) {
        return $b;
    } else {
        return $a;
    }
}

(I’m glossing over the short-circuiting feature of || and &&, but that’s not so important for this post).

Noting that == has higher precedence than || and &&, we can rewrite all your expressions in terms of AND and OR:

( 1 || 2 == 1 )     -->   (OR(1,2==1))  --->  OR(1,0) --> 1  TRUE
( 1 || 2 == 2 )     -->   (OR(1,2==2))  --->  OR(1,1) --> 1  TRUE

These two expressions are both true, but NOT for the reason you were expecting.

( 1 == ( 1 || 2 ) ) -->   (1 == OR(1,2))   --->   (1==1) TRUE
( 2 == ( 1 || 2 ) ) -->   (2 == OR(1,2))   --->   (2==1) FALSE


( 2 == 1 || 2 )     -->   OR(2==1,2)   --->  OR(0,2) -->  2  TRUE  
( 3 == 1 || 2 )     -->   OR(3==1,2)   --->  OR(0,3) -->  3  TRUE


( 1 == ( 1 || 2 ) ) -->   (1==OR(1,2)) --->  (1==1)  TRUE, not FALSE
( 2 == ( 1 || 2 ) ) -->   (2==OR(1,2)) --->  (2==1)  FALSE, not TRUE

( 1 == ( 1 && 2 ) ) -->   (1==AND(1,2)) ---> (1==1) TRUE
( 2 == ( 1 && 2 ) ) -->   (2==AND(1,2)) ---> (2==1) FALSE

And your original idea of checking

$status == ( 2 || 4 || 7 || 9)

is equivalent to

$status == OR(OR(OR(2,4),7),9)
$status == OR(OR(2,7),9)
$status == OR(2,9)
$status == 2