in reply to BUG when parsing defined-or ?

Insights?

Precedence.

The sub (&), form attempts to parse what follows the block into a list of values to pass to the subroutine, thus it is attempting to parse // "is undef" before it calls the sub and as there is no lvalue to a binary operator, it crunches.

You need parens -- either around the arguments for the sub, or around the entire subcall -- to ensure that the returned value from the sub becomes the lvalue to the operator. 


sub block(&) {
    return undef;
};;

print block {'whatever'} // "is undef";;
String found where operator expected at (eval 10) line 1, near "// "is
+ undef""
        (Missing operator before  "is undef"?)
[Too many arguments for main::block at (eval 10) line 1, near "// "is 
+undef""
syntax error at (eval 10) line 1, near "// "is undef""

print +( block {'whatever'} ) // "is undef";;
is undef

print block( sub{'whatever'} ) // "is undef";;
is undef
"is undef";;

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Re^2: BUG when parsing defined-or ?
by LanX (Saint) on Jan 24, 2016 at 22:41 UTC
    > The sub (&), form attempts to parse what follows the block into a list of values to pass to the subroutine,

    this shouldn't be!

    The prototype is explicitly declaring that only one value (i.e. the block) is passed to the sub.

    > as there is no lvalue to a binary operator, it crunches.

    || is a binary operator too and according to perldoc it has the same precedence.

    and as demonstrated || works as expected.

    Simply phrased: please explain: why || and not //.

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