Passing hash slice to subroutine expecting array reference - arrays

Question

Consider the following perl script:
use strict;
use warnings;
use Data::Dumper;
# Expects an array reference and an undef replacement value
sub undef_to_value(\#$) {
my $array_ref = shift(#_);
my $default = shift(#_);
# Map each element in the array referenced by $array_ref to...
# If the element is defined, itself
# Otherwise, the $default value
return map {
defined($_) ? $_ : $default
} #{ $array_ref };
}
my %grades = ("Alice" => 100, "Bob" => 85, "Carol" => 92);
my #students = ("Alice", "Bob", "Eve");
The following code works as expected:
my #student_grades = #grades{ #students };
#student_grades = undef_to_value(#student_grades, 0);
print Dumper(\#student_grades);
# $VAR1 = [
# 100,
# 85,
# 0
# ];
However, trying to pass a hash slice results in Type of arg 1 to main::undef_to_value must be array (not hash slice):
my #student_grades = undef_to_value( #grades{ #students }, 0 );
How is it that a hash slice can be coaxed into an array with assignment, but not during a subroutine call?
Is there any way to get the failing example to work as a single assignment?

Answer 1

How is it that a hash slice can be coaxed into an array with assignment
It's not. A hash slice in list context evaluates to a number of scalars, and the list assignment operator is perfectly happy with that.
In other words,
#L{qw( a b )} = #R{qw( a b )};
is equivalent to
($L{a}, $L{b}) = ($R{a}, $R{b});
As your comment says, undef_to_value expects an array reference. The prototype provides this if you provide an array, but you are providing" an hash slice instead. That's not a type a variable, so you can't take a reference to it[1].
Just accept scalars instead:
sub undef_to_value {
my $default = shift(#_);
return map { $_ // $default } #_;
}
my #student_grades = undef_to_value(0, #grades{ #students });
Of course, you could simply use
my #student_grades = map { $_ // 0 } #grades{ #students };
\#h{LIST} is equivalent to map { \$_ } #h{LIST}.

Answer 2

You can fool the prototype by referencing and then dereferencing an arbitrary list
undef_to_value( #{[#grades{ #students }]}, 0 );
but this will only modify a copy of your input, so it's not that helpful.
undef_to_value( #{[#grades{ #students }]}, 0 );
print Dumper([#grades{#students}]);
---
$VAR1 = [
100,
85,
undef
];
Fortunately, your undef_to_value function also returns the set of updated values, so you can say
#grades{#students} = undef_to_value( #{[#grades{ #students }]}, 0 );
print Dumper(\%grades);
---
$VAR1 = {
'Bob' => 85,
'Carol' => 92,
'Eve' => 0,
'Alice' => 100
};