Why does combining Perl hashes in and each expression not work?-CodePudding

I recently encountered this and became most displeased:

while (my ($key, $value) = each (%hash1, %hash2)) {
}

Gave this error: Experimental each on scalar is now forbidden at ...

But this, which seems to be the same operation using a superfluous variable:

my %h = (%hash1, %hash2);
while (my ($key, $value) = each %h) {
}

Compiled and worked just fine.

What's the reason for this, and is my displeasure warranted?

CodePudding user response：

There are a couple of issues that come up here. First, let's deal with your immediate problem.

my %h = (%hash1, %hash2);
while (my ($key, $value) = each(%hash1, %hash2)) { ... }

each is actually an ordinary function in Perl, not a special syntax or something. So, as far as Perl is concerned, you're calling each with two arguments, not one. each expects either an array or hash value (basically, something that begins with % or @), which is why each(%h) works. You can create a local hash and pass that using a bit more convoluted syntax

while (my ($key, $value) = each(%{{%hash1, %hash2}})) { ... }

Here, we use the hashref constructor to make a new hash {%hash1, %hash2}. This is a scalar value that happens to point to a hash. Then we immediately dereference it with %{...}. Unfortunately, this causes another problem. If you try to run this code, it'll compile fine but then infinitely loop forever. To see why this is, we'll need to take a brief tangent.

each is a bit of an oddball in Perl. It's actually stateful and stores the so-called state of its call in the hash object. So

my %h = (a => 1, b => 2);
say each(%h);
say each(%h);

These two calls to each will return different values. One will return ("a", 1) and the other will return ("b", 2) (the order of the two returns is unspecified).

Now, your while condition is going to run anew every time it loops, so if we create a temporary hash at every loop iteration, and Perl is trying to store its each state in the hash every time, then you'll never reach the end of iteration since you'll never iterate more than once on any given hash before it's erased and replaced with a new one.

My recommendation is to just use the temporary. Even if you could do it with each and the merged hash, you'd be making a new merged hash at every loop iteration. Alternatively, you can use keys to simply get all of the keys as a single list. This will only happen once, since it's happening as the head of a for loop.

for my $key (keys %{{%hash1, %hash2}}) {
    my $value = $hash1{$key} // $hash2{$key};
    ...
}

CodePudding user response：

Syntax for each:

each HASH

This means

each %NAME
each %BLOCK
each EXPR->%*

What you have does not match any of those patterns.

For a while, there was an experimental feature that allowed one to use

each EXPR

as long as the expression returned a reference to a hash or array.

The experiment was a failure, so this is no longer allowed. But your code wouldn't work even in a version of Perl with this feature. Your expression (%hash1, %hash2 in scalar context) returns the size of %hash2, which is neither a reference to a hash nor a reference to an array.

Now, you might be tempted to use

each %{ { %hash1, %hash2 } }

Unfortunately, that creates a new hash each time it's evaluated, so you will perpetually get the first element of this new hash.