Using a hash with object keys in Perl 6

Question

I'm trying to make a Hash with non-string keys, in my case arrays or lists.

> my %sum := :{(1, 3, 5) => 9, (2, 4, 6) => 12}
{(1 3 5) => 9, (2 4 6) => 12}

Now, I don't understand the following.

How to retrieve an existing element?

> %sum{(1, 3, 5)}
((Any) (Any) (Any))

> %sum{1, 3, 5}
((Any) (Any) (Any))

How to add a new element?

> %sum{2, 4} = 6
(6 (Any))

Answer 1

Several things are going on here: first of all, if you use (1,2,3) as a key, Rakudo Perl 6 will consider this to be a slice of 3 keys: 1, 2 and 3. Since neither of these exist in the object hash, you get ((Any) (Any) (Any)).

So you need to indicate that you want the list to be seen as single key of which you want the value. You can do this with $(), so %sum{$(1,3,5)}. This however does not give you the intended result. The reason behind that is the following:

> say (1,2,3).WHICH eq (1,2,3).WHICH
False

Object hashes internally key the object to its .WHICH value. At the moment, Lists are not considered value types, so each List has a different .WHICH. Which makes them unfit to be used as keys in object hashes, or in other cases where they are used by default (e.g. .unique and Sets, Bags and Mixes).

I'm actually working on making this the above eq return True before long: this should make it to the 2018.01 compiler release, on which also a Rakudo Star release will be based.

BTW, any time you're using object hashes and integer values, you will probably be better of using Bags. Alas not yet in this case either for the above reason.

You could actually make this work by using augment class List and adding a .WHICH method on that, but I would recommend against that as it will interfere with any future fixes.

Answer 2

Elizabeth's answer is solid, but until that feature is created, I don't see why you can't create a Key class to use as the hash key, which will have an explicit hash function which is based on its values rather than its location in memory. This hash function, used for both placement in the list and equality testing, is .WHICH. This function must return an ObjAt object, which is basically just a string.

class Key does Positional {
  has Int @.list handles <elems AT-POS EXISTS-POS ASSIGN-POS BIND-POS push>;
  method new(*@list) { self.bless(:@list); }
  method WHICH() { ObjAt.new(@!list.join('|')); }
}

my %hsh{Key};
%hsh{Key.new(1, 3)} = 'result';
say %hsh{Key.new(1, 3)}; # output: result

Note that I only allowed the key to contain Int. This is an easy way of being fairly confident no element's string value contains the '|' character, which could make two keys look the same despite having different elements. However, this is not hardened against naughty users--4 but role :: { method Str() { '|' } } is an Int that stringifies to the illegal value. You can make the code stronger if you use .WHICH recursively, but I'll leave that as an exercise.

This Key class is also a little fancier than you strictly need. It would be enough to have a @.list member and define .WHICH. I defined AT-POS and friends so the Key can be indexed, pushed to, and otherwise treated as an Array.