I'd like to propose
Array#product_set to return the product set of arrays (aka cartesian product)
deck = [1..13, %i(spades hearts diamond clubs)].product_set # => <#Enumerator ...> deck.first(2) # => [[1, :spades], [2, :spades]]
product_set would return an enumerator if no block is given. It should raise an error if an element of the array is not an Enumerable, like Array#transpose or #zip do.
Array.product would be acceptable too, I feel that an instance method of array is best in the case, in the same way that
transpose is an instance method and not a class method.
The name "product_set" is a correct mathematical term. Although the synonym "cartesian_product" would also be acceptable, I propose "product_set" because it is shorter and cute too. I feel it is even clearer than
product; the first time I head of
product I was convinced that
[2,3,7].product # => 42.
Addressing objections raised in #6499:
1) This is not for the sake of symmetry, but because often we have an array of the arrays we want a product of.
It is cumbersome to write
arrays.first.product(*arrays[1..-1]) or similar and it hides what is going on.
arrays.product_set is much nicer.
2) The goal is not mainly to get a lazy version, but more to make the API better. The fact that it returns an Enumerator if no block is given is just a bonus :-)
3) .product_set.to_a # => []
This can be seen from a cardinality argument, or for example because
array.repeated_permutation(n) == Array.new(n, array).product_set.to_a and
array.repeated_permutation(0) == [].