intersection

Retains elements that are also in the supplied collection

The notion of a set intersection isn't one that's fundamental to collection pipelines — it naturally fits in better with nested operator expressons — but it is sometimes useful.

Rather than use an explicit intersection function, it's often more straightforward to use filter

[1,1,2,2,3,4].select{|i| [1,3].include? i}
# => [1, 1, 3]

(filter #{1 3} [1 1 2 2 3 4])
;; => (1 1 3)

When operating on a list like this, using a filter naturally results in a result that includes duplicates from the source collection, as well as preserving ordering. If you need to remove duplicates, as you would expect with a set operation, you need to follow the intersection with distinct.

Ruby provides an operator for set intersection.

[1,1,2,2,3,4] & [1,3]
# => [1, 3]

As it's expected to be used in the context of set manipulation, the intersection operator does remove duplicates. Like any infix operator, however, it's awkward to use in a pipeline.

Clojure supplies an intersection function defined on its set data structure. To use it in a pipeline with a sequence, you need to convert the sequence to a set first, and convert back to a sequence when you're done if you need that as your result.

(->> [1 1 2 2 3 4]
     (set)
     (intersection #{1 3})
     (into []))
;; => [1 3]