difference

Remove the contents of the supplied list from the pipeline

ruby…
[1,1,2,2,3,4] - [1,3]
# => [2, 2, 4]
clojure…
(remove #{1 3} [1 1 2 2 3 4])
;; => (2 2 4)

Difference, as an operator, is something that's primarily used in the context of a nested operator expresson, but is also useful in a pipeline.

Strictly it isn't needed if you have filter since you can always use an appropriately configured filter to remove a given list of elements. (Indeed this is what the clojure example is actually doing since a clojure set can be used as a function which tests its argument to see if it's a member of the list.)

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

In Ruby's case, using reject is more verbose but avoids the messy tangles that can appear when using infix operators in pipelines

Difference is often seen as a set operation, but commonly these kinds of operations do attempt to remove duplicates. (Clojure does have a proper set difference function in clojure.set, but it only works on sets and it's argument ordering doesn't work with the threading operator.)

This page describes an operation in the collection pipeline pattern. For more context read: