Clojure 1.7 introduced a new abstraction called transducers for "composable algorithmic transformations", commonly used to apply a series of transformations over collections. The idea of transducers follows from the reducing function, which accepts arguments of the form (result, input) and returns result. A reducing function is what we typically use with reduce. A transducer accepts a reducing function, wraps/composes over its functionality to provide something extra, and returns another reducing function.

The functions in clojure.core that deal with collections have acquired an arity-1 variant, which returns a transducer, namely map, cat, mapcat, filter, remove, take, take-while, take-nth, drop, drop-while, replace, partition-by, partition-all, keep, keep-indexed, dedupe and random-sample.

Consider the following few examples, all of which do the same thing:

user=> (reduce ((filter odd?) +) [1 2 3 4 5])
9
user=> (transduce (filter odd?) + [1 2 3 4 5])
9
user=> (defn filter...