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Clojure's sequence library and the threading macro make lazy sequence operations like map, filter, and reduce composable, and their immutable semantics allow efficient re-use of intermediate results. Core.reducers combine multiple map, filter, takes, et al into a single *fold*, taking advantage of stream fusion--and in the upcoming Clojure 1.7, transducers abstract away the underlying collection entirely.
I've been working on concurrent folds, where we sacrifice some order in exchange for parallelism. Tesser generalizes reducers to a two-dimensional fold: concurrent reductions over independent chunks of a sequence, and a second reduction over those values. Higher-order fold combinators allow us to build up faceted data structures which compute many properties of a dataset in a single pass. The same fold can be run efficiently on multicore systems or transparently distributed--e.g. over Hadoop.
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Kyle Kingsbury (aphyr) is a software engineer at Factual, the lead author of the Riemann monitoring system, and principal instigator behind Jepsen: an exploration of distributed systems safety. He's obsessed with Clojure, databases, and networks.