clojure[script] transducers: basic use

simple-transducers.clj

;; attempted translation of the basic parts of the clojure documentation page on transducers http://clojure.org/reference/transducers 
;; into more comprehensible terms that don't require you to be as smart as Rich Hickey to understand.

;; transducers are a way to compose sequence functions with less than their normal arity into a combined transformation 
;; which then gets applied more efficiently than just nesting calls to the transformation, using ->, etc. 
;; In the below, xf is a transducer. The composition goes left to right, hence tests for oddness before testing for less-than-5ness 

(def xf 
  (comp 
        (filter odd?)
        (filter #(> 5 %))))

;; to just apply the composed transformation to a collection, the easiest thing is to use the `into` idiom

(def example [1 2 3 4 5 6 7 8 9 10])

(into [] xf example)
;; [1 3]

;; `transduce` is just like normal `reduce` but with the xf composed with the reducing function first. The docs are a little obscure 
;; on what that means ("with the transducer xform applied to the reducing function f"---huh?) but it seems to mean composition in 
;; that sense.

(transduce xf + example)
;; 4 

(transduce xf conj [] example)
;; [1 3] 

;; I actually find this last one a bit magical. The docs say: 
;; "f supplies the knowledge of how to accumulate the result, which occurs in the (potentially stateful) context of the reduce."
;; and what that seems to mean is "it somehow manages to act as if you applied the transducer to the whole collection before reducing." 

;; `eduction` seems to basically just mean "create a lazy sequence from applying the transducer to the collection" 

(take 2 (eduction xf example))
;; (1 3)

;; for future reference, here's the list of the built-in higher-order sequence functions that make transducers with one less arity, 
;; straight from the docs: 
;;  map cat mapcat filter remove take take-while take-nth drop drop-while replace partition-by partition-all keep keep-indexed 
;; map-indexed distinct interpose dedupe random-sample

;; Christophe Grand has also written tranducerific support for more core functions at https://github.com/cgrand/xforms 
;; right now they seem a little hard to understand (especially x/reduce!!) but reading the code is interesting...

;; this is a nice SO: http://stackoverflow.com/questions/26317325/can-someone-explain-clojure-transducers-to-me-in-simple-terms

Typo on line 2: "comprensible" -> "comprehensible"

thanks!