An accumulator is the result-so-far in a reduction, often named `acc`. A reducing function takes that result and the next collection item, then returns the result to carry forward. The accumulator need not be a number: it can be a map or another value suited to the task. section 3.5.4
See each result-so-far
`(reductions + (map #(* % %) (range 5)))` returns `(0 1 5 14 30)`. Each value includes one more square; `reduce` returns only the final value, `30`. section 3.5.4
(defn next-average [[cnt sum avg] x] ; #1
(let [new-cnt (inc cnt)
new-sum (+ sum x)
new-avg (/ new-sum (double new-cnt))]
[new-cnt new-sum new-avg]))
(defn stock-prices [values]
(reductions next-average [0 0 0] values)) ; #2
(stock-prices [5.4 3.4 7 8.2 11]) ; #3
;; ([0 0 0]
;; [1 5.4 5.4]
;; [2 8.8 4.4]
;; [3 15.8 5.266666666666667]
;; [4 24.0 6.0]
;; [5 35.0 7.0])
A running average carries a vector of count, sum, and average as its accumulator. An explicit initial value, `[0 0 0]`, also appears first in the results from `reductions`. section 3.5.4
Without an initial value, `reduce` starts with the collection’s first item. On an empty or nil collection, it instead calls the reducing function with no arguments: `(reduce + nil)` returns `0`, while `(reduce / [])` raises an `ArityException`. section 3.5.4
