A monoid combines a binary operation with an identity value. In Clojure's reducers, r/monoid adapts a two-argument reducing function so it also supplies the zero-argument initial value required by fold. section 7.1.3
Use r/monoid when a fold needs a reducing function with both arities:
- Provide op as a function accepting two arguments. section 7.1.3
- Provide ctor as a zero-argument function returning the initial value; wrap constants with constantly. section 7.1.3
- Receive a function callable with either zero arguments or two arguments; the latter are passed to op in order. section 7.1.3
Concatenating with fold
(require '[clojure.core.reducers :as r]) (r/fold (r/monoid str (constantly "Concatenate ")) ["th" "is " "str" "ing"]) ;; "Concatenate this string" Passing a value such as 0 instead of (constantly 0) causes a ClassCastException because ctor must be callable. section 7.1.3
(r/fold (r/monoid merge (constantly {})) ; #1
(fn [m k v] (assoc m k (str v)))
(zipmap (range 10) (range 10)))
;; {0 "0", 7 "7", 1 "1", 4 "4",
6 "6", 3 "3", 2 "2", 9 "9",
5 "5", 8 "8"}
Browse a map-fold example that uses merge with an empty-map constructor and combines transformed entries. section 7.1.3
