odyss009 · March 1, 2017 15:56 · Mar 1, 2017
diff --git a/function.js b/function.js
@@ -0,0 +1,52 @@
+// Count the iterations
+let count = 0;
+
+const isMultipleOf3Or5 = n => (n % 3 === 0 || n % 5 === 0);
+
+// Reducer functions can be composed if we abstract the differences.
+// The filter's predicate function will need to be abstracted
+// and curried in for function composition:
+const filterReducer = predicate => reducer => (
+  acc, item
+) => predicate(item) ? reducer(acc, item) : acc;
+
+// The mapping function needs to be abstracted and curried in
+// to make a map reducer composable:
+const mapReducer = mapper => reducer => (
+  acc, item
+) => {
+  count++;
+  return reducer(acc, mapper(item));
+};
+
+// A shared reducing function abstracts how individual values
+// get accumulated together:
+const arrReducer = (acc = [], item) => acc.concat([item]);
+
+const double = x => x * 2;
+
+// Generic function composition
+const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x);
+
+// Compose curried reducers, passing in the shared
+// reducing function. The returned reducer can be directly used
+// by any standard reduce() utility:
+const composeReducers = reducingFn => (...fns) => compose(...fns)(reducingFn);
+
+// Create a transducer: A function which can feed values
+// through multiple reducers without traversing the collection
+// more than once. If you filter the collection in a previous
+// step, the total work will not exceed the size of the filtered
+// set.
+const transducer = composeReducers(arrReducer)(
+  filterReducer(isMultipleOf3Or5),
+  mapReducer(x => x + 1),
+);
+
+// Usage
+const range = (start, end) => Array.from({ length: end - start + 1 }, (x, i) => i + start);
+
+const doubled = range(1,9)
+  .reduce(transducer, []);
+
+console.log(doubled, count);