feat: add reduce function

src/__tests__/builtins.test.js

@@ -1,6 +1,16 @@
 import builtIns from '../builtins'
 
-const { projection, join, map, sortBy, filter, get, assign, reverse } = builtIns
+const {
+  projection,
+  join,
+  map,
+  sortBy,
+  filter,
+  get,
+  assign,
+  reverse,
+  reduce
+} = builtIns
 
 describe('built ins', () => {
   describe('projection', () => {
@@ -46,6 +56,13 @@ describe('built ins', () => {
     })
   })
 
+  describe('reduce', () => {
+    it('returns correct value', () => {
+      const x = (a, b) => a * b + 1; // eslint-disable-line
+      expect(reduce([x, 4])([1, 2])).toEqual(11)
+    })
+  })
+
   describe('sortBy', () => {
     it('returns correct value', () => {
       const id = a => a; // eslint-disable-line

src/builtins.js

@@ -65,5 +65,8 @@ export default {
   ): {[string]: mixed} {
     return Object.assign({}, context, { [variable]: value })
   },
-  reverse: (input: Array<mixed>): Array<mixed> => input.slice().reverse()
+  reverse: (input: Array<mixed>): Array<mixed> => input.slice().reverse(),
+  reduce: ([f, x]: [(mixed) => mixed, mixed]): ((Array<mixed>) => mixed) => (
+    input: Array<mixed>
+  ): mixed => input.reduce(f, x)
 }