array: add reduce() method

vlib/builtin/array.v

@@ -354,3 +354,20 @@ pub fn (a []int) filter(predicate fn(p_val int, p_i int, p_arr []int) bool) []in
 	}
 	return res
 }
+
+////////////// REDUCE //////////////
+
+// method executes a reducer function (that you provide) on each element of the array, 
+// resulting in a single output value.
+pub fn (a []int) reduce(
+	iter fn (accum int, curr int) int,
+	accum_start int
+) int
+{
+	mut _accum := 0
+	_accum = accum_start
+	for i := 0; i < a.len; i++ {
+			_accum = iter(_accum, a[i])
+	}
+	return _accum
+}

vlib/builtin/array_test.v

@@ -314,3 +314,27 @@ fn test_filter() {
 	assert d[0] == 'is'
 	assert d[1] == 'awesome'
 }
+
+fn sum(prev int, curr int) int {
+	return prev + curr
+}
+
+fn sub(prev int, curr int) int {
+	return prev - curr
+}
+
+fn test_reduce() {
+	a := [1, 2, 3, 4, 5]
+	b := a.reduce(sum, 0)
+	c := a.reduce(sum, 5)
+	d := a.reduce(sum, -1)
+	assert b == 15
+	assert c == 20
+	assert d == 14
+
+	e := [1, 2, 3]
+	f := e.reduce(sub, 0)
+	g := e.reduce(sub, -1)
+	assert f == -6
+	assert g == -7
+}