fn: add reduce func

fn/func.go

@@ -0,0 +1,17 @@
+package fn
+
+// Reducer represents a function that takes an accumulator and the value, then
+// returns a new accumulator.
+type Reducer[T, V any] func(accum T, value V) T
+
+// Reduce takes a slice of something, and a reducer, and produces a final
+// accumulated value.
+func Reduce[T any, V any, S []V](s S, f Reducer[T, V]) T {
+	var accum T
+
+	for _, x := range s {
+		accum = f(accum, x)
+	}
+
+	return accum
+}