learned I didn't need git for external modules and different folders can have different mains

cmd/sum/sum.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+)
+
+func OpenFile(fileName string) (*os.File, error) {
+	fmt.Println("Opening", fileName)
+	return os.Open(fileName)
+}
+
+func CloseFile(file *os.File) {
+	fmt.Println("Closing file")
+	file.Close()
+}
+
+func GetFloats(fileName string) ([]float64, error) {
+	var numbers []float64
+	file, err := OpenFile(fileName)
+	if err != nil {
+		return nil, err
+	}
+	defer CloseFile(file)
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		number, err := strconv.ParseFloat(scanner.Text(), 64)
+		if err != nil {
+			return nil, err
+		}
+		numbers = append(numbers, number)
+	}
+	if scanner.Err() != nil {
+		return nil, scanner.Err()
+	}
+	return numbers, nil
+}
+
+func main() {
+	numbers, err := GetFloats(os.Args[1])
+	if err != nil {
+		log.Fatal(err)
+	}
+	var sum float64 = 0
+	for _, number := range numbers {
+		sum += number
+	}
+	fmt.Printf("Sum: %0.2f\n", sum)
+}