added code to force to implement all interface functions

input_file.txt

-Hello World !!! From the file
\ No newline at end of file

main.go

 package main
 
-import (
-	"bufio"
-	"fmt"
-	"os"
-)
+import "fmt"
 
-func main() {
-	fileName := os.Args[1]
+type ShapeCalculator interface {
+	Area() int
+	Perimeter() int
+}
 
-	f, err := os.Open(fileName)
-	if err != nil {
-		fmt.Println("Error : ", err)
-		os.Exit(1)
-	}
+type Rectangle struct {
+	Width  int
+	Height int
+}
 
-	defer f.Close()
+func (r *Rectangle) Area() int {
+	return r.Width * r.Height
+}
 
-	scanner := bufio.NewScanner(f)
+// func (r *Rectangle) Perimeter() int {
+// 	return r.Width * r.Height
+// }
 
-	for scanner.Scan() {
+// uncomment the following line to guarantee that Implementation implements all methods of SomeInterface
+// var _ ShapeCalculator = (*Rectangle)(nil) // ← this is the line
 
-		fmt.Println(scanner.Text())
+func main() {
+	rect := &Rectangle{
+		Width:  10,
+		Height: 3,
 	}
+	fmt.Println(rect.Area())
 }
-
-//Execute with file_name as service argument