go by example part 3

5-go-by-example/22-errors.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+)
+
+// by convention, errors are the last return value and have type error, a built-in interface
+func f1(arg int) (int, error) {
+	if arg == 42 {
+		// errors.New constructs a basic error value with the given error message
+		return -1, errors.New("can't work with 42")
+	}
+	// a nil value in the error position indicates that there was no error
+	return arg + 3, nil
+}
+
+// it’s possible to use custom types as errors by implementing the Error() method on them
+type argError struct {
+	arg  int
+	prob string
+}
+
+func (e *argError) Error() string {
+	return fmt.Sprintf("%d - %s", e.arg, e.prob)
+}
+
+func f2(arg int) (int, error) {
+	if arg == 42 {
+		// use &argError syntax to build a new struct
+		return -1, &argError{arg, "can't work with it"}
+	}
+	return arg + 3, nil
+}
+
+func main() {
+	// loops below test out each of our error-returning functions
+	for _, i := range []int{7, 42} {
+		if r, e := f1(i); e != nil {
+			fmt.Println("f1 failed:", e)
+		} else {
+			fmt.Println("f1 worked:", r)
+		}
+	}
+	for _, i := range []int{7, 42} {
+		if r, e := f2(i); e != nil {
+			fmt.Println("f2 failed:", e)
+		} else {
+			fmt.Println("f2 worked:", r)
+		}
+	}
+
+	// programmatically use the data in a custom error
+	_, e := f2(42)
+	if ae, ok := e.(*argError); ok {
+		fmt.Println(ae.arg)
+		fmt.Println(ae.prob)
+	}
+}