create package

04-errors-semantics/20-nil-interface-gotcha/myerror.go

@@ -0,0 +1,38 @@
+package myerror
+
+import (
+	"fmt"
+)
+
+type MyError struct {
+	Op string
+}
+
+func (e *MyError) Error() string {
+	if e == nil {
+		return "<nil MyError>"
+	}
+	return fmt.Sprintf("operation failed: %s", e.Op)
+}
+
+// BuggyDoThing returns a *MyError(nil) as error (typed nil pitfall)
+func BuggyDoThing(fail bool) error {
+	if fail {
+		return &MyError{Op: "fail"}
+	}
+	var e *MyError = nil
+	return e // BAD: interface is non-nil!
+}
+
+// FixedDoThing returns a true nil error when no error
+func FixedDoThing(fail bool) error {
+	if fail {
+		return &MyError{Op: "fail"}
+	}
+	return nil // GOOD: interface is nil
+}
+
+// Wraps error for errors.As demonstration
+func WrapError(err error) error {
+	return fmt.Errorf("wrapped: %w", err)
+}