Error values in depth

1type error interface {
2	Error() string
3}

1// 1. Ad-hoc — a message, wrapping what went wrong underneath
2func parseLine(n int, err error) error {
3	return fmt.Errorf("parsing config line %d: %w", n, err)
4}
5
6// 2. Sentinel — the caller needs to branch on this exact case
7var ErrNotFound = errors.New("not found")
8
9// 3. Custom type — the caller needs DATA out of the error
10type ValidationError struct{ Field, Msg string }
11
12func (e *ValidationError) Error() string { return e.Field + ": " + e.Msg }
13
14// 4. Joined — report every failure, not just the first
15func validate(errs []error) error {
16	return errors.Join(errs...) // nil if errs is empty
17}
18
19func handle(err error) {
20	if errors.Is(err, ErrNotFound) { // is this exact error anywhere in the chain?
21		fmt.Println("not found")
22	}
23	var ve *ValidationError
24	if errors.As(err, &ve) { // is there a *ValidationError in the chain? give it to me
25		fmt.Println("bad field:", ve.Field)
26	}
27}

1package main
2
3import (
4	"errors"
5	"fmt"
6)
7
8type ValidationError struct{ Field, Msg string }
9
10func (e *ValidationError) Error() string { return e.Field + ": " + e.Msg }
11
12func saveUser(age int) error {
13	if age < 0 {
14		return &ValidationError{Field: "age", Msg: "cannot be negative"}
15	}
16	return nil
17}
18
19func main() {
20	err := fmt.Errorf("signup: %w", saveUser(-3)) // wrapped once
21
22	var ve *ValidationError
23	if errors.As(err, &ve) { // found a *ValidationError in the chain
24		fmt.Println("tell the user to fix:", ve.Field) // age
25	}
26	fmt.Println("full message:", err) // signup: age: cannot be negative
27}