main.go

// Demonstrates interfaces, satisfaction, type switches, custom errors.
package main

import (
	"errors"
	"fmt"
)

type Speaker interface {
	Speak() string
}

type Dog struct{ Name string }

func (d Dog) Speak() string { return d.Name + ": woof" }

type Cat struct{ Name string }

func (c Cat) Speak() string { return c.Name + ": meow" }

type Money struct{ Cents int }

func (m Money) String() string {
	return fmt.Sprintf("$%d.%02d", m.Cents/100, m.Cents%100)
}

type ValidationError struct {
	Field, Reason string
}

func (e *ValidationError) Error() string {
	return fmt.Sprintf("validation: %s: %s", e.Field, e.Reason)
}

func validate(name string) error {
	if name == "" {
		return &ValidationError{Field: "name", Reason: "required"}
	}
	return nil
}

func describe(x any) string {
	switch v := x.(type) {
	case nil:
		return "nil"
	case int:
		return fmt.Sprintf("int %d", v)
	case string:
		return fmt.Sprintf("string %q", v)
	case fmt.Stringer:
		return "stringer: " + v.String()
	default:
		return fmt.Sprintf("other %T", v)
	}
}

func main() {
	animals := []Speaker{Dog{"Rex"}, Cat{"Mia"}}
	for _, a := range animals {
		fmt.Println(a.Speak())
	}

	fmt.Println(describe(42))
	fmt.Println(describe("go"))
	fmt.Println(describe(Money{Cents: 1099}))
	fmt.Println(describe(3.14))
	fmt.Println(describe(nil))

	if err := validate(""); err != nil {
		var ve *ValidationError
		if errors.As(err, &ve) {
			fmt.Println("got validation err on field:", ve.Field)
		}
	}
}