Generic interfaces blog post

// This program demonstrates a generic interface with a type parameter.
// It compiles with Go 1.27.1, which already supports generic interfaces.

package main

import (
	"fmt"
	"time"
)

// Comparer is a generic interface that requires a Compare method
// accepting a value of the same type.
type Comparer[T any] interface {
	Compare(T) int
}

// TimeWrapper wraps time.Time to implement Comparer[TimeWrapper].
type TimeWrapper struct{ t time.Time }

// Compare compares two TimeWrapper values using the underlying time.Time.
func (tw TimeWrapper) Compare(other TimeWrapper) int {
	if tw.t.Before(other.t) {
		return -1
	}
	if tw.t.After(other.t) {
		return 1
	}
	return 0
}

// Max returns the larger of two values that satisfy Comparer.
func Max[T Comparer[T]](a, b T) T {
	if a.Compare(b) >= 0 {
		return a
	}
	return b
}

func main() {
	now := TimeWrapper{t: time.Now()}
	earlier := TimeWrapper{t: time.Now().Add(-time.Hour)}

	// Max uses the generic Comparer interface to pick the later time.
	later := Max(now, earlier)
	fmt.Println("Later time:", later.t)
}