func main() {
    hmacKey := []byte("secret")

    h := sha256.New()
    hmacInstance := hmac.New(func() hash.Hash { return h }, hmacKey)

    var h2 hash.Hash
    hmacInstance2 := hmac.New(func() hash.Hash { return h2 }, hmacKey)

    h3 := sha512.New()
    f := func() hash.Hash { return h3 }
    hmacInstance3 := hmac.New(f, hmacKey)
}

func main() {
    hmacKey := []byte("secret")
    h := hmac.New(func() hash.Hash { return sha256.New() }, hmacKey)
}