Tipe Data Generik
We use generics to create definitions for item declarations, such as structs and functions, which we can then use with many different concrete data types. In Cairo, we can use generics when defining functions, structs, enums, traits, implementations and methods. In this chapter, we are going to take a look at how to effectively use generic types with all of them.
Generics allow us to write reusable code that works with many types, thus avoiding code duplication, while enhancing code maintainability.
Fungsi Generik
Making a function generic means it can operate on different types, avoiding the need for multiple, type-specific implementations. This leads to significant code reduction and increases the flexibility of the code.
Saat mendefinisikan sebuah fungsi yang menggunakan generics, kita menempatkan generics dalam tanda tangan fungsi, di mana kita biasanya akan menentukan tipe data dari parameter dan nilai kembalian. Sebagai contoh, bayangkan kita ingin membuat sebuah fungsi yang, diberikan dua Array item, akan mengembalikan yang terbesar. Jika kita perlu melakukan operasi ini untuk daftar berbagai jenis, maka kita harus mendefinisikan ulang fungsi tersebut setiap kali. Untungnya, kita dapat mengimplementasikan fungsi ini sekali menggunakan generics dan melanjutkan ke tugas-tugas lainnya.
// Specify generic type T between the angulars
fn largest_list<T>(l1: Array<T>, l2: Array<T>) -> Array<T> {
if l1.len() > l2.len() {
l1
} else {
l2
}
}
fn main() {
let mut l1 = array![1, 2];
let mut l2 = array![3, 4, 5];
// There is no need to specify the concrete type of T because
// it is inferred by the compiler
let l3 = largest_list(l1, l2);
}
The largest_list function compares two lists of the same type and returns the one with more elements and drops the other. If you compile the previous code, you will notice that it will fail with an error saying that there are no traits defined for dropping an array of a generic type. This happens because the compiler has no way to guarantee that an Array<T> is droppable when executing the main function. In order to drop an array of T, the compiler must first know how to drop T. This can be fixed by specifying in the function signature of largest_list that T must implement the Drop trait. The correct function definition of largest_list is as follows:
fn largest_list<T, impl TDrop: Drop<T>>(l1: Array<T>, l2: Array<T>) -> Array<T> {
if l1.len() > l2.len() {
l1
} else {
l2
}
}
The new largest_list function includes in its definition the requirement that whatever generic type is placed there, it must be droppable. This is what we call trait bounds. The main function remains unchanged, the compiler is smart enough to deduce which concrete type is being used and if it implements the Drop trait.
Kendala untuk Generic Types
When defining generic types, it is useful to have information about them. Knowing which traits a generic type implements allows us to use it more effectively in a function's logic at the cost of constraining the generic types that can be used with the function. We saw an example of this previously by adding the TDrop implementation as part of the generic arguments of largest_list. While TDrop was added to satisfy the compiler's requirements, we can also add constraints to benefit our function logic.
Bayangkan bahwa kita ingin, diberikan sebuah daftar elemen dari suatu tipe generik T, untuk menemukan elemen terkecil di antara mereka. Awalnya, kita tahu bahwa untuk sebuah elemen tipe T agar dapat dibandingkan, ia harus mengimplementasikan trait PartialOrd. Fungsi yang dihasilkan akan menjadi:
// Given a list of T get the smallest one
// The PartialOrd trait implements comparison operations for T
fn smallest_element<T, impl TPartialOrd: PartialOrd<T>>(list: @Array<T>) -> T {
// This represents the smallest element through the iteration
// Notice that we use the desnap (*) operator
let mut smallest = *list[0];
// The index we will use to move through the list
let mut index = 1;
// Iterate through the whole list storing the smallest
while index < list.len() {
if *list[index] < smallest {
smallest = *list[index];
}
index = index + 1;
};
smallest
}
fn main() {
let list: Array<u8> = array![5, 3, 10];
// We need to specify that we are passing a snapshot of `list` as an argument
let s = smallest_element(@list);
assert!(s == 3);
}
The smallest_element function uses a generic type T that implements the PartialOrd trait, takes a snapshot of an Array<T> as a parameter and returns a copy of the smallest element. Because the parameter is of type @Array<T>, we no longer need to drop it at the end of the execution and so we are not required to implement the Drop trait for T as well. Why does it not compile then?
When indexing on list, the value results in a snap of the indexed element, and unless PartialOrd is implemented for @T we need to desnap the element using *. The * operation requires a copy from @T to T, which means that T needs to implement the Copy trait. After copying an element of type @T to T, there are now variables with type T that need to be dropped, requiring T to implement the Drop trait as well. We must then add both Drop and Copy traits implementation for the function to be correct. After updating the smallest_element function the resulting code would be:
fn smallest_element<T, impl TPartialOrd: PartialOrd<T>, impl TCopy: Copy<T>, impl TDrop: Drop<T>>(
list: @Array<T>,
) -> T {
let mut smallest = *list[0];
let mut index = 1;
while index < list.len() {
if *list[index] < smallest {
smallest = *list[index];
}
index = index + 1;
};
smallest
}
Anonymous Generic Implementation Parameter (+ Operator)
Until now, we have always specified a name for each implementation of the required generic trait: TPartialOrd for PartialOrd<T>, TDrop for Drop<T>, and TCopy for Copy<T>.
However, most of the time, we don't use the implementation in the function body; we only use it as a constraint. In these cases, we can use the + operator to specify that the generic type must implement a trait without naming the implementation. This is referred to as an anonymous generic implementation parameter.
For example, +PartialOrd<T> is equivalent to impl TPartialOrd: PartialOrd<T>.
We can rewrite the smallest_element function signature as follows:
fn smallest_element<T, +PartialOrd<T>, +Copy<T>, +Drop<T>>(list: @Array<T>) -> T {
let mut smallest = *list[0];
let mut index = 1;
loop {
if index >= list.len() {
break smallest;
}
if *list[index] < smallest {
smallest = *list[index];
}
index = index + 1;
}
}
Structs
We can also define structs to use a generic type parameter for one or more fields using the <> syntax, similar to function definitions. First, we declare the name of the type parameter inside the angle brackets just after the name of the struct. Then we use the generic type in the struct definition where we would otherwise specify concrete data types. The next code example shows the definition Wallet<T> which has a balance field of type T.
#[derive(Drop)]
struct Wallet<T> {
balance: T,
}
fn main() {
let w = Wallet { balance: 3 };
}
Kode di atas menghasilkan trait Drop untuk tipe Wallet secara otomatis. Ini setara dengan menulis kode berikut:
struct Wallet<T> {
balance: T,
}
impl WalletDrop<T, +Drop<T>> of Drop<Wallet<T>>;
fn main() {
let w = Wallet { balance: 3 };
}
Kami menghindari menggunakan makro derive untuk implementasi Drop dari Wallet dan sebaliknya mendefinisikan implementasi WalletDrop kami sendiri. Perhatikan bahwa kita harus mendefinisikan, sama seperti fungsi, tipe generik tambahan untuk WalletDrop yang menyatakan bahwa T juga mengimplementasikan trait Drop. Pada dasarnya, kami mengatakan bahwa struct Wallet<T> dapat dihapus selama T juga dapat dihapus.
Terakhir, jika kita ingin menambahkan bidang ke Wallet yang mewakili alamatnya dan kita ingin bidang tersebut berbeda dari T tetapi juga generik, kita hanya perlu menambahkan tipe generik lain di antara <>:
#[derive(Drop)]
struct Wallet<T, U> {
balance: T,
address: U,
}
fn main() {
let w = Wallet { balance: 3, address: 14 };
}
We add to the Wallet struct definition a new generic type U and then assign this type to the new field member address. Notice that the derive attribute for the Drop trait works for U as well.
Enum
Seperti yang kita lakukan dengan struct, kita dapat mendefinisikan enums untuk menyimpan tipe data generik dalam variasi mereka. Sebagai contoh, enum Option<T> yang disediakan oleh perpustakaan inti Cairo:
enum Option<T> {
Some: T,
None,
}
Enum Option<T> adalah generic terhadap tipe T dan memiliki dua variasi: Some, yang menyimpan satu nilai tipe T, dan None yang tidak menyimpan nilai apa pun. Dengan menggunakan enum Option<T>, kita dapat menyatakan konsep abstrak dari nilai opsional dan karena nilai memiliki tipe generik T, kita dapat menggunakan abstraksi ini dengan tipe apa pun.
Enums can use multiple generic types as well, like the definition of the Result<T, E> enum that the core library provides:
enum Result<T, E> {
Ok: T,
Err: E,
}
Enum Result<T, E> memiliki dua tipe generik, T dan E, dan dua variasi: Ok yang menyimpan nilai tipe T dan Err yang menyimpan nilai tipe E. Definisi ini membuatnya nyaman untuk menggunakan enum Result di mana pun kita memiliki operasi yang mungkin berhasil (dengan mengembalikan nilai tipe T) atau gagal (dengan mengembalikan nilai tipe E).
Metode Generic
We can implement methods on structs and enums, and use the generic types in their definitions, too. Using our previous definition of Wallet<T> struct, we define a balance method for it:
#[derive(Copy, Drop)]
struct Wallet<T> {
balance: T,
}
trait WalletTrait<T> {
fn balance(self: @Wallet<T>) -> T;
}
impl WalletImpl<T, +Copy<T>> of WalletTrait<T> {
fn balance(self: @Wallet<T>) -> T {
return *self.balance;
}
}
fn main() {
let w = Wallet { balance: 50 };
assert!(w.balance() == 50);
}
We first define WalletTrait<T> trait using a generic type T which defines a method that returns the value of the field balance from Wallet. Then we give an implementation for the trait in WalletImpl<T>. Note that you need to include a generic type in both definitions of the trait and the implementation.
We can also specify constraints on generic types when defining methods on the type. We could, for example, implement methods only for Wallet<u128> instances rather than Wallet<T>. In the code example, we define an implementation for wallets which have a concrete type of u128 for the balance field.
#[derive(Copy, Drop)]
struct Wallet<T> {
balance: T,
}
/// Generic trait for wallets
trait WalletTrait<T> {
fn balance(self: @Wallet<T>) -> T;
}
impl WalletImpl<T, +Copy<T>> of WalletTrait<T> {
fn balance(self: @Wallet<T>) -> T {
return *self.balance;
}
}
/// Trait for wallets of type u128
trait WalletReceiveTrait {
fn receive(ref self: Wallet<u128>, value: u128);
}
impl WalletReceiveImpl of WalletReceiveTrait {
fn receive(ref self: Wallet<u128>, value: u128) {
self.balance += value;
}
}
fn main() {
let mut w = Wallet { balance: 50 };
assert!(w.balance() == 50);
w.receive(100);
assert!(w.balance() == 150);
}
The new method receive increments the size of balance of any instance of a Wallet<u128>. Notice that we changed the main function making w a mutable variable in order for it to be able to update its balance. If we were to change the initialization of w by changing the type of balance the previous code wouldn't compile.
Cairo allows us to define generic methods inside generic traits as well. Using the past implementation from Wallet<U, V> we are going to define a trait that picks two wallets of different generic types and creates a new one with a generic type of each. First, let's rewrite the struct definition:
struct Wallet<T, U> {
balance: T,
address: U,
}
Next, we are going to naively define the mixup trait and implementation:
// This does not compile!
trait WalletMixTrait<T1, U1> {
fn mixup<T2, U2>(self: Wallet<T1, U1>, other: Wallet<T2, U2>) -> Wallet<T1, U2>;
}
impl WalletMixImpl<T1, U1> of WalletMixTrait<T1, U1> {
fn mixup<T2, U2>(self: Wallet<T1, U1>, other: Wallet<T2, U2>) -> Wallet<T1, U2> {
Wallet { balance: self.balance, address: other.address }
}
}
We are creating a trait WalletMixTrait<T1, U1> with the mixup<T2, U2> method which given an instance of Wallet<T1, U1> and Wallet<T2, U2> creates a new Wallet<T1, U2>. As mixup signature specifies, both self and other are getting dropped at the end of the function, which is why this code does not compile. If you have been following from the start until now you would know that we must add a requirement for all the generic types specifying that they will implement the Drop trait for the compiler to know how to drop instances of Wallet<T, U>. The updated implementation is as follows:
trait WalletMixTrait<T1, U1> {
fn mixup<T2, +Drop<T2>, U2, +Drop<U2>>(
self: Wallet<T1, U1>, other: Wallet<T2, U2>,
) -> Wallet<T1, U2>;
}
impl WalletMixImpl<T1, +Drop<T1>, U1, +Drop<U1>> of WalletMixTrait<T1, U1> {
fn mixup<T2, +Drop<T2>, U2, +Drop<U2>>(
self: Wallet<T1, U1>, other: Wallet<T2, U2>,
) -> Wallet<T1, U2> {
Wallet { balance: self.balance, address: other.address }
}
}
Kami menambahkan persyaratan agar T1 dan U1 dapat di-drop pada deklarasi WalletMixImpl. Selanjutnya, kami melakukan hal yang sama untuk T2 dan U2, kali ini sebagai bagian dari tanda tangan mixup. Sekarang kita dapat mencoba fungsi mixup:
fn main() {
let w1: Wallet<bool, u128> = Wallet { balance: true, address: 10 };
let w2: Wallet<felt252, u8> = Wallet { balance: 32, address: 100 };
let w3 = w1.mixup(w2);
assert!(w3.balance);
assert!(w3.address == 100);
}
Pertama, kita membuat dua instans: satu dari Wallet<bool, u128> dan yang lain dari Wallet<felt252, u8>. Kemudian, kita memanggil mixup dan membuat instans baru Wallet<bool, u8>.