Communicating Intention with Functional TypeScript

Communicating Intention
with Functional TypeScript

What don’t you like
about TypeScript?

Thiago Temple
Software Engineer
@SurveyMonkey

Function
Signatures
The Any
Type
Strings
Invalid
Object
States
Immutability

function add(n1: number, n2: number) { }
function processOrder(order: Order, options: Orde

type OrderResult =
ProcessedOrder |
OrderAction |
string;
function processOrder
( order: Order,
options: OrderOptions): OrderResult {

type OrderResult =
ProcessedOrder |
OrderAction |
string;

type OrderProcessingError = string;
type OrderResult =
ProcessedOrder |
OrderAction |
string;

type OrderProcessingError = string;
type OrderResult =
ProcessedOrder |
OrderAction |
OrderProcessingError;

function processOrder
( order: Order,
options: OrderOptions) {

type Result<TResult, TError> =
Success<TResult> |
Failure<TError>;
type Success<T> = {
kind: "successfulResult";
value: T;
};
type Failure<T> = {
kind: "failedResult";
error: T;
};

type OrderResult =
Result<ProcessedOrder | OrderAction,
OrderProcessingError>;
function processOrder(
order: Order,
options: OrderOptions): OrderResult {

const result =
processOrder(order, orderOptions);
if (result.kind === "failedResult") {
result.error;
// OrderProcessingError A.K.A. string
} else {
result.value;
// ProcessedOrder | OrderAction
}

• Not Found
• An Error Has Occurred
• Unpredicted Case
• Something is Not Required
• Condition as False-y
• And More…
Versatile

function findUserById(id: string):
User | null {

type Option<T> =
Some<T> | None;
type Some<T> = {
kind: "someOption";
value: T;
};
type None = {
kind: "noneOption";
}

type Option<T> =
Some<T> | None;
function findUserById(id: string):
Option<User> {

const user =
findUserById(userId);
if (user.kind === "someOption") {
user.value; // User
}

–Thiago Temple
“Don’t use any as a catch-all for types you’re unsure
of, also, don’t be lazy.”

Valid Uses of any
• When you know nothing about the given value
• When you don’t care about the given value

function exec(args: unknown) {
args["someKey"] // Compile Error
}

function exec<T extends { someKey: string }>(args: T) {
args["someKey"] // OK
}

const val1 = lib.someFun();
lib2.otherFunc(val1);
// Err: val2 is of != type

const val1: any = lib.someFun();
lib2.otherFunc(val1);

const val1 = lib.someFun();
lib2.otherFunc(val1 as any);

function findByGenre(genre: string): Movie[]

type Genre =
"Drama" |
"Comedy" |
"Action";
function findByGenre(genre: Genre): Movie[] {

type OrderStatus =
"New" |
"Paid" |
"Shipped" |
"Completed";
type Order = {
status: OrderStatus
// ...
};

function processOrder(order: Order): Order {
switch(order.status) {
case "New":
return ...;
case "Paid":
return ...;
case "Shipped":
return ...;
case "Completed":
return ...;
}
}

nvalid States Impossible to Re

type Payment = {
amount: number;
creditCardInfo?: CreditCardInfo;
payPalInfo?: PayPalInfo;
cash: boolean;
}

const payment: Payment = {
amount: 42,
cash: false
};

type Payment = {
amount: number;
paymentInfo: PaymentInfo;
}
type PaymentInfo =
CreditCardInfo |
PayPalInfo |
Cash;

type CreditCardInfo = {
kind: "creditCardInfo";
// ...
};
type PayPalInfo = {
kind: "payPalInfo";
// ...
};
type Cash = {
kind: "cash";
}

const payment: Payment = {
amount: 42,
paymentInfo: {
kind: "creditCardInfo",
number: 12321312312312312312312
}
};

type Order = {
orderId: string;
billingAddress: Address;
shippingAddress?: Address;
};

type Order = {
orderId: string;
shipping: ShippingInfo;
};
type ShippingInfo =
SameAsBilling |
DownloadableContent |
ShippingAddress;

type SameAsBilling = {
kind: "sameAsBilling";
}
type DownloadableContent = {
kind: "downloadableContent";
}
type ShippingAddress = {
kind: "shippingAddress";
address: Address;
}

ShippingResult {
switch(order.shipping.kind) {
case "downloadableContent":
return ...;
case "sameAsBilling":
return ...;
case "shippingAddress":
order.shipping.address; // Address

function processOrders(orders: readonly Order[]) {
orders.push({...}) // Error
orders.pop() // Error
orders.reverse() // Error
}

function processOrders(order: readonly Order) {

function processOrders
(order: Readonly<Order>) {
order.orderId = 123; // Error
order.customer.customerId = 123; // OK
}

type Immutable<T> = {
readonly [K in keyof T]: Immutable<T[K]>;
}
type Order = Immutable<{
orderId: string;
customer: Customer;
shipping: ShippingInfo;
}>;

function processOrder(order: Immutable<Order>)

function processOrder(order: Immutable<Order>) {
order.orderId = ""; // Error
order.customer.customerId = ""; // Error
order.shipping = { kind: "sameAsBilling" }; // Error
}

Compiled Code Doesn’t Get Stale

Think About The Next Person
(and yourself)

Be Mindful With Your Use of any

Communicating Intention with Functional TypeScript

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