handle-missing-values-with-option

description: Use Option instead of null/undefined to make missing values explicit and type-safe. globs: "**/*.ts" alwaysApply: true

Handle Missing Values with Option

Rule: Use Option instead of null/undefined to make missing values explicit and type-safe.

Example

import { Option, Effect } from "effect"

// ============================================
// 1. Creating Options
// ============================================

// Some - a value is present
const hasValue = Option.some(42)

// None - no value
const noValue = Option.none<number>()

// From nullable - null/undefined becomes None
const fromNull = Option.fromNullable(null)        // None
const fromValue = Option.fromNullable("hello")    // Some("hello")

// ============================================
// 2. Checking and extracting values
// ============================================

const maybeUser = Option.some({ name: "Alice", age: 30 })

// Check if value exists
if (Option.isSome(maybeUser)) {
  console.log(`User: ${maybeUser.value.name}`)
}

// Get with default
const name = Option.getOrElse(
  Option.map(maybeUser, u => u.name),
  () => "Anonymous"
)

// ============================================
// 3. Transforming Options
// ============================================

const maybeNumber = Option.some(5)

// Map - transform the value if present
const doubled = Option.map(maybeNumber, n => n * 2)  // Some(10)

// FlatMap - chain operations that return Option
const safeDivide = (a: number, b: number): Option.Option<number> =>
  b === 0 ? Option.none() : Option.some(a / b)

const result = Option.flatMap(maybeNumber, n => safeDivide(10, n))  // Some(2)

// ============================================
// 4. Domain modeling example
// ============================================

interface User {
  readonly id: string
  readonly name: string
  readonly email: Option.Option<string>  // Email is optional
  readonly phone: Option.Option<string>  // Phone is optional
}

const createUser = (name: string): User => ({
  id: crypto.randomUUID(),
  name,
  email: Option.none(),
  phone: Option.none(),
})

const addEmail = (user: User, email: string): User => ({
  ...user,
  email: Option.some(email),
})

const getContactInfo = (user: User): string => {
  const email = Option.getOrElse(user.email, () => "no email")
  const phone = Option.getOrElse(user.phone, () => "no phone")
  return `${user.name}: ${email}, ${phone}`
}

// ============================================
// 5. Use in Effects
// ============================================

const findUser = (id: string): Effect.Effect<Option.Option<User>> =>
  Effect.succeed(
    id === "123"
      ? Option.some({ id, name: "Alice", email: Option.none(), phone: Option.none() })
      : Option.none()
  )

const program = Effect.gen(function* () {
  const maybeUser = yield* findUser("123")

  if (Option.isSome(maybeUser)) {
    yield* Effect.log(`Found: ${maybeUser.value.name}`)
  } else {
    yield* Effect.log("User not found")
  }
})

Effect.runPromise(program)