fp-backend

What this skill does

# fp-ts Backend Patterns

Functional programming patterns for building type-safe, testable backend services using fp-ts.

## When to Use
- You are building or refactoring a Node.js or Deno backend with fp-ts.
- The task involves dependency injection, service composition, or typed backend errors with `ReaderTaskEither`.
- You need functional backend architecture patterns rather than isolated utility snippets.

## Core Concepts

### ReaderTaskEither (RTE)

The `ReaderTaskEither<R, E, A>` type is the backbone of functional backend development:
- **R** (Reader): Dependencies/environment (database, config, logger)
- **E** (Either left): Error type
- **A** (Either right): Success value

```typescript
import * as RTE from 'fp-ts/ReaderTaskEither'
import * as TE from 'fp-ts/TaskEither'
import { pipe } from 'fp-ts/function'

// Define your dependencies
type Deps = {
  db: DatabaseClient
  logger: Logger
  config: Config
}

// Define domain errors
type AppError =
  | { _tag: 'NotFound'; resource: string; id: string }
  | { _tag: 'ValidationError'; message: string }
  | { _tag: 'DatabaseError'; cause: unknown }
  | { _tag: 'Unauthorized'; reason: string }

// A service function
const getUser = (id: string): RTE.ReaderTaskEither<Deps, AppError, User> =>
  pipe(
    RTE.ask<Deps>(),
    RTE.flatMap(({ db, logger }) =>
      pipe(
        RTE.fromTaskEither(db.users.findById(id)),
        RTE.mapLeft((e): AppError => ({ _tag: 'DatabaseError', cause: e })),
        RTE.flatMap(user =>
          user
            ? RTE.right(user)
            : RTE.left({ _tag: 'NotFound', resource: 'User', id })
        ),
        RTE.tap(user => RTE.fromIO(() => logger.info(`Found user: ${user.id}`)))
      )
    )
  )
```

## Service Layer Patterns

### Defining Service Modules

Structure services as modules exporting RTE functions:

```typescript
// src/services/user.service.ts
import * as RTE from 'fp-ts/ReaderTaskEither'
import * as TE from 'fp-ts/TaskEither'
import * as A from 'fp-ts/Array'
import { pipe } from 'fp-ts/function'

type UserDeps = {
  db: DatabaseClient
  hasher: PasswordHasher
  mailer: EmailService
}

type UserError =
  | { _tag: 'UserNotFound'; id: string }
  | { _tag: 'EmailExists'; email: string }
  | { _tag: 'InvalidPassword' }

// Create user
export const create = (
  input: CreateUserInput
): RTE.ReaderTaskEither<UserDeps, UserError, User> =>
  pipe(
    RTE.ask<UserDeps>(),
    RTE.flatMap(({ db, hasher }) =>
      pipe(
        // Check email uniqueness
        checkEmailUnique(input.email),
        RTE.flatMap(() =>
          RTE.fromTaskEither(hasher.hash(input.password))
        ),
        RTE.flatMap(hashedPassword =>
          RTE.fromTaskEither(
            db.users.create({
              ...input,
              password: hashedPassword,
            })
          )
        )
      )
    )
  )

// Find by ID
export const findById = (
  id: string
): RTE.ReaderTaskEither<UserDeps, UserError, User> =>
  pipe(
    RTE.ask<UserDeps>(),
    RTE.flatMap(({ db }) =>
      pipe(
        RTE.fromTaskEither(db.users.findUnique({ where: { id } })),
        RTE.flatMap(user =>
          user
            ? RTE.right(user)
            : RTE.left({ _tag: 'UserNotFound' as const, id })
        )
      )
    )
  )

// Find many with pagination
export const findMany = (
  params: PaginationParams
): RTE.ReaderTaskEither<UserDeps, UserError, PaginatedResult<User>> =>
  pipe(
    RTE.ask<UserDeps>(),
    RTE.flatMap(({ db }) =>
      RTE.fromTaskEither(
        pipe(
          TE.Do,
          TE.bind('users', () => db.users.findMany({
            skip: params.offset,
            take: params.limit,
          })),
          TE.bind('total', () => db.users.count()),
          TE.map(({ users, total }) => ({
            data: users,
            total,
            ...params,
          }))
        )
      )
    )
  )

const checkEmailUnique = (
  email: string
): RTE.ReaderTaskEither<UserDeps, UserError, void> =>
  pipe(
    RTE.ask<UserDeps>(),
    RTE.flatMap(({ db }) =>
      pipe(
        RTE.fromTaskEither(db.users.findUnique({ where: { email } })),
        RTE.flatMap(existing =>
          existing
            ? RTE.left({ _tag: 'EmailExists' as const, email })
            : RTE.right(undefined)
        )
      )
    )
  )
```

### Composing Services

```typescript
// src/services/order.service.ts
import * as UserService from './user.service'
import * as ProductService from './product.service'
import * as PaymentService from './payment.service'

type OrderDeps = UserService.UserDeps &
  ProductService.ProductDeps &
  PaymentService.PaymentDeps & {
    db: DatabaseClient
  }

export const createOrder = (
  userId: string,
  items: OrderItem[]
): RTE.ReaderTaskEither<OrderDeps, OrderError, Order> =>
  pipe(
    RTE.Do,
    // Validate user exists
    RTE.bind('user', () =>
      pipe(
        UserService.findById(userId),
        RTE.mapLeft(toOrderError)
      )
    ),
    // Validate and get products
    RTE.bind('products', () =>
      pipe(
        items,
        A.traverse(RTE.ApplicativePar)(item =>
          ProductService.findById(item.productId)
        ),
        RTE.mapLeft(toOrderError)
      )
    ),
    // Calculate total
    RTE.bind('total', ({ products }) =>
      RTE.right(calculateTotal(products, items))
    ),
    // Process payment
    RTE.bind('payment', ({ user, total }) =>
      pipe(
        PaymentService.charge(user, total),
        RTE.mapLeft(toOrderError)
      )
    ),
    // Create order
    RTE.flatMap(({ user, products, total, payment }) =>
      createOrderRecord(user, products, items, total, payment)
    )
  )
```

## Functional Dependency Injection

### Building the Dependency Container

```typescript
// src/deps.ts
import { pipe } from 'fp-ts/function'
import * as TE from 'fp-ts/TaskEither'
import * as RTE from 'fp-ts/ReaderTaskEither'

// Layer 0: Config (no dependencies)
type Config = {
  database: { url: string; poolSize: number }
  redis: { url: string }
  jwt: { secret: string; expiresIn: string }
}

const loadConfig = (): TE.TaskEither<Error, Config> =>
  TE.tryCatch(
    async () => ({
      database: {
        url: process.env.DATABASE_URL!,
        poolSize: parseInt(process.env.DB_POOL_SIZE || '10'),
      },
      redis: { url: process.env.REDIS_URL! },
      jwt: {
        secret: process.env.JWT_SECRET!,
        expiresIn: process.env.JWT_EXPIRES || '1d',
      },
    }),
    (e) => new Error(`Config error: ${e}`)
  )

// Layer 1: Infrastructure (depends on config)
type Infrastructure = {
  config: Config
  db: PrismaClient
  redis: RedisClient
  logger: Logger
}

const buildInfrastructure = (
  config: Config
): TE.TaskEither<Error, Infrastructure> =>
  pipe(
    TE.Do,
    TE.bind('db', () =>
      TE.tryCatch(
        async () => {
          const prisma = new PrismaClient({
            datasources: { db: { url: config.database.url } },
          })
          await prisma.$connect()
          return prisma
        },
        (e) => new Error(`Database error: ${e}`)
      )
    ),
    TE.bind('redis', () =>
      TE.tryCatch(
        async () => createRedisClient(config.redis.url),
        (e) => new Error(`Redis error: ${e}`)
      )
    ),
    TE.bind('logger', () => TE.right(createLogger())),
    TE.map(({ db, redis, logger }) => ({
      config,
      db,
      redis,
      logger,
    }))
  )

// Layer 2: Services (depends on infrastructure)
type Services = {
  hasher: PasswordHasher
  jwt: JwtService
  mailer: EmailService
}

const buildServices = (infra: Infrastructure): Services => ({
  hasher: createBcryptHasher(),
  jwt: createJwtService(infra.config.jwt),
  mailer: createEmailService(infra.config),
})

// Full application dependencies
export type AppDeps = Infrastructure & Services

export const buildDeps = (): TE.TaskEither<Error, AppDeps> =>
  pipe(
    loadConfig(),
    TE.flatMap(buildInfrastructure),
    TE.map(infra => ({
      ...infra,
      ...buildServices(infra),
    }))
  )

// Cleanup
expor