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

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:

// 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

// 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

// 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
export const destroyDeps = (deps: AppDeps): TE.TaskEither<Error, void> =>
  pipe(
    TE.tryCatch(
      async () => {
        await deps.db.$disconnect()
        await deps.redis.quit()
      },
      (e) => new Error(`Cleanup error: ${e}`)
    )
  )

Running Programs with Dependencies

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

const program: RTE.ReaderTaskEither<AppDeps, AppError, void> = pipe(
  RTE.ask<AppDeps>(),
  RTE.flatMap(deps =>
    pipe(
      startServer(deps),
      RTE.fromTaskEither
    )
  )
)

const main = async () => {
  const result = await pipe(
    buildDeps(),
    TE.mapLeft((e): AppError => ({ _tag: 'StartupError', cause: e })),
    TE.flatMap(deps =>
      pipe(
        program(deps),
        TE.tap(() => TE.fromIO(() => console.log('Server running'))),
        // Cleanup on exit
        TE.tapError(() => destroyDeps(deps))
      )
    )
  )()

  if (result._tag === 'Left') {
    console.error('Failed to start:', result.left)
    process.exit(1)
  }
}

main()

Database Operations

Prisma Wrappers

// src/lib/db.ts
import * as TE from 'fp-ts/TaskEither'
import * as O from 'fp-ts/Option'
import { PrismaClient, Prisma } from '@prisma/client'

type DbError =
  | { _tag: 'RecordNotFound'; model: string; id: string }
  | { _tag: 'UniqueViolation'; field: string }
  | { _tag: 'ForeignKeyViolation'; field: string }
  | { _tag: 'UnknownDbError'; cause: unknown }

// Wrap Prisma operations
const wrapPrisma = <A>(
  operation: () => Promise<A>
): TE.TaskEither<DbError, A> =>
  TE.tryCatch(operation, (error): DbError => {
    if (error instanceof Prisma.PrismaClientKnownRequestError) {
      switch (error.code) {
        case 'P2002':
          return {
            _tag: 'UniqueViolation',
            field: (error.meta?.target as string[])?.join(', ') || 'unknown',
          }
        case 'P2003':
          return {
            _tag: 'ForeignKeyViolation',
            field: error.meta?.field_name as string || 'unknown',
          }
        case 'P2025':
          return {
            _tag: 'RecordNotFound',
            model: error.meta?.modelName as string || 'unknown',
            id: 'unknown',
          }
      }
    }
    return { _tag: 'UnknownDbError', cause: error }
  })

// Repository factory
export const createRepository = <
  Model,
  CreateInput,
  UpdateInput,
  WhereUnique,
  WhereMany
>(
  db: PrismaClient,
  delegate: {
    findUnique: (args: { where: WhereUnique }) => Promise<Model | null>
    findMany: (args: { where?: WhereMany; skip?: number; take?: number }) => Promise<Model[]>
    create: (args: { data: CreateInput }) => Promise<Model>
    update: (args: { where: WhereUnique; data: UpdateInput }) => Promise<Model>
    delete: (args: { where: WhereUnique }) => Promise<Model>
    count: (args?: { where?: WhereMany }) => Promise<number>
  }
) => ({
  findUnique: (where: WhereUnique): TE.TaskEither<DbError, O.Option<Model>> =>
    pipe(
      wrapPrisma(() => delegate.findUnique({ where })),
      TE.map(O.fromNullable)
    ),

  findMany: (
    where?: WhereMany,
    pagination?: { skip: number; take: number }
  ): TE.TaskEither<DbError, Model[]> =>
    wrapPrisma(() => delegate.findMany({ where, ...pagination })),

  create: (data: CreateInput): TE.TaskEither<DbError, Model> =>
    wrapPrisma(() => delegate.create({ data })),

  update: (
    where: WhereUnique,
    data: UpdateInput
  ): TE.TaskEither<DbError, Model> =>
    wrapPrisma(() => delegate.update({ where, data })),

  delete: (where: WhereUnique): TE.TaskEither<DbError, Model> =>
    wrapPrisma(() => delegate.delete({ where })),

  count: (where?: WhereMany): TE.TaskEither<DbError, number> =>
    wrapPrisma(() => delegate.count({ where })),
})

// Usage
const userRepo = createRepository(prisma, prisma.user)

Transaction Handling

// src/lib/transaction.ts
import * as TE from 'fp-ts/TaskEither'
import * as RTE from 'fp-ts/ReaderTaskEither'
import { PrismaClient } from '@prisma/client'
import { pipe } from 'fp-ts/function'

type TxClient = Omit<
  PrismaClient,
  '$connect' | '$disconnect' | '$on' | '$transaction' | '$use'
>

type TxDeps = { tx: TxClient }

// Transaction wrapper
export const withTransaction = <R extends { db: PrismaClient }, E, A>(
  program: RTE.ReaderTaskEither<R & TxDeps, E, A>
): RTE.ReaderTaskEither<R, E | DbError, A> =>
  pipe(
    RTE.ask<R>(),
    RTE.flatMap(deps =>
      RTE.fromTaskEither(
        TE.tryCatch(
          () =>
            deps.db.$transaction(async tx => {
              const result = await program({ ...deps, tx })()
              if (result._tag === 'Left') {
                throw result.left // Rollback
              }
              return result.right
            }),
          (error): E | DbError => {
            // Re-throw domain errors
            if (typeof error === 'object' && error !== null && '_tag' in error) {
              return error as E
            }
            return { _tag: 'UnknownDbError', cause: error }
          }
        )
      )
    )
  )

// Usage in service
export const transferFunds = (
  fromId: string,
  toId: string,
  amount: number
): RTE.ReaderTaskEither<AppDeps, TransferError, Transfer> =>
  withTransaction(
    pipe(
      RTE.Do,
      RTE.bind('from', () => debitAccount(fromId, amount)),
      RTE.bind('to', () => creditAccount(toId, amount)),
      RTE.bind('transfer', ({ from, to }) =>
        createTransferRecord(from, to, amount)
      ),
      RTE.map(({ transfer }) => transfer)
    )
  )

// Inside transaction, use tx instead of db
const debitAccount = (
  accountId: string,
  amount: number
): RTE.ReaderTaskEither<TxDeps, TransferError, Account> =>
  pipe(
    RTE.ask<TxDeps>(),
    RTE.flatMap(({ tx }) =>
      RTE.fromTaskEither(
        pipe(
          TE.tryCatch(
            () =>
              tx.account.update({
                where: { id: accountId },
                data: { balance: { decrement: amount } },
              }),
            toDbError
          ),
          TE.flatMap(account =>
            account.balance < 0
              ? TE.left({ _tag: 'InsufficientFunds' as const, accountId })
              : TE.right(account)
          )
        )
      )
    )
  )

Middleware Patterns

Express Middleware

// src/middleware/fp-express.ts
import { Request, Response, NextFunction, RequestHandler } from 'express'
import * as TE from 'fp-ts/TaskEither'
import * as RTE from 'fp-ts/ReaderTaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

// Convert RTE handler to Express middleware
export const toHandler =
  <R, E, A>(
    getDeps: (req: Request) => R,
    handler: (req: Request) => RTE.ReaderTaskEither<R, E, A>,
    onError: (error: E, res: Response) => void
  ): RequestHandler =>
  async (req, res, next) => {
    const deps = getDeps(req)
    const result = await handler(req)(deps)()

    pipe(
      result,
      E.fold(
        error => onError(error, res),
        data => res.json(data)
      )
    )
  }

// Error handler
const handleError = (error: AppError, res: Response): void => {
  switch (error._tag) {
    case 'NotFound':
      res.status(404).json({ error: error.resource + ' not found' })
      break
    case 'ValidationError':
      res.status(400).json({ error: error.message })
      break
    case 'Unauthorized':
      res.status(401).json({ error: error.reason })
      break
    default:
      res.status(500).json({ error: 'Internal server error' })
  }
}

// Usage
const getUserHandler = toHandler(
  req => req.app.locals.deps as AppDeps,
  req => UserService.findById(req.params.id),
  handleError
)

app.get('/users/:id', getUserHandler)

Hono Middleware

// src/middleware/fp-hono.ts
import { Hono, Context, MiddlewareHandler } from 'hono'
import * as RTE from 'fp-ts/ReaderTaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

// Store deps in context
declare module 'hono' {
  interface ContextVariableMap {
    deps: AppDeps
  }
}

// Dependency injection middleware
export const withDeps = (deps: AppDeps): MiddlewareHandler =>
  async (c, next) => {
    c.set('deps', deps)
    await next()
  }

// Convert RTE to Hono handler
export const toHonoHandler =
  <E, A>(
    handler: (c: Context) => RTE.ReaderTaskEither<AppDeps, E, A>,
    onError: (error: E, c: Context) => Response
  ) =>
  async (c: Context): Promise<Response> => {
    const deps = c.get('deps')
    const result = await handler(c)(deps)()

    return pipe(
      result,
      E.fold(
        error => onError(error, c),
        data => c.json(data)
      )
    )
  }

// Validation middleware
export const validate =
  <T>(schema: z.ZodSchema<T>): MiddlewareHandler =>
  async (c, next) => {
    const body = await c.req.json()
    const result = schema.safeParse(body)

    if (!result.success) {
      return c.json(
        { error: 'Validation failed', details: result.error.flatten() },
        400
      )
    }

    c.set('validatedBody', result.data)
    await next()
  }

// Auth middleware using RTE
export const requireAuth: MiddlewareHandler = async (c, next) => {
  const deps = c.get('deps')
  const token = c.req.header('Authorization')?.replace('Bearer ', '')

  if (!token) {
    return c.json({ error: 'No token provided' }, 401)
  }

  const result = await pipe(
    deps.jwt.verify(token),
    TE.mapLeft(() => ({ _tag: 'Unauthorized' as const, reason: 'Invalid token' }))
  )()

  if (E.isLeft(result)) {
    return c.json({ error: result.left.reason }, 401)
  }

  c.set('user', result.right)
  await next()
}

// Usage
const app = new Hono()

app.use('*', withDeps(deps))
app.use('/api/*', requireAuth)

app.get(
  '/api/users/:id',
  toHonoHandler(
    c => UserService.findById(c.req.param('id')),
    (error, c) => {
      if (error._tag === 'UserNotFound') {
        return c.json({ error: 'User not found' }, 404)
      }
      return c.json({ error: 'Internal error' }, 500)
    }
  )
)

Request Context Pattern

// src/context.ts
import * as RTE from 'fp-ts/ReaderTaskEither'
import { pipe } from 'fp-ts/function'

// Request-scoped context
type RequestContext = {
  requestId: string
  userId: O.Option<string>
  startTime: number
}

type ContextDeps = AppDeps & { ctx: RequestContext }

// Logging with context
const logWithContext =
  (level: 'info' | 'warn' | 'error') =>
  (message: string, meta?: object): RTE.ReaderTaskEither<ContextDeps, never, void> =>
    pipe(
      RTE.ask<ContextDeps>(),
      RTE.flatMap(({ logger, ctx }) =>
        RTE.fromIO(() =>
          loggerlevel,
            elapsed: Date.now() - ctx.startTime,
          })
        )
      )
    )

export const log = {
  info: logWithContext('info'),
  warn: logWithContext('warn'),
  error: logWithContext('error'),
}

// Middleware to create context
export const withContext: MiddlewareHandler = async (c, next) => {
  const deps = c.get('deps')
  const ctx: RequestContext = {
    requestId: crypto.randomUUID(),
    userId: O.fromNullable(c.get('user')?.id),
    startTime: Date.now(),
  }

  c.set('deps', { ...deps, ctx })

  // Log request start
  deps.logger.info('Request started', {
    requestId: ctx.requestId,
    method: c.req.method,
    path: c.req.path,
  })

  await next()

  // Log request end
  deps.logger.info('Request completed', {
    requestId: ctx.requestId,
    status: c.res.status,
    elapsed: Date.now() - ctx.startTime,
  })
}

Error Handling Patterns

Typed Error Hierarchy

// src/errors.ts
import * as E from 'fp-ts/Either'
import * as O from 'fp-ts/Option'

// Base error types
type DomainError =
  | NotFoundError
  | ValidationError
  | ConflictError
  | AuthError
  | InfrastructureError

type NotFoundError = {
  _tag: 'NotFoundError'
  resource: string
  id: string
}

type ValidationError = {
  _tag: 'ValidationError'
  field: string
  message: string
  value?: unknown
}

type ConflictError = {
  _tag: 'ConflictError'
  resource: string
  field: string
  value: string
}

type AuthError =
  | { _tag: 'Unauthenticated' }
  | { _tag: 'Unauthorized'; required: string }
  | { _tag: 'TokenExpired' }

type InfrastructureError = {
  _tag: 'InfrastructureError'
  service: string
  cause: unknown
}

// Smart constructors
export const notFound = (resource: string, id: string): NotFoundError => ({
  _tag: 'NotFoundError',
  resource,
  id,
})

export const validation = (
  field: string,
  message: string,
  value?: unknown
): ValidationError => ({
  _tag: 'ValidationError',
  field,
  message,
  value,
})

export const conflict = (
  resource: string,
  field: string,
  value: string
): ConflictError => ({
  _tag: 'ConflictError',
  resource,
  field,
  value,
})

// Error to HTTP status mapping
export const toHttpStatus = (error: DomainError): number => {
  switch (error._tag) {
    case 'NotFoundError':
      return 404
    case 'ValidationError':
      return 400
    case 'ConflictError':
      return 409
    case 'Unauthenticated':
      return 401
    case 'Unauthorized':
      return 403
    case 'TokenExpired':
      return 401
    case 'InfrastructureError':
      return 503
    default:
      return 500
  }
}

// Error to response body
export const toResponseBody = (
  error: DomainError
): { error: string; details?: unknown } => {
  switch (error._tag) {
    case 'NotFoundError':
      return { error: `${error.resource} not found` }
    case 'ValidationError':
      return {
        error: 'Validation failed',
        details: { field: error.field, message: error.message },
      }
    case 'ConflictError':
      return {
        error: `${error.resource} with ${error.field} already exists`,
      }
    case 'Unauthenticated':
      return { error: 'Authentication required' }
    case 'Unauthorized':
      return { error: `Permission denied: ${error.required}` }
    case 'TokenExpired':
      return { error: 'Token expired' }
    case 'InfrastructureError':
      return { error: 'Service temporarily unavailable' }
  }
}

Error Recovery

// src/lib/recovery.ts
import * as RTE from 'fp-ts/ReaderTaskEither'
import * as TE from 'fp-ts/TaskEither'
import { pipe } from 'fp-ts/function'

// Retry with exponential backoff
export const withRetry =
  <R, E, A>(
    maxAttempts: number,
    baseDelayMs: number,
    shouldRetry: (error: E) => boolean
  ) =>
  (
    operation: RTE.ReaderTaskEither<R, E, A>
  ): RTE.ReaderTaskEither<R, E, A> =>
    pipe(
      RTE.ask<R>(),
      RTE.flatMap(deps => {
        const attempt = (
          remaining: number,
          delay: number
        ): TE.TaskEither<E, A> =>
          pipe(
            operation(deps),
            TE.orElse(error => {
              if (remaining <= 0 || !shouldRetry(error)) {
                return TE.left(error)
              }
              return pipe(
                TE.fromTask(() => new Promise(r => setTimeout(r, delay))),
                TE.flatMap(() => attempt(remaining - 1, delay * 2))
              )
            })
          )

        return RTE.fromTaskEither(attempt(maxAttempts - 1, baseDelayMs))
      })
    )

// Fallback to cached value
export const withFallback =
  <R extends { cache: CacheClient }, E, A>(
    cacheKey: string,
    ttlSeconds: number
  ) =>
  (
    operation: RTE.ReaderTaskEither<R, E, A>
  ): RTE.ReaderTaskEither<R, E, A> =>
    pipe(
      RTE.ask<R>(),
      RTE.flatMap(({ cache, ...rest }) =>
        pipe(
          operation,
          // On success, cache the result
          RTE.tap(result =>
            RTE.fromTaskEither(cache.set(cacheKey, result, ttlSeconds))
          ),
          // On failure, try to get cached value
          RTE.orElse(error =>
            pipe(
              RTE.fromTaskEither(cache.get<A>(cacheKey)),
              RTE.flatMap(cached =>
                cached ? RTE.right(cached) : RTE.left(error)
              )
            )
          )
        )
      )
    )

// Circuit breaker
type CircuitState = 'closed' | 'open' | 'half-open'

export const createCircuitBreaker = <E>(
  failureThreshold: number,
  resetTimeoutMs: number,
  isFailure: (error: E) => boolean
) => {
  let state: CircuitState = 'closed'
  let failures = 0
  let lastFailure = 0

  return <R, A>(
    operation: RTE.ReaderTaskEither<R, E, A>
  ): RTE.ReaderTaskEither<R, E | { _tag: 'CircuitOpen' }, A> =>
    pipe(
      RTE.ask<R>(),
      RTE.flatMap(deps => {
        // Check if circuit should reset
        if (
          state === 'open' &&
          Date.now() - lastFailure > resetTimeoutMs
        ) {
          state = 'half-open'
        }

        if (state === 'open') {
          return RTE.left({ _tag: 'CircuitOpen' as const })
        }

        return pipe(
          operation,
          RTE.tap(() => {
            if (state === 'half-open') {
              state = 'closed'
              failures = 0
            }
            return RTE.right(undefined)
          }),
          RTE.tapError(error => {
            if (isFailure(error)) {
              failures++
              lastFailure = Date.now()
              if (failures >= failureThreshold) {
                state = 'open'
              }
            }
            return RTE.right(undefined)
          })
        )
      })
    )
}

Testing Strategies

Mocking Dependencies

// src/services/__tests__/user.service.test.ts
import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import * as O from 'fp-ts/Option'
import { describe, it, expect, vi } from 'vitest'
import * as UserService from '../user.service'

// Create mock dependencies
const createMockDeps = (overrides: Partial<UserDeps> = {}): UserDeps => ({
  db: {
    users: {
      findUnique: vi.fn(() => Promise.resolve(null)),
      create: vi.fn(data => Promise.resolve({ id: '1', ...data })),
      update: vi.fn((where, data) => Promise.resolve({ id: where.id, ...data })),
    },
  },
  hasher: {
    hash: vi.fn(password => TE.right(`hashed_${password}`)),
    verify: vi.fn(() => TE.right(true)),
  },
  mailer: {
    send: vi.fn(() => TE.right(undefined)),
  },
  ...overrides,
})

describe('UserService', () => {
  describe('create', () => {
    it('should create a user with hashed password', async () => {
      const deps = createMockDeps()
      const input = {
        email: 'test@example.com',
        password: 'secret123',
        name: 'Test User',
      }

      const result = await UserService.create(input)(deps)()

      expect(E.isRight(result)).toBe(true)
      if (E.isRight(result)) {
        expect(result.right.email).toBe(input.email)
      }
      expect(deps.hasher.hash).toHaveBeenCalledWith('secret123')
    })

    it('should fail when email already exists', async () => {
      const existingUser = { id: '1', email: 'test@example.com' }
      const deps = createMockDeps({
        db: {
          users: {
            findUnique: vi.fn(() => Promise.resolve(existingUser)),
            create: vi.fn(),
          },
        },
      })

      const result = await UserService.create({
        email: 'test@example.com',
        password: 'secret',
        name: 'Test',
      })(deps)()

      expect(E.isLeft(result)).toBe(true)
      if (E.isLeft(result)) {
        expect(result.left._tag).toBe('EmailExists')
      }
    })
  })

  describe('findById', () => {
    it('should return user when found', async () => {
      const user = { id: '1', email: 'test@example.com', name: 'Test' }
      const deps = createMockDeps({
        db: {
          users: {
            findUnique: vi.fn(() => Promise.resolve(user)),
          },
        },
      })

      const result = await UserService.findById('1')(deps)()

      expect(E.isRight(result)).toBe(true)
      if (E.isRight(result)) {
        expect(result.right).toEqual(user)
      }
    })

    it('should return NotFound when user does not exist', async () => {
      const deps = createMockDeps()

      const result = await UserService.findById('nonexistent')(deps)()

      expect(E.isLeft(result)).toBe(true)
      if (E.isLeft(result)) {
        expect(result.left._tag).toBe('UserNotFound')
        expect(result.left.id).toBe('nonexistent')
      }
    })
  })
})

Integration Testing with Test Containers

// src/__tests__/integration/user.integration.test.ts
import { PostgreSqlContainer } from '@testcontainers/postgresql'
import { PrismaClient } from '@prisma/client'
import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
import { describe, it, expect, beforeAll, afterAll } from 'vitest'
import { buildDeps, destroyDeps, AppDeps } from '../../deps'
import * as UserService from '../../services/user.service'

describe('UserService Integration', () => {
  let container: PostgreSqlContainer
  let deps: AppDeps

  beforeAll(async () => {
    // Start PostgreSQL container
    container = await new PostgreSqlContainer().start()

    // Build real dependencies with test database
    process.env.DATABASE_URL = container.getConnectionUri()

    const depsResult = await buildDeps()()
    if (E.isLeft(depsResult)) {
      throw new Error(`Failed to build deps: ${depsResult.left}`)
    }
    deps = depsResult.right

    // Run migrations
    await deps.db.$executeRaw`CREATE EXTENSION IF NOT EXISTS "uuid-ossp"`
    // ... run Prisma migrations
  }, 60000)

  afterAll(async () => {
    await destroyDeps(deps)()
    await container.stop()
  })

  it('should create and retrieve a user', async () => {
    // Create user
    const createResult = await UserService.create({
      email: 'integration@test.com',
      password: 'password123',
      name: 'Integration Test',
    })(deps)()

    expect(E.isRight(createResult)).toBe(true)
    if (E.isLeft(createResult)) return

    const user = createResult.right

    // Retrieve user
    const findResult = await UserService.findById(user.id)(deps)()

    expect(E.isRight(findResult)).toBe(true)
    if (E.isRight(findResult)) {
      expect(findResult.right.email).toBe('integration@test.com')
    }
  })
})

Property-Based Testing

// src/__tests__/property/user.property.test.ts
import * as fc from 'fast-check'
import * as E from 'fp-ts/Either'
import { describe, it, expect } from 'vitest'
import { validateEmail, validatePassword } from '../../validation'

describe('Validation Properties', () => {
  it('valid emails should pass validation', () => {
    fc.assert(
      fc.property(fc.emailAddress(), email => {
        const result = validateEmail(email)
        return E.isRight(result)
      })
    )
  })

  it('passwords meeting requirements should pass', () => {
    const validPassword = fc
      .tuple(
        fc.stringOf(fc.constantFrom(...'abcdefghijklmnopqrstuvwxyz'), {
          minLength: 4,
        }),
        fc.stringOf(fc.constantFrom(...'ABCDEFGHIJKLMNOPQRSTUVWXYZ'), {
          minLength: 1,
        }),
        fc.stringOf(fc.constantFrom(...'0123456789'), { minLength: 1 }),
        fc.stringOf(fc.constantFrom(...'!@#$%^&*'), { minLength: 1 })
      )
      .map(parts => parts.join(''))

    fc.assert(
      fc.property(validPassword, password => {
        const result = validatePassword(password)
        return E.isRight(result)
      })
    )
  })

  it('empty strings should fail email validation', () => {
    const result = validateEmail('')
    expect(E.isLeft(result)).toBe(true)
  })
})

Quick Reference

Common Imports

import * as RTE from 'fp-ts/ReaderTaskEither'
import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import * as O from 'fp-ts/Option'
import * as A from 'fp-ts/Array'
import * as T from 'fp-ts/Task'
import { pipe, flow } from 'fp-ts/function'

RTE Cheat Sheet

Operation	Description
RTE.right(a)	Lift value into success
RTE.left(e)	Create error
RTE.ask<R>()	Get dependencies
RTE.fromTaskEither(te)	Lift TaskEither
RTE.fromEither(e)	Lift Either
RTE.fromOption(onNone)(o)	Lift Option
RTE.flatMap(f)	Chain operations
RTE.map(f)	Transform success
RTE.mapLeft(f)	Transform error
RTE.tap(f)	Side effect on success
RTE.tapError(f)	Side effect on error
RTE.orElse(f)	Recover from error
RTE.getOrElse(f)	Extract with fallback

Service Template

// Template for a new service
import * as RTE from 'fp-ts/ReaderTaskEither'
import { pipe } from 'fp-ts/function'

type MyServiceDeps = {
  db: DatabaseClient
  // ... other dependencies
}

type MyServiceError =
  | { _tag: 'NotFound'; id: string }
  | { _tag: 'ValidationFailed'; reason: string }

export const myOperation = (
  input: Input
): RTE.ReaderTaskEither<MyServiceDeps, MyServiceError, Output> =>
  pipe(
    RTE.ask<MyServiceDeps>(),
    RTE.flatMap(deps =>
      // Your implementation here
      RTE.right(output)
    )
  )

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.