fp-backend

name	fp-backend
description	Functional programming patterns for Node.js/Deno backend development using fp-ts, ReaderTaskEither, and functional dependency injection
risk	unknown
source	community
version	1.0.0
author	kadu
tags	["fp-ts","typescript","backend","functional-programming","node","deno","dependency-injection","reader-task-either"]

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/enprojectnment (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(
    .<>(),
    .(
      (
        .(db..(id)),
        .((e):  ({ : , : e })),
        .(
          user
            ? .(user)
            : .({ : , : , id })
        ),
        .( .( logger.()))
      )
    )
  )

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.(
      (
        
        (input.),
        .(
          .(hasher.(input.))
        ),
        .(
          .(
            db..({
              ...input,
              : hashedPassword,
            })
          )
        )
      )
    )
  )


  findById = (
  : 
): .<, , > =>
  (
    .<>(),
    .(
      (
        .(db..({ : { id } })),
        .(
          user
            ? .(user)
            : .({ :   , id })
        )
      )
    )
  )


  findMany = (
  : 
): .<, , <>> =>
  (
    .<>(),
    .(
      .(
        (
          .,
          .(,  db..({
            : params.,
            : params.,
          })),
          .(,  db..()),
          .( ({
            : users,
            total,
            ...params,
          }))
        )
      )
    )
  )

 checkEmailUnique = (
  : 
): .<, , > =>
  (
    .<>(),
    .(
      (
        .(db..({ : { email } })),
        .(
          existing
            ? .({ :   , email })
            : .()
        )
      )
    )
  )

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)
      )
    ),
    
    .(, 
      (
        items,
        A.(.)(
          .(item.)
        ),
        .(toOrderError)
      )
    ),
    
    .(, 
      .((products, items))
    ),
    
    .(, 
      (
        .(user, total),
        .(toOrderError)
      )
    ),
    
    .(
      (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..!,
        : process.. || ,
      },
    }),
      ()
  )


  = {
  : 
  : 
  : 
  : 
}

 buildInfrastructure = (
  : 
): .<, > =>
  (
    .,
    .(, 
      .(
         () => {
           prisma =  ({
            : { : { : config.. } },
          })
           prisma.$connect()
           prisma
        },
          ()
      )
    ),
    .(, 
      .(
         () => (config..),
          ()
      )
    ),
    .(,  .(())),
    .( ({
      config,
      db,
      redis,
      logger,
    }))
  )


  = {
  : 
  : 
  : 
}

 buildServices = (: ):  ({
  : (),
  : (infra..),
  : (infra.),
})


   =  & 

  buildDeps = (): .<, > =>
  (
    (),
    .(buildInfrastructure),
    .( ({
      ...infra,
      ...(infra),
    }))
  )


  destroyDeps = (: ): .<, > =>
  (
    .(
       () => {
         deps..$disconnect()
         deps..()
      },
        ()
    )
  )

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(
        (deps),
        .( .( .())),
        
        .( (deps))
      )
    )
  )()

   (result. === ) {
    .(, result.)
    process.()
  }
}

()

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.) {
         :
           {
            : ,
            : (error.?.  [])?.() || ,
          }
         :
           {
            : ,
            : error.?.   || ,
          }
         :
           {
            : ,
            : error.?.   || ,
            : ,
          }
      }
    }
     { : , : error }
  })


  createRepository = <
  ,
  ,
  ,
  ,
  
> ({
  : (: ): .<, O.<>> =>
    (
      ( delegate.({ where })),
      .(O.)
    ),

  : (
    ?: ,
    ?: { : ; :  }
  ): .<, []> =>
    ( delegate.({ where, ...pagination })),

  : (: ): .<, > =>
    ( delegate.({ data })),

  : (
    : ,
    : 
  ): .<, > =>
    ( delegate.({ where, data })),

  : (: ): .<, > =>
    ( delegate.({ where })),

  : (?: ): .<, > =>
    ( delegate.({ where })),
})


 userRepo = (prisma, prisma.)

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.(
        .(
          
            deps..$transaction( tx => {
               result =  ({ ...deps, tx })()
               (result. === ) {
                 result. 
              }
               result.
            }),
          (error): E |  {
            
             ( error ===  && error !==  &&   error) {
               error  E
            }
             { : , : error }
          }
        )
      )
    )
  )


  transferFunds = (
  : ,
  : ,
  : 
): .<, , > =>
  (
    (
      .,
      .(,  (fromId, amount)),
      .(,  (toId, amount)),
      .(, 
        (, to, amount)
      ),
      .( transfer)
    )
  )


 debitAccount = (
  : ,
  : 
): .<, , > =>
  (
    .<>(),
    .(
      .(
        (
          .(
            
              tx..({
                : { : accountId },
                : { : { : amount } },
              }),
            toDbError
          ),
          .(
            account. < 
              ? .({ :   , accountId })
              : .(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 =  (req)(deps)()

    (
      result,
      E.(
         (error, res),
         res.(data)
      )
    )
  }


 handleError = (: , : ):  {
   (error.) {
     :
      res.().({ : error. +  })
      
     :
      res.().({ : error. })
      
     :
      res.().({ : error. })
      
    :
      res.().({ :  })
  }
}


 getUserHandler = (
   req...  ,
   .(req..),
  handleError
)

app.(, 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: ): <> => {
     deps = c.()
     result =  (c)(deps)()

     (
      result,
      E.(
         (error, c),
         c.(data)
      )
    )
  }


  validate =
  <T>(: z.<T>): 
   (c, next) => {
     body =  c..()
     result = schema.(body)

     (!result.) {
       c.(
        { : , : result..() },
        
      )
    }

    c.(, result.)
     ()
  }


  :  =  (c, next) => {
   deps = c.()
   token = c..()?.(, )

   (!token) {
     c.({ :  }, )
  }

   result =  (
    deps..(token),
    .( ({ :   , :  }))
  )()

   (E.(result)) {
     c.({ : result.. }, )
  }

  c.(, result.)
   ()
}


 app =  ()

app.(, (deps))
app.(, requireAuth)

app.(
  ,
  (
     .(c..()),
     {
       (error. === ) {
         c.({ :  }, )
      }
       c.({ :  }, )
    }
  )
)

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,
            : .() - ctx.,
          })
        )
      )
    )

  log = {
  : (),
  : (),
  : (),
}


  :  =  (c, next) => {
   deps = c.()
   :  = {
    : crypto.(),
    : O.(c.()?.),
    : .(),
  }

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

  
  deps..(, {
    : ctx.,
    : c..,
    : c..,
  })

   ()

  
  deps..(, {
    : ctx.,
    : c..,
    : .() - ctx.,
  })
}

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 }
  | { :  }

  = {
  : 
  : 
  : 
}


  notFound = (: , : ):  ({
  : ,
  resource,
  id,
})

  validation = (
  : ,
  : ,
  ?: 
):  ({
  : ,
  field,
  message,
  value,
})

  conflict = (
  : ,
  : ,
  : 
):  ({
  : ,
  resource,
  field,
  value,
})


  toHttpStatus = (: ):  {
   (error.) {
     :
       
     :
       
     :
       
     :
       
     :
       
     :
       
     :
       
    :
       
  }
}


  toResponseBody = (
  : 
): { : ; ?:  } => {
   (error.) {
     :
       { :  }
     :
       {
        : ,
        : { : error., : error. },
      }
     :
       {
        : ,
      }
     :
       { :  }
     :
       { :  }
     :
       { :  }
     :
       { :  }
  }
}

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 => {
               (remaining <=  || !(error)) {
                 .(error)
              }
               (
                .(  ( (r, delay))),
                .( (remaining - , delay * ))
              )
            })
          )

         .((maxAttempts - , baseDelayMs))
      })
    )


  withFallback =
  <R  { :  }, E, A>
  (
    : .<R, E, A>
  ): .<R, E, A> =>
    (
      .<R>(),
      .(
        (
          operation,
          
          .(
            .(cache.(cacheKey, result, ttlSeconds))
          ),
          
          .(
            (
              .(cache.<A>(cacheKey)),
              .(
                cached ? .(cached) : .(error)
              )
            )
          )
        )
      )
    )


  =  |  | 

  createCircuitBreaker = <E> {
   :  = 
   failures = 
   lastFailure = 

   <R, A>(
    : .<R, E, A>
  ): .<R, E | { :  }, A> =>
    (
      .<R>(),
      .( {
        
         (
          state ===  &&
          .() - lastFailure > resetTimeoutMs
        ) {
          state = 
        }

         (state === ) {
           .({ :    })
        }

         (
          operation,
          .( {
             (state === ) {
              state = 
              failures = 
            }
             .()
          }),
          .( {
             ((error)) {
              failures++
              lastFailure = .()
               (failures >= failureThreshold) {
                state = 
              }
            }
             .()
          })
        )
      })
    )
}

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 })),
    },
  },
  : {
    : vi.( .()),
    : vi.( .()),
  },
  : {
    : vi.( .()),
  },
  ...overrides,
})

(,  {
  (,  {
    (,  () => {
       deps = ()
       input = {
        : ,
        : ,
        : ,
      }

       result =  .(input)(deps)()

      (E.(result)).()
       (E.(result)) {
        (result..).(input.)
      }
      (deps..).()
    })

    (,  () => {
       existingUser = { : , :  }
       deps = ({
        : {
          : {
            : vi.( .(existingUser)),
            : vi.(),
          },
        },
      })

       result =  .({
        : ,
        : ,
        : ,
      })(deps)()

      (E.(result)).()
       (E.(result)) {
        (result..).()
      }
    })
  })

  (,  {
    (,  () => {
       user = { : , : , :  }
       deps = ({
        : {
          : {
            : vi.( .(user)),
          },
        },
      })

       result =  .()(deps)()

      (E.(result)).()
       (E.(result)) {
        (result.).(user)
      }
    })

    (,  () => {
       deps = ()

       result =  .()(deps)()

      (E.(result)).()
       (E.(result)) {
        (result..).()
        (result..).()
      }
    })
  })
})

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.. = container.()

     depsResult =  ()()
     (E.(depsResult)) {
        ()
    }
    deps = depsResult.

    
     deps..
    
  }, )

  ( () => {
     (deps)()
     container.()
  })

  (,  () => {
    
     createResult =  .({
      : ,
      : ,
      : ,
    })(deps)()

    (E.(createResult)).()
     (E.(createResult)) 

     user = createResult.

    
     findResult =  .(user.)(deps)()

    (E.(findResult)).()
     (E.(findResult)) {
      (findResult..).()
    }
  })
})

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(...), { :  }),
        fc.(fc.(...), { :  })
      )
      .( parts.())

    fc.(
      fc.(validPassword,  {
         result = (password)
         E.(result)
      })
    )
  })

  (,  {
     result = ()
    (E.(result)).()
  })
})

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 enprojectnment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.