fp-backend

# 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 => ...