name: fp-refactor
description: Comprehensive guide for refactoring imperative TypeScript code to fp-ts functional patterns
version: 1.0.0
author: fp-ts-skills
tags:
- fp-ts
- refactoring
- functional-programming
- typescript
- migration
- either
- option
- task
- reader

Refactoring Imperative Code to fp-ts

This skill provides comprehensive patterns and strategies for migrating existing imperative TypeScript code to fp-ts functional programming patterns.

Table of Contents

1. Converting try-catch to Either/TaskEither
2. Converting null checks to Option
3. Converting callbacks to Task
4. Converting class-based DI to Reader
5. Converting imperative loops to functional operations
6. Migrating Promise chains to TaskEither
7. Common Pitfalls
8. Gradual Adoption Strategies
9. When NOT to Refactor

1. Converting try-catch to Either/TaskEither

The Problem with try-catch

Traditional try-catch blocks have several issues:
- Error handling is implicit and easy to forget
- The type system doesn't track which functions can throw
- Control flow is non-linear and harder to reason about
- Composing multiple fallible operations is verbose

Pattern: Synchronous try-catch to Either

Before (Imperative)

function parseJSON(input: string): unknown {
  try {
    return JSON.parse(input);
  } catch (error) {
    throw new Error(`Invalid JSON: ${error}`);
  }
}

function validateUser(data: unknown): User {
  try {
    if (!data || typeof data !== 'object') {
      throw new Error('Data must be an object');
    }
    const obj = data as Record<string, unknown>;
    if (typeof obj.name !== 'string') {
      throw new Error('Name is required');
    }
    if (typeof obj.age !== 'number') {
      throw new Error('Age must be a number');
    }
    return { name: obj.name, age: obj.age };
  } catch (error) {
    throw error;
  }
}

// Usage with nested try-catch
function processUserInput(input: string): User | null {
  try {
    const data = parseJSON(input);
    const user = validateUser(data);
    return user;
  } catch (error) {
    console.error('Failed to process user:', error);
    return null;
  }
}

After (fp-ts Either)

import * as E from 'fp-ts/Either';
import * as J from 'fp-ts/Json';
import { pipe } from 'fp-ts/function';

interface User {
  name: string;
  age: number;
}

// Use Json.parse which returns Either<Error, Json>
const parseJSON = (input: string): E.Either<Error, unknown> =>
  pipe(
    J.parse(input),
    E.mapLeft((e) => new Error(`Invalid JSON: ${e}`))
  );

// Validation returns Either, making errors explicit in types
const validateUser = (data: unknown): E.Either<Error, User> => {
  if (!data || typeof data !== 'object') {
    return E.left(new Error('Data must be an object'));
  }
  const obj = data as Record<string, unknown>;
  if (typeof obj.name !== 'string') {
    return E.left(new Error('Name is required'));
  }
  if (typeof obj.age !== 'number') {
    return E.left(new Error('Age must be a number'));
  }
  return E.right({ name: obj.name, age: obj.age });
};

// Compose with pipe and flatMap - errors propagate automatically
const processUserInput = (input: string): E.Either<Error, User> =>
  pipe(
    parseJSON(input),
    E.flatMap(validateUser)
  );

// Handle both cases explicitly
pipe(
  processUserInput('{"name": "Alice", "age": 30}'),
  E.match(
    (error) => console.error('Failed to process user:', error.message),
    (user) => console.log('User:', user)
  )
);

Step-by-Step Refactoring Guide

1. Identify the error type: Determine what errors can occur and create appropriate error types
2. Change return type: From T to Either<E, T> where E is your error type
3. Replace throw statements: Convert throw new Error(...) to E.left(new Error(...))
4. Replace return statements: Convert return value to E.right(value)
5. Remove try-catch blocks: They're no longer needed
6. Update callers: Use pipe with E.flatMap to chain operations

Pattern: Async try-catch to TaskEither

Before (Imperative)

async function fetchUser(id: string): Promise<User> {
  try {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) {
      throw new Error(`HTTP error: ${response.status}`);
    }
    const data = await response.json();
    return validateUser(data);
  } catch (error) {
    throw new Error(`Failed to fetch user: ${error}`);
  }
}

async function fetchUserPosts(userId: string): Promise<Post[]> {
  try {
    const response = await fetch(`/api/users/${userId}/posts`);
    if (!response.ok) {
      throw new Error(`HTTP error: ${response.status}`);
    }
    return await response.json();
  } catch (error) {
    throw new Error(`Failed to fetch posts: ${error}`);
  }
}

// Complex orchestration with try-catch
async function getUserWithPosts(id: string): Promise<{ user: User; posts: Post[] } | null> {
  try {
    const user = await fetchUser(id);
    const posts = await fetchUserPosts(id);
    return { user, posts };
  } catch (error) {
    console.error(error);
    return null;
  }
}

After (fp-ts TaskEither)

import * as TE from 'fp-ts/TaskEither';
import * as E from 'fp-ts/Either';
import { pipe } from 'fp-ts/function';

// Wrap fetch in TaskEither
const fetchUser = (id: string): TE.TaskEither<Error, User> =>
  pipe(
    TE.tryCatch(
      () => fetch(`/api/users/${id}`),
      (reason) => new Error(`Network error: ${reason}`)
    ),
    TE.flatMap((response) =>
      response.ok
        ? TE.right(response)
        : TE.left(new Error(`HTTP error: ${response.status}`))
    ),
    TE.flatMap((response) =>
      TE.tryCatch(
        () => response.json(),
        (reason) => new Error(`JSON parse error: ${reason}`)
      )
    ),
    TE.flatMap((data) => TE.fromEither(validateUser(data)))
  );

const fetchUserPosts = (userId: string): TE.TaskEither<Error, Post[]> =>
  pipe(
    TE.tryCatch(
      () => fetch(`/api/users/${userId}/posts`),
      (reason) => new Error(`Network error: ${reason}`)
    ),
    TE.flatMap((response) =>
      response.ok
        ? TE.right(response)
        : TE.left(new Error(`HTTP error: ${response.status}`))
    ),
    TE.flatMap((response) =>
      TE.tryCatch(
        () => response.json(),
        (reason) => new Error(`JSON parse error: ${reason}`)
      )
    )
  );

// Clean composition with automatic error propagation
const getUserWithPosts = (
  id: string
): TE.TaskEither<Error, { user: User; posts: Post[] }> =>
  pipe(
    TE.Do,
    TE.bind('user', () => fetchUser(id)),
    TE.bind('posts', () => fetchUserPosts(id))
  );

// Execute and handle results
const main = async () => {
  const result = await getUserWithPosts('123')();
  pipe(
    result,
    E.match(
      (error) => console.error('Failed:', error.message),
      ({ user, posts }) => console.log('Success:', user, posts)
    )
  );
};

Helper: tryCatch Utility

Create a reusable wrapper for functions that might throw:

import * as E from 'fp-ts/Either';
import * as TE from 'fp-ts/TaskEither';

// For sync functions
const tryCatchSync = <A>(f: () => A): E.Either<Error, A> =>
  E.tryCatch(f, (e) => (e instanceof Error ? e : new Error(String(e))));

// For async functions
const tryCatchAsync = <A>(f: () => Promise<A>): TE.TaskEither<Error, A> =>
  TE.tryCatch(f, (e) => (e instanceof Error ? e : new Error(String(e))));

2. Converting null checks to Option

The Problem with null/undefined

• TypeScript's strict null checks help, but null still spreads through code
• Chained property access requires verbose null guards
• The distinction between "missing" and "present but null" is unclear
• Easy to forget null checks leading to runtime errors

Pattern: Simple null checks to Option

Before (Imperative)

interface Config {
  database?: {
    host?: string;
    port?: number;
    credentials?: {
      username?: string;
      password?: string;
    };
  };
}

function getDatabaseUrl(config: Config): string | null {
  if (!config.database) {
    return null;
  }
  if (!config.database.host) {
    return null;
  }
  const port = config.database.port ?? 5432;

  let auth = '';
  if (config.database.credentials) {
    if (config.database.credentials.username && config.database.credentials.password) {
      auth = `${config.database.credentials.username}:${config.database.credentials.password}@`;
    }
  }

  return `postgres://${auth}${config.database.host}:${port}`;
}

// Usage requires null check
const url = getDatabaseUrl(config);
if (url !== null) {
  connectToDatabase(url);
} else {
  console.error('Database URL not configured');
}

After (fp-ts Option)

import * as O from 'fp-ts/Option';
import { pipe } from 'fp-ts/function';

const getDatabaseUrl = (config: Config): O.Option<string> =>
  pipe(
    O.fromNullable(config.database),
    O.flatMap((db) =>
      pipe(
        O.fromNullable(db.host),
        O.map((host) => {
          const port = db.port ?? 5432;
          const auth = pipe(
            O.fromNullable(db.credentials),
            O.flatMap((creds) =>
              pipe(
                O.Do,
                O.bind('username', () => O.fromNullable(creds.username)),
                O.bind('password', () => O.fromNullable(creds.password)),
                O.map(({ username, password }) => `${username}:${password}@`)
              )
            ),
            O.getOrElse(() => '')
          );
          return `postgres://${auth}${host}:${port}`;
        })
      )
    )
  );

// Usage is explicit about the optional nature
pipe(
  getDatabaseUrl(config),
  O.match(
    () => console.error('Database URL not configured'),
    (url) => connectToDatabase(url)
  )
);

Pattern: Array find operations

Before (Imperative)

interface User {
  id: string;
  name: string;
  email: string;
}

function findUserById(users: User[], id: string): User | undefined {
  return users.find((u) => u.id === id);
}

function getUserEmail(users: User[], id: string): string | null {
  const user = findUserById(users, id);
  if (!user) {
    return null;
  }
  return user.email;
}

// Chained lookups get messy
function getManagerEmail(users: User[], employee: { managerId?: string }): string | null {
  if (!employee.managerId) {
    return null;
  }
  const manager = findUserById(users, employee.managerId);
  if (!manager) {
    return null;
  }
  return manager.email;
}

After (fp-ts Option)

import * as O from 'fp-ts/Option';
import * as A from 'fp-ts/Array';
import { pipe } from 'fp-ts/function';

const findUserById = (users: User[], id: string): O.Option<User> =>
  A.findFirst<User>((u) => u.id === id)(users);

const getUserEmail = (users: User[], id: string): O.Option<string> =>
  pipe(
    findUserById(users, id),
    O.map((user) => user.email)
  );

const getManagerEmail = (
  users: User[],
  employee: { managerId?: string }
): O.Option<string> =>
  pipe(
    O.fromNullable(employee.managerId),
    O.flatMap((managerId) => findUserById(users, managerId)),
    O.map((manager) => manager.email)
  );

Step-by-Step Refactoring Guide

1. Identify nullable values: Find all T | null, T | undefined, or optional properties
2. Wrap with fromNullable: Convert nullable values to Option at system boundaries
3. Change return types: From T | null to Option<T>
4. Replace null checks: Use O.map, O.flatMap, O.filter instead of if statements
5. Handle at boundaries: Use O.getOrElse, O.match, or O.toNullable when interfacing with non-fp code

Converting Between Option and Either

import * as O from 'fp-ts/Option';
import * as E from 'fp-ts/Either';
import { pipe } from 'fp-ts/function';

// Option to Either: provide error for None case
const optionToEither = <E, A>(onNone: () => E) => (
  option: O.Option<A>
): E.Either<E, A> =>
  pipe(
    option,
    E.fromOption(onNone)
  );

// Example
const findUser = (id: string): O.Option<User> => /* ... */;

const getUser = (id: string): E.Either<Error, User> =>
  pipe(
    findUser(id),
    E.fromOption(() => new Error(`User ${id} not found`))
  );

3. Converting callbacks to Task

The Problem with Callbacks

• Callback hell makes code hard to read
• Error handling is inconsistent
• Difficult to compose and sequence
• No standard way to handle async operations

Pattern: Node-style callbacks to Task

Before (Imperative)

import * as fs from 'fs';

function readFileCallback(
  path: string,
  callback: (error: Error | null, data: string | null) => void
): void {
  fs.readFile(path, 'utf-8', (err, data) => {
    if (err) {
      callback(err, null);
    } else {
      callback(null, data);
    }
  });
}

function processFile(
  inputPath: string,
  outputPath: string,
  callback: (error: Error | null) => void
): void {
  readFileCallback(inputPath, (err, data) => {
    if (err) {
      callback(err);
      return;
    }
    const processed = data!.toUpperCase();
    fs.writeFile(outputPath, processed, (writeErr) => {
      if (writeErr) {
        callback(writeErr);
      } else {
        callback(null);
      }
    });
  });
}

// Callback hell
function processMultipleFiles(
  files: Array<{ input: string; output: string }>,
  callback: (error: Error | null) => void
): void {
  let completed = 0;
  let hasError = false;

  files.forEach(({ input, output }) => {
    if (hasError) return;
    processFile(input, output, (err) => {
      if (hasError) return;
      if (err) {
        hasError = true;
        callback(err);
        return;
      }
      completed++;
      if (completed === files.length) {
        callback(null);
      }
    });
  });
}

After (fp-ts Task/TaskEither)

import * as fs from 'fs/promises';
import * as TE from 'fp-ts/TaskEither';
import * as A from 'fp-ts/Array';
import { pipe } from 'fp-ts/function';

// Wrap fs.promises in TaskEither
const readFile = (path: string): TE.TaskEither<Error, string> =>
  TE.tryCatch(
    () => fs.readFile(path, 'utf-8'),
    (e) => (e instanceof Error ? e : new Error(String(e)))
  );

const writeFile = (path: string, data: string): TE.TaskEither<Error, void> =>
  TE.tryCatch(
    () => fs.writeFile(path, data),
    (e) => (e instanceof Error ? e : new Error(String(e)))
  );

// Clean composition
const processFile = (
  inputPath: string,
  outputPath: string
): TE.TaskEither<Error, void> =>
  pipe(
    readFile(inputPath),
    TE.map((data) => data.toUpperCase()),
    TE.flatMap((processed) => writeFile(outputPath, processed))
  );

// Process multiple files in parallel or sequence
const processMultipleFilesParallel = (
  files: Array<{ input: string; output: string }>
): TE.TaskEither<Error, void[]> =>
  pipe(
    files,
    A.traverse(TE.ApplicativePar)(({ input, output }) =>
      processFile(input, output)
    )
  );

const processMultipleFilesSequential = (
  files: Array<{ input: string; output: string }>
): TE.TaskEither<Error, void[]> =>
  pipe(
    files,
    A.traverse(TE.ApplicativeSeq)(({ input, output }) =>
      processFile(input, output)
    )
  );

Pattern: Converting callback-based APIs

import * as TE from 'fp-ts/TaskEither';

// Generic callback-to-TaskEither converter
const fromCallback = <A>(
  f: (callback: (error: Error | null, result: A | null) => void) => void
): TE.TaskEither<Error, A> =>
  () =>
    new Promise((resolve) => {
      f((error, result) => {
        if (error) {
          resolve({ _tag: 'Left', left: error });
        } else {
          resolve({ _tag: 'Right', right: result as A });
        }
      });
    });

// Usage
const readFileLegacy = (path: string): TE.TaskEither<Error, string> =>
  fromCallback((cb) => fs.readFile(path, 'utf-8', cb));

4. Converting class-based DI to Reader

The Problem with Class-based DI

• Tight coupling between classes and their dependencies
• Testing requires mocking entire class hierarchies
• Dependency injection containers add runtime complexity
• Hard to trace data flow through the application

Pattern: Service classes to Reader

Before (Imperative with Classes)

// Traditional class-based approach
interface Logger {
  log(message: string): void;
  error(message: string): void;
}

interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
}

interface EmailService {
  send(to: string, subject: string, body: string): Promise<void>;
}

class UserService {
  constructor(
    private readonly logger: Logger,
    private readonly userRepo: UserRepository,
    private readonly emailService: EmailService
  ) {}

  async updateEmail(userId: string, newEmail: string): Promise<void> {
    this.logger.log(`Updating email for user ${userId}`);

    const user = await this.userRepo.findById(userId);
    if (!user) {
      this.logger.error(`User ${userId} not found`);
      throw new Error(`User ${userId} not found`);
    }

    const oldEmail = user.email;
    user.email = newEmail;

    await this.userRepo.save(user);

    await this.emailService.send(
      oldEmail,
      'Email Changed',
      `Your email has been changed to ${newEmail}`
    );

    this.logger.log(`Email updated for user ${userId}`);
  }
}

// Manual DI setup
const logger = new ConsoleLogger();
const userRepo = new PostgresUserRepository(dbConnection);
const emailService = new SmtpEmailService(smtpConfig);
const userService = new UserService(logger, userRepo, emailService);

After (fp-ts Reader)

import * as R from 'fp-ts/Reader';
import * as RTE from 'fp-ts/ReaderTaskEither';
import * as TE from 'fp-ts/TaskEither';
import { pipe } from 'fp-ts/function';

// Define the environment/dependencies as an interface
interface AppEnv {
  logger: {
    log: (message: string) => void;
    error: (message: string) => void;
  };
  userRepo: {
    findById: (id: string) => TE.TaskEither<Error, User | null>;
    save: (user: User) => TE.TaskEither<Error, void>;
  };
  emailService: {
    send: (to: string, subject: string, body: string) => TE.TaskEither<Error, void>;
  };
}

// Helper to access environment
const ask = RTE.ask<AppEnv, Error>();

// Service functions using ReaderTaskEither
const logInfo = (message: string): RTE.ReaderTaskEither<AppEnv, Error, void> =>
  pipe(
    ask,
    RTE.map((env) => env.logger.log(message))
  );

const logError = (message: string): RTE.ReaderTaskEither<AppEnv, Error, void> =>
  pipe(
    ask,
    RTE.map((env) => env.logger.error(message))
  );

const findUser = (id: string): RTE.ReaderTaskEither<AppEnv, Error, User | null> =>
  pipe(
    ask,
    RTE.flatMapTaskEither((env) => env.userRepo.findById(id))
  );

const saveUser = (user: User): RTE.ReaderTaskEither<AppEnv, Error, void> =>
  pipe(
    ask,
    RTE.flatMapTaskEither((env) => env.userRepo.save(user))
  );

const sendEmail = (
  to: string,
  subject: string,
  body: string
): RTE.ReaderTaskEither<AppEnv, Error, void> =>
  pipe(
    ask,
    RTE.flatMapTaskEither((env) => env.emailService.send(to, subject, body))
  );

// The updateEmail function using Reader composition
const updateEmail = (
  userId: string,
  newEmail: string
): RTE.ReaderTaskEither<AppEnv, Error, void> =>
  pipe(
    logInfo(`Updating email for user ${userId}`),
    RTE.flatMap(() => findUser(userId)),
    RTE.flatMap((user) => {
      if (!user) {
        return pipe(
          logError(`User ${userId} not found`),
          RTE.flatMap(() => RTE.left(new Error(`User ${userId} not found`)))
        );
      }
      const oldEmail = user.email;
      const updatedUser = { ...user, email: newEmail };

      return pipe(
        saveUser(updatedUser),
        RTE.flatMap(() =>
          sendEmail(
            oldEmail,
            'Email Changed',
            `Your email has been changed to ${newEmail}`
          )
        ),
        RTE.flatMap(() => logInfo(`Email updated for user ${userId}`))
      );
    })
  );

// Build the environment
const createAppEnv = (): AppEnv => ({
  logger: {
    log: (msg) => console.log(`[INFO] ${msg}`),
    error: (msg) => console.error(`[ERROR] ${msg}`),
  },
  userRepo: {
    findById: (id) => TE.tryCatch(
      () => postgresClient.query('SELECT * FROM users WHERE id = $1', [id]),
      (e) => new Error(String(e))
    ),
    save: (user) => TE.tryCatch(
      () => postgresClient.query('UPDATE users SET email = $1 WHERE id = $2', [user.email, user.id]),
      (e) => new Error(String(e))
    ),
  },
  emailService: {
    send: (to, subject, body) => TE.tryCatch(
      () => smtpClient.send({ to, subject, body }),
      (e) => new Error(String(e))
    ),
  },
});

// Run the program
const main = async () => {
  const env = createAppEnv();
  const result = await updateEmail('user-123', '[email protected]')(env)();

  pipe(
    result,
    E.match(
      (error) => console.error('Failed:', error),
      () => console.log('Success!')
    )
  );
};

Testing with Reader

// Easy to test with mock environment
const createTestEnv = (): AppEnv => {
  const logs: string[] = [];
  const savedUsers: User[] = [];
  const sentEmails: Array<{ to: string; subject: string; body: string }> = [];

  return {
    logger: {
      log: (msg) => logs.push(`[INFO] ${msg}`),
      error: (msg) => logs.push(`[ERROR] ${msg}`),
    },
    userRepo: {
      findById: (id) =>
        TE.right(id === 'existing-user' ? { id, email: '[email protected]', name: 'Test' } : null),
      save: (user) => {
        savedUsers.push(user);
        return TE.right(undefined);
      },
    },
    emailService: {
      send: (to, subject, body) => {
        sentEmails.push({ to, subject, body });
        return TE.right(undefined);
      },
    },
  };
};

// Test
describe('updateEmail', () => {
  it('should update email and send notification', async () => {
    const env = createTestEnv();
    const result = await updateEmail('existing-user', '[email protected]')(env)();

    expect(E.isRight(result)).toBe(true);
    // Assert on captured side effects
  });
});

5. Converting imperative loops to functional operations

Pattern: for loops to map/filter/reduce

Before (Imperative)

interface Product {
  id: string;
  name: string;
  price: number;
  category: string;
  inStock: boolean;
}

function processProducts(products: Product[]): {
  totalValue: number;
  categoryCounts: Record<string, number>;
  expensiveProducts: string[];
} {
  let totalValue = 0;
  const categoryCounts: Record<string, number> = {};
  const expensiveProducts: string[] = [];

  for (let i = 0; i < products.length; i++) {
    const product = products[i];

    // Skip out of stock
    if (!product.inStock) {
      continue;
    }

    // Sum total value
    totalValue += product.price;

    // Count categories
    if (categoryCounts[product.category] === undefined) {
      categoryCounts[product.category] = 0;
    }
    categoryCounts[product.category]++;

    // Collect expensive products
    if (product.price > 100) {
      expensiveProducts.push(product.name);
    }
  }

  return { totalValue, categoryCounts, expensiveProducts };
}

After (fp-ts functional operations)

import * as A from 'fp-ts/Array';
import * as R from 'fp-ts/Record';
import { pipe } from 'fp-ts/function';
import * as N from 'fp-ts/number';
import * as Monoid from 'fp-ts/Monoid';

const processProducts = (products: Product[]) => {
  const inStockProducts = pipe(
    products,
    A.filter((p) => p.inStock)
  );

  const totalValue = pipe(
    inStockProducts,
    A.map((p) => p.price),
    A.reduce(0, (acc, price) => acc + price)
  );

  const categoryCounts = pipe(
    inStockProducts,
    A.reduce({} as Record<string, number>, (acc, product) => ({
      ...acc,
      [product.category]: (acc[product.category] ?? 0) + 1,
    }))
  );

  const expensiveProducts = pipe(
    inStockProducts,
    A.filter((p) => p.price > 100),
    A.map((p) => p.name)
  );

  return { totalValue, categoryCounts, expensiveProducts };
};

// Or using a single pass with foldMap for efficiency
import { Monoid as M } from 'fp-ts/Monoid';

interface ProductStats {
  totalValue: number;
  categoryCounts: Record<string, number>;
  expensiveProducts: string[];
}

const productStatsMonoid: M<ProductStats> = {
  empty: { totalValue: 0, categoryCounts: {}, expensiveProducts: [] },
  concat: (a, b) => ({
    totalValue: a.totalValue + b.totalValue,
    categoryCounts: pipe(
      a.categoryCounts,
      R.union({ concat: (x, y) => x + y })(b.categoryCounts)
    ),
    expensiveProducts: [...a.expensiveProducts, ...b.expensiveProducts],
  }),
};

const processProductsSinglePass = (products: Product[]): ProductStats =>
  pipe(
    products,
    A.filter((p) => p.inStock),
    A.foldMap(productStatsMonoid)((product) => ({
      totalValue: product.price,
      categoryCounts: { [product.category]: 1 },
      expensiveProducts: product.price > 100 ? [product.name] : [],
    }))
  );

Pattern: Nested loops to flatMap

Before (Imperative)

interface Order {
  id: string;
  items: OrderItem[];
}

interface OrderItem {
  productId: string;
  quantity: number;
}

function getAllProductIds(orders: Order[]): string[] {
  const productIds: string[] = [];

  for (const order of orders) {
    for (const item of order.items) {
      if (!productIds.includes(item.productId)) {
        productIds.push(item.productId);
      }
    }
  }

  return productIds;
}

After (fp-ts)

import * as A from 'fp-ts/Array';
import { pipe } from 'fp-ts/function';
import * as S from 'fp-ts/Set';
import * as Str from 'fp-ts/string';

const getAllProductIds = (orders: Order[]): string[] =>
  pipe(
    orders,
    A.flatMap((order) => order.items),
    A.map((item) => item.productId),
    A.uniq(Str.Eq)
  );

// Or using Set for better performance with large datasets
const getAllProductIdsSet = (orders: Order[]): Set<string> =>
  pipe(
    orders,
    A.flatMap((order) => order.items),
    A.map((item) => item.productId),
    (ids) => new Set(ids)
  );

Pattern: while loops to recursion/unfold

Before (Imperative)

function paginate<T>(
  fetchPage: (cursor: string | null) => Promise<{ items: T[]; nextCursor: string | null }>
): Promise<T[]> {
  const allItems: T[] = [];
  let cursor: string | null = null;

  while (true) {
    const { items, nextCursor } = await fetchPage(cursor);
    allItems.push(...items);

    if (nextCursor === null) {
      break;
    }
    cursor = nextCursor;
  }

  return allItems;
}

After (fp-ts)

import * as TE from 'fp-ts/TaskEither';
import * as A from 'fp-ts/Array';
import { pipe } from 'fp-ts/function';

interface Page<T> {
  items: T[];
  nextCursor: string | null;
}

const paginate = <T>(
  fetchPage: (cursor: string | null) => TE.TaskEither<Error, Page<T>>
): TE.TaskEither<Error, T[]> => {
  const go = (
    cursor: string | null,
    accumulated: T[]
  ): TE.TaskEither<Error, T[]> =>
    pipe(
      fetchPage(cursor),
      TE.flatMap(({ items, nextCursor }) => {
        const newAccumulated = [...accumulated, ...items];
        return nextCursor === null
          ? TE.right(newAccumulated)
          : go(nextCursor, newAccumulated);
      })
    );

  return go(null, []);
};

// Using unfold for generating sequences
import * as RA from 'fp-ts/ReadonlyArray';

const range = (start: number, end: number): readonly number[] =>
  RA.unfold(start, (n) => (n <= end ? O.some([n, n + 1]) : O.none));

6. Migrating Promise chains to TaskEither

Pattern: Promise.then chains to pipe

Before (Imperative)

function fetchUserData(userId: string): Promise<UserProfile> {
  return fetch(`/api/users/${userId}`)
    .then((response) => {
      if (!response.ok) {
        throw new Error(`HTTP ${response.status}`);
      }
      return response.json();
    })
    .then((data) => validateUserData(data))
    .then((validData) => enrichUserProfile(validData))
    .catch((error) => {
      console.error('Failed to fetch user data:', error);
      throw error;
    });
}

// Chained promises with conditionals
function processOrder(orderId: string): Promise<OrderResult> {
  return getOrder(orderId)
    .then((order) => {
      if (order.status === 'cancelled') {
        throw new Error('Order is cancelled');
      }
      return order;
    })
    .then((order) => validateInventory(order))
    .then((validOrder) => processPayment(validOrder))
    .then((paidOrder) => shipOrder(paidOrder))
    .catch((error) => {
      logError(error);
      return { success: false, error: error.message };
    });
}

After (fp-ts TaskEither)

import * as TE from 'fp-ts/TaskEither';
import * as E from 'fp-ts/Either';
import { pipe } from 'fp-ts/function';

const fetchUserData = (userId: string): TE.TaskEither<Error, UserProfile> =>
  pipe(
    TE.tryCatch(
      () => fetch(`/api/users/${userId}`),
      (e) => new Error(`Network error: ${e}`)
    ),
    TE.flatMap((response) =>
      response.ok
        ? TE.tryCatch(
            () => response.json(),
            (e) => new Error(`Parse error: ${e}`)
          )
        : TE.left(new Error(`HTTP ${response.status}`))
    ),
    TE.flatMap((data) => TE.fromEither(validateUserData(data))),
    TE.flatMap((validData) => enrichUserProfile(validData))
  );

// Conditionals are explicit
const processOrder = (orderId: string): TE.TaskEither<Error, OrderResult> =>
  pipe(
    getOrder(orderId),
    TE.filterOrElse(
      (order) => order.status !== 'cancelled',
      () => new Error('Order is cancelled')
    ),
    TE.flatMap(validateInventory),
    TE.flatMap(processPayment),
    TE.flatMap(shipOrder),
    TE.map((shipped) => ({ success: true, order: shipped })),
    TE.orElse((error) =>
      pipe(
        TE.fromIO(() => logError(error)),
        TE.map(() => ({ success: false, error: error.message }))
      )
    )
  );

Pattern: Promise.all to traverse

Before (Imperative)

async function fetchAllUsers(ids: string[]): Promise<User[]> {
  const promises = ids.map((id) => fetchUser(id));
  return Promise.all(promises);
}

// With error handling for individual items
async function fetchUsersWithFallback(ids: string[]): Promise<Array<User | null>> {
  const promises = ids.map(async (id) => {
    try {
      return await fetchUser(id);
    } catch {
      return null;
    }
  });
  return Promise.all(promises);
}

After (fp-ts)

import * as TE from 'fp-ts/TaskEither';
import * as A from 'fp-ts/Array';
import * as T from 'fp-ts/Task';
import { pipe } from 'fp-ts/function';

// Parallel execution - fails fast on first error
const fetchAllUsers = (ids: string[]): TE.TaskEither<Error, User[]> =>
  pipe(
    ids,
    A.traverse(TE.ApplicativePar)(fetchUser)
  );

// Sequential execution
const fetchAllUsersSequential = (ids: string[]): TE.TaskEither<Error, User[]> =>
  pipe(
    ids,
    A.traverse(TE.ApplicativeSeq)(fetchUser)
  );

// Collect successes, ignore failures (using Task instead of TaskEither)
const fetchUsersWithFallback = (ids: string[]): T.Task<Array<User | null>> =>
  pipe(
    ids,
    A.traverse(T.ApplicativePar)((id) =>
      pipe(
        fetchUser(id),
        TE.match(
          () => null,
          (user) => user
        )
      )
    )
  );

// Or keep track of which failed
const fetchUsersPartitioned = (
  ids: string[]
): T.Task<{ successes: User[]; failures: Array<{ id: string; error: Error }> }> =>
  pipe(
    ids,
    A.traverse(T.ApplicativePar)((id) =>
      pipe(
        fetchUser(id),
        TE.bimap(
          (error) => ({ id, error }),
          (user) => user
        ),
        (te) => te
      )
    ),
    T.map(A.separate),
    T.map(({ left: failures, right: successes }) => ({ successes, failures }))
  );

Pattern: Promise.race to alternative

import * as TE from 'fp-ts/TaskEither';
import * as T from 'fp-ts/Task';
import { pipe } from 'fp-ts/function';

// Race - first to complete wins
const raceTaskEithers = <E, A>(
  tasks: Array<TE.TaskEither<E, A>>
): TE.TaskEither<E, A> =>
  () => Promise.race(tasks.map((te) => te()));

// Try alternatives on failure (like Promise.any but typed)
const tryAlternatives = <E, A>(
  primary: TE.TaskEither<E, A>,
  fallback: TE.TaskEither<E, A>
): TE.TaskEither<E, A> =>
  pipe(
    primary,
    TE.orElse(() => fallback)
  );

// Chain of fallbacks
const withFallbacks = <E, A>(
  tasks: Array<TE.TaskEither<E, A>>
): TE.TaskEither<E, A> =>
  tasks.reduce((acc, task) => pipe(acc, TE.orElse(() => task)));

7. Common Pitfalls

Pitfall 1: Forgetting to run Tasks

// WRONG: Task is not executed
const fetchData = (): TE.TaskEither<Error, Data> => /* ... */;
const result = fetchData(); // This is still a Task, not the result!

// CORRECT: Execute the Task
const result = await fetchData()(); // Note the double invocation

Pitfall 2: Mixing async/await with fp-ts incorrectly

// WRONG: Breaking out of the fp-ts ecosystem
const processData = async (input: string): Promise<Result> => {
  const parsed = parseInput(input); // Returns Either
  if (E.isLeft(parsed)) {
    throw new Error(parsed.left.message); // Don't do this!
  }
  return await fetchData(parsed.right)();
};

// CORRECT: Stay in the ecosystem
const processData = (input: string): TE.TaskEither<Error, Result> =>
  pipe(
    parseInput(input),
    TE.fromEither,
    TE.flatMap(fetchData)
  );

Pitfall 3: Using map when flatMap is needed

// WRONG: Results in nested Either
const result: E.Either<Error, E.Either<Error, User>> = pipe(
  parseUserId(input), // E.Either<Error, string>
  E.map(fetchUser) // Returns E.Either<Error, User>, so we get nested Either
);

// CORRECT: Use flatMap to flatten
const result: E.Either<Error, User> = pipe(
  parseUserId(input),
  E.flatMap(fetchUser)
);

Pitfall 4: Losing error information

// WRONG: Original error context is lost
const fetchData = (): TE.TaskEither<Error, Data> =>
  pipe(
    TE.tryCatch(
      () => fetch('/api/data'),
      () => new Error('Failed') // Lost the original error!
    )
  );

// CORRECT: Preserve error context
const fetchData = (): TE.TaskEither<Error, Data> =>
  pipe(
    TE.tryCatch(
      () => fetch('/api/data'),
      (reason) => new Error(`Network request failed: ${reason}`)
    )
  );

// BETTER: Use typed errors
type FetchError =
  | { _tag: 'NetworkError'; cause: unknown }
  | { _tag: 'ParseError'; cause: unknown }
  | { _tag: 'ValidationError'; message: string };

const fetchData = (): TE.TaskEither<FetchError, Data> =>
  pipe(
    TE.tryCatch(
      () => fetch('/api/data'),
      (cause): FetchError => ({ _tag: 'NetworkError', cause })
    ),
    TE.flatMap((response) =>
      TE.tryCatch(
        () => response.json(),
        (cause): FetchError => ({ _tag: 'ParseError', cause })
      )
    )
  );

Pitfall 5: Overusing fromNullable

// WRONG: Unnecessary wrapping and unwrapping
const getName = (user: User | null): string => {
  const optUser = O.fromNullable(user);
  const name = pipe(optUser, O.map(u => u.name), O.toNullable);
  return name ?? 'Unknown';
};

// CORRECT: Use Option only when you need its composition benefits
const getName = (user: User | null): string => user?.name ?? 'Unknown';

// BETTER: Use Option when chaining multiple operations
const getManagerName = (user: User | null): O.Option<string> =>
  pipe(
    O.fromNullable(user),
    O.flatMap(u => O.fromNullable(u.manager)),
    O.map(m => m.name)
  );

Pitfall 6: Not handling the left case

// WRONG: Ignoring potential errors
const processUser = (input: string): User => {
  const result = parseUser(input); // E.Either<Error, User>
  return (result as E.Right<User>).right; // Unsafe cast!
};

// CORRECT: Always handle both cases
const processUser = (input: string): User =>
  pipe(
    parseUser(input),
    E.getOrElse((error) => {
      console.error('Parse failed:', error);
      return defaultUser;
    })
  );

8. Gradual Adoption Strategies

Strategy 1: Start at the Boundaries

Begin by converting functions at the edges of your system:
- API response handlers
- Database query results
- File system operations
- User input validation

// Wrap external API calls first
const fetchUserApi = (id: string): TE.TaskEither<ApiError, UserDto> =>
  pipe(
    TE.tryCatch(
      () => externalApiClient.getUser(id),
      (e) => ({ type: 'api_error' as const, cause: e })
    )
  );

// Internal code can stay imperative initially
async function handleUserRequest(userId: string) {
  const result = await fetchUserApi(userId)();
  if (E.isRight(result)) {
    // Process user with existing code
    return processUser(result.right);
  } else {
    throw new Error(`API error: ${result.left.type}`);
  }
}

Strategy 2: Create Bridge Functions

Build helpers to convert between fp-ts and imperative code:

// Bridge from Either to thrown errors
const unsafeUnwrap = <E, A>(either: E.Either<E, A>): A =>
  pipe(
    either,
    E.getOrElseW((e) => {
      throw e instanceof Error ? e : new Error(String(e));
    })
  );

// Bridge from thrown errors to Either
const catchSync = <A>(f: () => A): E.Either<Error, A> =>
  E.tryCatch(f, (e) => (e instanceof Error ? e : new Error(String(e))));

// Bridge from Promise to TaskEither
const fromPromise = <A>(p: Promise<A>): TE.TaskEither<Error, A> =>
  TE.tryCatch(() => p, (e) => (e instanceof Error ? e : new Error(String(e))));

// Bridge from TaskEither to Promise (throws on Left)
const toPromise = <E, A>(te: TE.TaskEither<E, A>): Promise<A> =>
  te().then(E.getOrElseW((e) => { throw e; }));

Strategy 3: Module-by-Module Migration

1. Pick a module with clear boundaries
2. Add fp-ts types to internal functions
3. Keep external API unchanged initially
4. Test thoroughly before moving on
5. Update external API once internals are stable

// Phase 1: Internal functions use fp-ts
// File: user-service.internal.ts
export const validateUser = (data: unknown): E.Either<ValidationError, User> => /* ... */;
export const enrichUser = (user: User): TE.TaskEither<Error, EnrichedUser> => /* ... */;

// File: user-service.ts (public API unchanged)
export async function getUser(id: string): Promise<User> {
  const result = await pipe(
    fetchUser(id),
    TE.flatMap(validateUser >>> TE.fromEither),
    TE.flatMap(enrichUser)
  )();

  if (E.isLeft(result)) {
    throw result.left;
  }
  return result.right;
}

// Phase 2: Update public API
// File: user-service.ts
export const getUser = (id: string): TE.TaskEither<UserError, User> =>
  pipe(
    fetchUser(id),
    TE.flatMap(validateUser >>> TE.fromEither),
    TE.flatMap(enrichUser)
  );

Strategy 4: Type-Driven Development

Use TypeScript's type system to guide the migration:

// Step 1: Change type signature first
type OldGetUser = (id: string) => Promise<User | null>;
type NewGetUser = (id: string) => TE.TaskEither<UserError, User>;

// Step 2: Compiler will show all call sites that need updating
const getUser: NewGetUser = (id) => /* implement */;

// Step 3: Update call sites one by one
// The compiler ensures you handle all cases

Strategy 5: Testing as Documentation

Write tests that demonstrate the expected behavior:

describe('UserService', () => {
  describe('getUser (fp-ts)', () => {
    it('returns Right with user on success', async () => {
      const result = await getUser('valid-id')();
      expect(E.isRight(result)).toBe(true);
      if (E.isRight(result)) {
        expect(result.right.id).toBe('valid-id');
      }
    });

    it('returns Left with NotFound error for unknown id', async () => {
      const result = await getUser('unknown')();
      expect(E.isLeft(result)).toBe(true);
      if (E.isLeft(result)) {
        expect(result.left._tag).toBe('NotFound');
      }
    });
  });
});

9. When NOT to Refactor

Simple Synchronous Code

Don't refactor straightforward code that doesn't benefit from fp-ts:

// This is fine as-is
function formatName(first: string, last: string): string {
  return `${first} ${last}`;
}

// Don't do this - it adds complexity without benefit
const formatName = (first: string, last: string): string =>
  pipe(
    first,
    (f) => `${f} ${last}`
  );

Performance-Critical Loops

fp-ts operations create intermediate arrays. For hot paths, keep imperative code:

// Keep this for performance-critical code processing millions of items
function sumLargeArray(numbers: number[]): number {
  let sum = 0;
  for (let i = 0; i < numbers.length; i++) {
    sum += numbers[i];
  }
  return sum;
}

// This creates intermediate arrays
const sumWithFpts = (numbers: number[]): number =>
  pipe(numbers, A.reduce(0, (acc, n) => acc + n));

Third-Party Library Interfaces

When working with libraries that expect specific patterns:

// Express middleware must match Express's interface
app.get('/users/:id', async (req, res) => {
  // Keep imperative here, convert at boundaries
  const result = await getUser(req.params.id)();

  if (E.isLeft(result)) {
    res.status(404).json({ error: result.left.message });
  } else {
    res.json(result.right);
  }
});

Code Touched by Non-FP Team Members

If your team isn't familiar with fp-ts, forced adoption will hurt productivity:

// If team doesn't know fp-ts, this is harder to maintain
const processOrder = (order: Order): TE.TaskEither<Error, Result> =>
  pipe(
    validateOrder(order),
    TE.fromEither,
    TE.flatMap(enrichOrder),
    TE.flatMap(submitOrder)
  );

// Familiar to all TypeScript developers
async function processOrder(order: Order): Promise<Result> {
  const validated = validateOrder(order);
  if (!validated.success) {
    throw new Error(validated.error);
  }
  const enriched = await enrichOrder(validated.data);
  return await submitOrder(enriched);
}

Trivial Null Checks

Don't use Option for simple, one-off null checks:

// This is fine
const name = user?.name ?? 'Anonymous';

// Overkill for simple cases
const name = pipe(
  O.fromNullable(user),
  O.map((u) => u.name),
  O.getOrElse(() => 'Anonymous')
);

When the Error Type Doesn't Matter

If you're going to throw/log anyway and don't need error composition:

// If this is your error handling anyway...
try {
  await doSomething();
} catch (e) {
  logger.error(e);
  throw e;
}

// ...then Either doesn't add much value
const result = await doSomethingTE()();
if (E.isLeft(result)) {
  logger.error(result.left);
  throw result.left;
}

Test Code

Test code should be readable, not necessarily functional:

// Clear test code
describe('UserService', () => {
  it('creates a user', async () => {
    const user = await createUser({ name: 'Alice' });
    expect(user.name).toBe('Alice');
  });
});

// Unnecessarily complex
describe('UserService', () => {
  it('creates a user', async () => {
    await pipe(
      createUser({ name: 'Alice' }),
      TE.map((user) => expect(user.name).toBe('Alice')),
      TE.getOrElse(() => T.of(fail('Should not fail')))
    )();
  });
});

Quick Reference: Imperative to fp-ts Mapping

Summary

Migrating to fp-ts is a journey, not a destination. Key principles:

1. Start small: Convert individual functions, not entire codebases
2. Be pragmatic: Not everything needs to be functional
3. Type-driven: Let the compiler guide your refactoring
4. Test thoroughly: Each conversion should be verified
5. Document patterns: Create team-specific guides for your codebase
6. Review benefits: Ensure the added complexity provides value

The goal is more maintainable, type-safe code—not functional programming for its own sake.