fp-refactor — quality + safety report

---
name: fp-refactor
description: Comprehensive guide for refactoring imperative TypeScript code to fp-ts functional patterns
risk: unknown
source: community
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.

## When to Use
- You are refactoring an existing imperative TypeScript codebase toward fp-ts patterns.
- The task involves converting `try/catch`, null checks, callbacks, DI, or loops into functional equivalents.
- You need migration guidance and tradeoffs, not just isolated fp-ts examples.

## Table of Contents

1. [Converting try-catch to Either/TaskEither](#1-converting-try-catch-to-eithertaskeither)
2. [Converting null checks to Option](#2-converting-null-checks-to-option)
3. [Converting callbacks to Task](#3-converting-callbacks-to-task)
4. [Converting class-based DI to Reader](#4-converting-class-based-di-to-reader)
5. [Converting imperative loops to functional operations](#5-converting-imperative-loops-to-functional-operations)
6. [Migrating Promise chains to TaskEither](#6-migrating-promise-chains-to-taskeither)
7. [Common Pitfalls](#7-common-pitfalls)
8. [Gradual Adoption Strategies](#8-gradual-adoption-strategies)
9. [When NOT to Refactor](#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)

```typescript
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)

```typescript
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)

```typescript
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)

```typescript
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:

```typescript
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)

```typescript
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)

```typescript
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)

```typescript
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)

```typescript
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 | undefin

… (truncated)