Fp refactor

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
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) {