在 Node.js 中使用 fp-ts 进行函数式编程

介绍

函数式编程 (FP) 因其可组合性、可测试性和稳健性而广受欢迎。在 JavaScript 生态系统中，**fp-ts** 等库将强大的 FP 概念引入 TypeScript，让您可以编写更简洁、更可靠的代码。

本文探讨了 **fp-ts** 概念，例如 `Option`、`Either`、`Task`、`Reader` 和 `ReaderTaskEither`。我们将使用 **fp-ts**、**pg**（PostgreSQL 客户端）和 **Express.js** 构建一个基本的 CRUD 应用程序，以了解这些抽象在实际应用程序中的表现。

关键概念

在深入研究该应用程序之前，让我们简单讨论一下主要概念：

设置项目

初始化项目

mkdir fp-ts-crud && cd fp-ts-crud
npm init -y
npm install express pg fp-ts io-ts
npm install --save-dev typescript @types/express ts-node-dev jest @types/jest ts-jest

设置 TypeScript

创建一个 `tsconfig.json`：

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "CommonJS",
    "outDir": "dist",
    "strict": true,
    "esModuleInterop": true
  },
  "include": ["src/**/*"]
}

项目结构

src/
  index.ts        # Entry point
  db.ts           # Database setup
  models/         # Data models and validation
  services/       # Business logic
  controllers/    # CRUD operations
  utils/          # fp-ts utilities
  errors/         # Custom error classes

实现 CRUD 应用程序

数据库设置（db.ts）

import { Pool } from 'pg';

export const pool = new Pool({
  user: 'postgres',
  host: 'localhost',
  database: 'fp_ts_crud',
  password: 'password',
  port: 5432,
});

定义模型和验证 (models/User.ts)

import * as t from 'io-ts';
import { isRight } from 'fp-ts/Either';

export const User = t.type({
  id: t.number,
  name: t.string,
  email: t.string,
});

export const validateUser = (data: unknown): t.TypeOf | null => {
  const result = User.decode(data);
  return isRight(result) ? result.right : null;
};

自定义错误处理 (errors/AppError.ts)

export class AppError extends Error {
  constructor(public statusCode: number, public code: string, public message: string) {
    super(message);
    this.name = 'AppError';
  }
}

export const createAppError = (statusCode: number, code: string, message: string): AppError => {
  return new AppError(statusCode, code, message);
};

服务层 (services/UserService.ts)

import { pool } from '../db';
import { ReaderTaskEither, right, left } from 'fp-ts/ReaderTaskEither';
import { pipe } from 'fp-ts/function';
import { createAppError, AppError } from '../errors/AppError';
import { validateUser } from '../models/User';

type Dependencies = { db: typeof pool };
type User = { name: string; email: string };

export const createUser = (
  user: User
): ReaderTaskEither => (deps) => async () => {
  // Validate the incoming user data
  const validatedUser = validateUser(user);

  if (!validatedUser) {
    return left(createAppError(400, 'INVALID_USER_DATA', 'Invalid user data provided'));
  }

  try {
    const result = await deps.db.query(
      'INSERT INTO users (name, email) VALUES ($1, $2) RETURNING id',
      [validatedUser.name, validatedUser.email]
    );
    return right(`User created with ID: ${result.rows[0].id}`);
  } catch (error) {
    return left(createAppError(500, 'USER_CREATION_FAILED', 'Failed to create user'));
  }
};

export const getUser = (
  id: number
): ReaderTaskEither => (deps) => async () => {
  try {
    const result = await deps.db.query('SELECT * FROM users WHERE id = $1', [id]);
    return result.rows[0]
      ? right(result.rows[0])
      : left(createAppError(404, 'USER_NOT_FOUND', 'User not found'));
  } catch {
    return left(createAppError(500, 'USER_FETCH_FAILED', 'Failed to fetch user'));
  }
};

CRUD 操作 (controllers/UserController.ts)

import { pipe } from 'fp-ts/function';
import { createUser, getUser } from '../services/UserService';
import { pool } from '../db';
import { AppError } from '../errors/AppError';

const errorHandler = (err: unknown, res: express.Response): void => {
  if (err instanceof AppError) {
    res.status(err.statusCode).json({ error: { code: err.code, message: err.message } });
  } else {
    res.status(500).json({ error: { code: 'UNKNOWN_ERROR', message: 'An unexpected error occurred' } });
  }
};

export const createUserHandler = (req: express.Request, res: express.Response): void => {
  pipe(
    createUser(req.body),
    (task) => task({ db: pool }),
    (promise) =>
      promise.then((result) =>
        result._tag === 'Left'
          ? errorHandler(result.left, res)
          : res.json({ message: result.right })
      )
  );
};

export const getUserHandler = (req: express.Request, res: express.Response): void => {
  pipe(
    getUser(parseInt(req.params.id, 10)),
    (task) => task({ db: pool }),
    (promise) =>
      promise.then((result) =>
        result._tag === 'Left'
          ? errorHandler(result.left, res)
          : res.json(result.right)
      )
  );
};

Express API（index.ts）

import express from 'express';
import { createUserHandler, getUserHandler } from './controllers/UserController';

const app = express();
app.use(express.json());

// Routes
app.post('/users', createUserHandler);
app.get('/users/:id', getUserHandler);

// Start Server
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

使用 Docker 和 Docker Compose 运行应用程序

Dockerfile

# Stage 1: Build
FROM node:22 AS builder
WORKDIR /app
COPY package*.json .
RUN npm install
COPY . .
RUN npm run build

# Stage 2: Run
FROM node:22
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY package*.json ./
RUN npm install --production
CMD ["node", "dist/index.js"]

docker-compose.yml

version: '3.8'
services:
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: password
      POSTGRES_DB: fp_ts_crud
    ports:
      - "5432:5432"
    volumes:
      - db_data:/var/lib/postgresql/data
volumes:
  db_data:

运行应用程序-开发模式

# Start the database
docker-compose up -d

# Run the app
npx ts-node-dev src/index.ts

运行应用程序 - 生产模式

# Build the docker image
docker build -t fp-ts-crud-app .

# Start the database
docker-compose up -d

# Run the container
docker run -p 3000:3000 fp-ts-crud-app

编写测试

设置 Jest

更新 `package.json` 脚本：

"scripts": {
  "test": "jest",
  "test:watch": "jest --watch"
}

示例测试（__tests__/UserService.test.ts）

import { createUser, getUser } from '../services/UserService';
import { pool } from '../db';

jest.mock('../db', () => ({
  pool: {
    query: jest.fn(),
  },
}));

describe('UserService', () => {
  afterEach(() => {
    jest.clearAllMocks();
  });

  it('should create a user', async () => {
    (pool.query as jest.Mock).mockResolvedValueOnce({ rows: [{ id: 1 }] });

    const result = await createUser({ name: 'Alice', email: 'alice@example.com' })({ db: pool })();

    expect(result._tag).toBe('Right');
    if (result._tag === 'Right') {
      expect(result.right).toBe('User created with ID: 1');
    }
  });

  it('should return error if user not found', async () => {
    (pool.query as jest.Mock).mockResolvedValueOnce({ rows: [] });

    const result = await getUser(1)({ db: pool })();

    expect(result._tag).toBe('Left');
    if (result._tag === 'Left') {
      expect(result.left.message).toBe('User not found');
    }
  });

  it('should return error for invalid user data during creation', async () => {
    const invalidData = { name: 123, email: 'invalid-email' };
    const result = await createUser(invalidData)({ db: pool })();
    expect(result._tag).toBe('Left');

    if (result._tag === 'Left') {
      expect(result.left.message).toBe('Invalid user data provided');
    }
  });
});

结论

通过利用 **fp-ts**、Docker 和强大的错误处理功能，我们构建了一个功能齐全、可扩展且可维护的 Node.js CRUD 应用程序。使用函数式编程模式可让您的代码更加可预测和可靠，尤其是在处理异步工作流时。