Simplify Your Code: Functional Core, Imperative Shell

By Arham Jain

Is your code a tangled mess of business logic and side effects? Mixing database calls, network requests, and other external interactions directly with your core logic can lead to code that’s difficult to test, reuse, and understand. Instead, consider writing a functional core that’s called from an imperativ​​e shell.

Separating your code into functional cores and imperative shells makes it more testable, maintainable, and adaptable. The core logic can be tested in isolation, and the imperati​​ve shell can be swapped out or modified as needed. Here’s some messy example code that mixes logic and side effects to send expiration notification emails to users:

// Bad: Logic and side effects are mixed function sendUserExpiryEmail(): void {   for (const user of db.getUsers()) {     if (user.subscriptionEndDate > Date.now()) continue;     if (user.isFreeTrial) continue;     email.send(user.email, "Your account has expired " + user.name + “.”);   } }

A functional core should contain pure, testable business logic, which is free of side effects (such as I/O or external state mutation). It operates only on the data it is given.

An imperative shell is responsible for side effects, like database calls and sending emails. It uses the functions in your functional core to perform the business logic.

Rewriting the above code to follow the functional core / imperative shell pattern might look like:

Functional core

function getExpiredUsers(users: User[], cutoff: Date): User[] {   return users.filter(user => user.subscriptionEndDate <= cutoff && !user.isFreeTrial); } function generateExpiryEmails(users: User[]): Array<[string, string]> {   return users.map(user =>      ([user.email, “Your account has expired “ + user.name + “.”])   ); }

Imperative shell

email.bulkSend(generateExpiryEmails(getExpiredUsers(db.getUsers(), Date.now())));

Now that the code is following this pattern, adding a feature to send a new type of email is as simple as writing a new pure function and reusing getExpiredUsers:

// Sending a reminder email to users function generateReminderEmails(users: User[], cutoff: Date): Array<[string, string]> {...} const fiveDaysFromNow = ... email.bulkSend(generateReminderEmails(getExpiredUsers(db.getUsers(), fiveDaysFromNow)));

Learn more in Gary Bernhardt’s original talk about functional core, imperative shell.