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Using fp-ts for Functional Programming in TypeScript

Bringing it Together

Explore how to bring together user registration validation and unified HTTP-inspired responses using fp-ts in TypeScript. Understand building pure helper functions, managing success and error flows with monads, and improving maintainability with pipe. This lesson helps you combine functional programming techniques to create clear, reliable user registration workflows.

We'll cover the following...

Creating a unified response type

Let’s combine all the things we’ve learned so far. The one element we should add is a unified response. We’ll use the following HTTP/REST-inspired type:

TypeScript 3.3.4
type Response = {
    status: number,
    message: string,
};

In the type above, the status will be a 2xx, 4xx, or 5xx number signifying success or failure (similar to HTTP response code), and a message will give additional information. Let’s look at them below. We’ll also add some pure helper functions to create responses.

TypeScript 3.3.4
const createdResponse = (message: string): Response => ({
    status: 201,
    message,
});
const badRequest = (exception: string): Response => ({
    status: 400,
    message: exception,
});
const internalServerError = (): Response => ({
    status: 500,
    message: 'failed to create',
});

We could factor out some duplication here, but this example is good enough for now. Let’s review an example where we use these functions:

C++
function userResponse(u: UserRegistrationDto) {
    return pipe(
        u,
        e => sequenceForOption(
            O.some(firstNameIso.wrap(e.firstName)),
            O.some(lastNameIso.wrap(e.lastName)),
            prismPositiveInteger.getOption(e.age),
            findGender(e.sex),
            findRegion(e.country),
        ),
        O.map(([f, l, a, g, c]) => createUser(f, l, a, g, c)),
        O.map(u => {
            console.log(`Created ${JSON.stringify(u)}. Could now save in db`);
            return createdResponse(`Created ${JSON.stringify(u)}`);
        }),
        O.getOrElse(internalServerError),
    );
} 
function flow(u: UserRegistrationDto) {
    return pipe(
        fieldsNotEmpty(u),
        E.chain(validateAge),
        E.chain(validateGender),
        E.map(userResponse),
        E.getOrElse(badRequest),
    );
} 
console.log(flow(exampleEvent));

Let’s explain what we did in the code above:

  • Lines 1–18: The userResponse function builds our user. It logs a message, which is a stand-in for a call to a database, and builds a response. The response is either a 201 in case of success or
...