Using fp-ts for Functional Programming in TypeScript/

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Bringing it Together

Bringing it Together

Understand the project’s code that we’ve written so far.

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
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