fp-ts sample

fp-sample.ts

import { pipe } from 'fp-ts/function';
import * as O from 'fp-ts/Option';
import * as E from 'fp-ts/Either';
import * as A from 'fp-ts/Array';
import { sequenceT } from 'fp-ts/Apply';
import { flow } from 'fp-ts/function';
import * as t from 'io-ts';

// Example of a complex validator using io-ts
const UserProfile = t.type({
  name: t.string,
  age: t.Int, // Int is a refinement of number which ensures it's an integer
  email: t.string,
});

// Let's assume we have some user input that needs validation
const userInput = {
  name: 'Alice',
  age: 30,
  email: '[email protected]',
};

// Function to simulate an API call that might fail
function fetchUserFromAPI(userId: number): E.Either<Error, unknown> {
  return Math.random() > 0.5
    ? E.right({ name: 'Bob', age: 28, email: '[email protected]' })
    : E.left(new Error('Failed to fetch user'));
}

// Function to validate user data with io-ts
function validateUser(input: unknown): E.Either<t.Errors, t.TypeOf<typeof UserProfile>> {
  return UserProfile.decode(input);
}

// Helper function to transform Either to Option
const eitherToOption = <E, A>(e: E.Either<E, A>): O.Option<A> =>
  pipe(
    e,
    E.fold(
      (_, __) => O.none,
      (a) => O.some(a)
    )
  );

// Simulate fetching multiple users and validating them
const userIds = [1, 2, 3, 4];
const validatedUsers = pipe(
  userIds,
  A.map(fetchUserFromAPI),
  A.map(eitherToOption),
  A.compact, // Remove None values
  A.map(validateUser)
);

// Function to combine several Options into one
const combineOptions = sequenceT(O.Apply);

// Example of using combinators to process data (e.g., finding users over 25)
const findUsersOver25 = (users: Array<E.Either<t.Errors, t.TypeOf<typeof UserProfile>>>): Array<t.TypeOf<typeof UserProfile>> =>
  pipe(
    users,
    A.map(E.fold(
      () => [],
      (user) => user.age > 25 ? [user] : []
    )),
    A.flatten
  );

// Using the functions to process the user data
const result = pipe(
  validatedUsers,
  (users) => findUsersOver25(users)
);

// Showcasing advanced composition using fp-ts
const processUserData = flow(
  A.map(fetchUserFromAPI),
  A.map(eitherToOption),
  A.compact,
  A.map(validateUser),
  findUsersOver25
);

// Process the list of userIds
const finalResult = processUserData(userIds);

// Print out the results of our operations
console.log('Validated Users:', JSON.stringify(result, null, 2));
console.log('Final Result:', JSON.stringify(finalResult, null, 2));