fp-ts

import { pipe } from 'fp-ts/function'; import * as O from 'fp-ts/Option'; import * as E from 'fp-ts/Either'; import * as TE from 'fp-ts/TaskEither'; interface User { id: number; name: string; email?: string; } const fetchUser = (userId: number): TE.TaskEither<Error, string> => TE.tryCatch( () => fetch(`https://jsonplaceholder.typicode.com/users/${userId}`).then( (res) => { if (!res.ok) { throw new Error(`HTTP error! status: ${res.status}`); } return res.text(); }, ), (reason) => new Error(String(reason)), ); const parseJson = (s: string): E.Either<Error, User> => E.tryCatch( () => JSON.parse(s) as User, (reason) => new Error(`Failed to parse JSON: ${String(reason)}`), ); const getUserEmail = (user: User): O.Option<string> => O.fromNullable(user.email); // Example usage: const run = async () => { console.log('Fetching user 1...'); const result = await pipe( fetchUser(1), // Fetch user with ID 1 TE.flatMapEither(parseJson), // Parse the response as JSON, converting TaskEither<Error, string> to TaskEither<Error, User> TE.map(getUserEmail), // Map the User to an Option<string> (email) TE.match( (error) => `Error fetching user 1: ${error.message}`, (emailOption) => pipe( emailOption, O.match( () => 'User 1 email not found.', (email) => `User 1 email: ${email}`, ), ), ), )(); console.log(result); console.log('\nAttempting to fetch non-existent user 999...'); const invalidResult = await pipe( fetchUser(999), // Fetch a non-existent user, likely resulting in an HTTP error TE.flatMapEither(parseJson), // This step will likely be skipped or fail due to the upstream error TE.map(getUserEmail), TE.match( (error) => `Error fetching user 999: ${error.message}`, (emailOption) => pipe( emailOption, O.match( () => 'User 999 email not found.', (email) => `User 999 email: ${email}`, ), ), ), )(); console.log(invalidResult); }; run();