Using ReaderTaskEither from fp-ts

reader-task-example.ts

import * as http from "http";
import { ReaderTaskEither } from "fp-ts/lib/ReaderTaskEither"
import { TaskEither, fromIO } from "fp-ts/lib/TaskEither";
import { IO } from "fp-ts/lib/IO";
import { Task } from "fp-ts/lib/Task";
import axios, { AxiosInstance } from 'axios';
import { Either, left, right } from "fp-ts/lib/Either";

export type Logger = (s: string) => TaskEither<never, void>;

function consoleLogger(s: string): TaskEither<never, void> {
  return fromIO(new IO(() => console.log(s)));
}

export interface WithLogger {
  logger: Logger
}

export interface WithClient {
  client: AxiosInstance
}


function log<E extends WithLogger, L = never>(s: string): ReaderTaskEither<E, L, void> {
  return new ReaderTaskEither(e => e.logger(s))
}

function request<E extends WithClient>(s: string): ReaderTaskEither<E, string, string> {
  return new ReaderTaskEither(e => 
    new TaskEither(
      new Task(
        () => 
          e.client.get<string>(s)
          .then(r => right<string, string>(r.data))
          .catch(e => left(e.toString())) as Promise<Either<string, string>>
      )
  ));
}

export function getWithFallback<E extends WithLogger & WithClient>(primary: string, secondary: string): ReaderTaskEither<E, string, void> {
  return request<E>(primary)
    .orElse((e) => log<E, string>(`primary failed with ${e}`).applySecond(request<E>(secondary)))
    .chain(result => log<E>(`result: ${result}`));
}


getWithFallback("http://www.google.com/does/not/exist", "http://www.google.com")
  .run({
    logger: consoleLogger,
    client: axios
  })
  .then(e => console.log("end "))