前三篇的快速連結
pipe與flow
option, map, flatten 與 chain
either, left & right

TaskEither就是Either方法的非同步版本，需要非同步實現的場景會用到TaskEither，如非同步http請求。 程式碼示例如下：

import axios from 'axios'
import { pipe } from 'fp-ts/lib/function'
import * as TE from 'fp-ts/lib/TaskEither'

(async () => {
  const ok = await pipe(
    TE.tryCatch(
      () => axios.get('https://httpstat.us/200'),
      (reason) => new Error(`${reason}`),
    ),
    TE.map((resp) => resp.data),
  )()

  console.log(ok)
  // { _tag: 'Right', right: { code: 200, description: 'OK' } }
})();

(async () => {
  const result = await pipe(
    TE.tryCatch(
      () => axios.get('https://httpstat.us/500'),
      (reason) => new Error(`${reason}`),
    ),
    TE.map((resp) => resp.data),
  )();

  console.log(result)
  /**
   * {
   *   _tag: 'Left',
   *   left: Error: Error: Request failed with status code 500
   *       at /tmp/either-demo/taskeither.ts:19:19
   *       at /tmp/either-demo/node_modules/fp-ts/lib/TaskEither.js:94:85
   *       at processTicksAndRejections (internal/process/task_queues.js:97:5)
   * }
   */
})()
 

tryCatch在Either下也有同樣的實現，只是因為這裡用到了async/await，所以才要用TaskEither。