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在打字稿中使用 fp-ts 时如何避免嵌套管道？做记号？这是我要避免的嵌套管道示例

  pipe(
  userId,
  O.fold(
    () => setUser('No user found'),
    (uId: string) => {
      fetchUser(uId).then((response: Either<NonEmptyArray<string>, User>) => {
        pipe(
          response,
          E.fold(
            () => setUser('Unable to fetch user'),
            (user: User) => setUser(JSON.stringify(user, null, 2))
          )
        );
      });
    }
  )
);
4

1 回答 1

4

一种方法是尽可能延迟fold，以避免不必要的嵌套。

无需知道值是否存在或操作是否失败，就可以组合代数效应。

例如 ，如果未找到用户，则TaskEither转换后的 with将保留第一个错误。chain否则，它包含fetchUser错误或User成功案例中的数据。

工作示例

import { pipeable as P, option as O, taskEither as TE, nonEmptyArray as NA } from "fp-ts";

type User = { name: string };

// global state (side effect)
let user: string | undefined = undefined;
const setUser = (usr: string) => (user = usr);

// given input
const userId: O.Option<string> = O.some("Arun");

const fetchUser: (uid: string) => TE.TaskEither<NA.NonEmptyArray<string>, User> = uid =>
  TE.taskEither.of({ name: "Arun" });
  // An error case would be: TE.left(NA.of("Unable to fetch user"))  

const res = P.pipe(
  userId,
  TE.fromOption(() => NA.of("No user found")),
  TE.chain(fetchUser),
  TE.map(user => JSON.stringify(user, null, 2)),
  TE.fold( // we fold at the very end before applying the effect
    err => TE.taskEither.fromIO(() => { setUser(err[0]); }),
    user => TE.taskEither.fromIO(() => { setUser(JSON.stringify(user, null, 2)); })
  ),
  TE.chain(() => TE.taskEither.fromIO(() => { console.log(user); }))
);

// run the effect
res();

PS：我在这里假设，你fetchUser是一个异步操作，它创建TaskEither. 如果需要，您可以将其切换回Either。

于 2020-06-13T09:59:24.430 回答