这个问题有几个层次，所以我将提出一个可能的解决方案，并介绍它是如何工作的。
首先，fp-ts使用Task来表示异步工作，因此在fp-ts中使用异步代码时，大多数情况下您首先将代码转换为Task或TaskEither（稍后会详细介绍）。首先，我们需要一个函数来执行getItem任务，并且在返回null时返回Either而不是null。

import * as TE from 'fp-ts/lib/TaskEither';
import * as T from 'fp-ts/lib/Task';
// flow is function composition and avoids some syntax.
// This can be thought of a "point-free" although internally I use a
// not point-free function.
const getItemTE: (id: number) => TE.TaskEither<string, Item> = flow(
  getItem,
  // A task must be a function with no arguments that returns a promise,
  // This takes the result of calling getItem and returns a function that
  // returns that response in order to match the Task interface
  (promisedItem) => () => promisedItem,
  // We convert that task into a `TaskEither`, to get the type we want,
  // but note that this value will never be `left` because the underlying
  // task was returning `Item | null`. Both of those values are considered
  // a Right.
  TE.fromTask,
  // Converting to a TaskEither allows us to chain, which will hand us the
  // unwrapped `right` value and expects us to return a new TaskEither.
  // In this case, we want the task either we get by converting null
  // values into Eithers using the `fromNullable` function.
  TE.chain(TE.fromNullable("can't find item")),
);

现在我们有了一个函数，它接受一个数字并产生一个TaskEither，它将获取该项目。
接下来，我们要对body对象使用这个函数，可以将新创建的函数与body沿着使用来获取一个项：

// Note: at this point the Promise will be created so
// any IO is going to be kicked off here, not when the value
// is eventually awaited.
const item: TE.TaskEither<string, Item> = pipe(
  body.itemId,
  // This will wrap up the value into a task either
  // which enables us to chain.
  TE.fromNullable('Item id is missed'),
  // Now we just chain with the newly created function.
  // If the value is `right` then the value is fetched,
  // otherwise the 'Item id is missed' value is stored
  // in left and is returned.
  TE.chain(getItemTE)
);

最后，您可以对该值执行一些操作，如下所示：

async function doSomethingWithItem(item: TE.TaskEither<string, Item>) {
  pipe(
    // The item will already have been fetched at this point due
    // to the way the code is written. If you want to avoid fetching
    // the object right away, you might need to change around
    // the line where I went from the `getItem` response to a Task
    await item(),
    E.fold(
      (err) => console.error(err),
      (item) => {
        console.log(`Got item with id: ${item.itemId}`);
      },
    )
  );
}

doSomethingWithItem(item);