使用fp-ts的sequenceS处理不同Left类型Either的类型推断问题

当使用fp-ts的sequenceS函数处理多个不同Left类型的Either时，TypeScript无法自动将多个Left类型推断为联合类型，导致出现类型不兼容错误。

示例代码：

import * as E from 'fp-ts/Either'

type EmailValidationError = { type: 'EmailError' }
type UsernameValidationError = { type: 'UsernameError' }
type Email = string

declare function validateEmail(email: string): E.Either<EmailValidationError, Email>
declare function validateUsername(username: string): E.Either<UsernameValidationError, Email>

function validateData(inputs: { email: string; username: string }) {
  return E.sequenceS(E.Apply)({
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  })
}

错误信息：

Type 'UsernameValidationError' is not assignable to type 'EmailValidationError'.

解决方案 #

1. 封装自动推断联合类型的sequenceS工具函数 #

封装通用工具函数，自动提取所有Either的Left类型并组成联合类型，无需手动指定：

import * as E from 'fp-ts/Either'

// 封装自动推断Left联合类型的sequenceS工具
const sequenceSEither = <T extends Record<string, E.Either<any, any>>>(validations: T) =>
  E.sequenceS(E.Apply)<
    T[keyof T] extends E.Either<infer L, any> ? L : never
  >()(validations) as E.Either<
    T[keyof T] extends E.Either<infer L, any> ? L : never,
    { [K in keyof T]: T[K] extends E.Either<any, infer R> ? R : never }
  >

// 使用工具函数
function validateData(inputs: { email: string; username: string }) {
  return sequenceSEither({
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  })
}

2. 显式放宽Apply类型约束 #

通过将E.Apply的类型显式放宽，让TypeScript自动推断联合类型：

function validateData(inputs: { email: string; username: string }) {
  return E.sequenceS(E.Apply as E.Apply<E.Either<any, any>>)({
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  })
}

3. 利用类型推断提取联合类型 #

先将验证结果存入变量，自动提取Left联合类型后传入sequenceS：

function validateData(inputs: { email: string; username: string }) {
  const validations = {
    email: validateEmail(inputs.email),
    username: validateUsername(inputs.username),
  }
  
  // 自动提取所有Left类型的联合
  type LeftErrors = typeof validations[keyof typeof validations] extends E.Either<infer L, any> ? L : never
  
  return E.sequenceS(E.Apply)<LeftErrors>(validations)
}

内容的提问来源于stack exchange，提问作者David Novák