如何在Scala Cats中无需类型别名实现Either的sequence操作

Hey there, let's break down why your sequence call is throwing that type error and how to fix it without relying on a type alias!

The Root Cause #

Scala's type inference can struggle with binary type constructors like Either[A, B] when working with higher-kinded functions like sequence. The sequence method expects a structure of type F[G[A]] (here, F is List, G needs to be a unary type constructor like Either[String, *]—a kind of * -> *).

When you define each element as Right(3):Either[String,Int], Scala sees each element as the concrete type Either[String, Int] (a kind of *), not as the higher-kinded Either[String, ?] that sequence needs to find the Applicative instance for. The type alias XorStr[X] = Either[String,X] fixes this because it explicitly wraps Either into a unary constructor, making type inference work smoothly.

Solutions Without Type Aliases #

1. Explicitly Define the List's Element Type

Tell Scala upfront that your list is of type List[Either[String, Int]]—this lets it infer the correct higher-kinded G for sequence:

import cats._, cats.data._, cats.implicits._

// Explicitly type the entire list
val les: List[Either[String, Int]] = List(Right(3), Right(2))
les.sequence // Now works! Returns: Either[String, List[Int]] = Right(List(3, 2))

2. Explicitly Specify sequence's Type Parameters

If you prefer not to type the list directly, you can pass the type parameters to sequence to clarify the higher-kinded constructor:

import cats._, cats.data._, cats.implicits._

val les = List(Right(3):Either[String,Int], Right(2):Either[String,Int])
// Use kind projector syntax to specify G as Either[String, *]
les.sequence[Either[String, *], Int] 

Note: For Scala 2, you'll need the Kind Projector compiler plugin to use the * syntax. If you don't have it, replace Either[String, *] with Either[String, ?].

About Implicit Imports #

You didn't miss any critical imports! The cats.implicits._ import already includes the Applicative instance for Either[E, *] (required for sequence). The issue was purely about type inference, not missing implicit values.

内容的提问来源于stack exchange，提问作者Karl