Question

鉴于List[Either[A,B]]，我可以在Haskell中使用sequence来提取整个List Right[B]或Left[A]。

Prelude> sequence [Left "bad", Right 555]
Left "bad"
Prelude> sequence [Right 4534, Right 555]
Right [4534,555]

我不确定此方法是否符合sequence的定义，但它是处理List[Either[A,B]] => Either[A, List[B]]的一个很小的功能。

scala> def f[A, B](es: List[Either[A, B]]): Either[A, List[B]] = es match {
     |   case Right(x) :: xs      => f(xs).right.map(y => x :: y)
     |   case Nil                 => Right(Nil)
     |   case left @ Left(_) :: _ => left
     | }

但是我收到了这个我不理解的错误。

<console>:10: error: type mismatch;
 found   : scala.collection.immutable.::[Either[?A3,?B3]] where type ?B3 <: B (this is a GADT skolem), type ?A3 <: A (this is a GADT skolem)
 required: Either[A,List[B]]
         case left @ Left(_) :: _ => left
                                     ^

我在这里做错了什么？

Answer 1

错误消息显示left的类型为Either[A, B]，但f的预期类型为Either[A, List[B]]。

你需要在案例中解构Left，然后在表达式中重构它。这看起来很愚蠢，但你要记住，分支中的Left(x)被标记为与你想要的类型不同。

| case Left(x) :: _ => Left(x)

Haskell会给你一个类似的错误：

f :: Either a b -> Either a [b]
f l@(Left x) = l

Couldn't match type ‘b’ with ‘[b]’
  ‘b’ is a rigid type variable bound by
      the type signature for f :: Either a b -> Either a [b]
      at /Users/Jake/Code/Haskell/L.hs:3:6
Expected type: Either a [b]
  Actual type: Either a b
Relevant bindings include
  l :: Either a b (bound at /Users/Jake/Code/Haskell/L.hs:4:3)
  f :: Either a b -> Either a [b]
    (bound at /Users/Jake/Code/Haskell/L.hs:4:1)
In the expression: l
In an equation for ‘f’: f l@(Left x) = l

Scala中的类序函数

1 个答案: