haskell - 尝试实现 Data.Either

Question

为了帮助我学习 Applicative Functors 和 Functors，我想看看 Either使用类型类 Functor 实现和 Applicative .显然我可以继续阅读代码，但我发现自己尝试实现事物以更好地理解事物更有用。

仅供引用，我正在尝试实现此演示文稿结果的 Haskell 版本 http://applicative-errors-scala.googlecode.com/svn/artifacts/0.6/chunk-html/index.html

无论如何，这就是我到目前为止所拥有的

 data Validation a b = Success a | Failure b deriving (Show, Eq)

 instance Functor (Validation a) where
     fmap f (Failure x) = Failure x
     fmap f (Success x) = Success (f x)

但是每当我尝试使用 ghci 运行它时我刚刚收到以下错误消息:-

[1 of 1] Compiling Main             ( t.hs, interpreted )

t.hs:5:35:
    Couldn't match type `b' with `a1'
      `b' is a rigid type variable bound by
          the type signature for
            fmap :: (a1 -> b) -> Validation a a1 -> Validation a b
          at t.hs:4:5
      `a1' is a rigid type variable bound by
           the type signature for
             fmap :: (a1 -> b) -> Validation a a1 -> Validation a b
           at t.hs:4:5
    Expected type: a
      Actual type: b
    In the return type of a call of `f'
    In the first argument of `Success', namely `(f x)'
    In the expression: Success (f x)

t.hs:5:37:
    Couldn't match type `a' with `a1'
      `a' is a rigid type variable bound by
          the instance declaration at t.hs:3:30
      `a1' is a rigid type variable bound by
           the type signature for
             fmap :: (a1 -> b) -> Validation a a1 -> Validation a b
           at t.hs:4:5
    In the first argument of `f', namely `x'
    In the first argument of `Success', namely `(f x)'
    In the expression: Success

我不确定这是为什么，有人可以帮忙吗？

Answer 1

您正在尝试制作 Functor Success 上的实例工作部分，这是正常的做法，但由于类型参数的顺序，它是在 Failure 中的类型上定义的。部分代替。

由于您已将其定义为

data Validation a b = Success a | Failure b

instance Functor (Validation a) where
    ...

这意味着您的 fmap 的实现应该有类型 (x -> y) -> Validation a x -> Validation a y .但由于第二个类型变量是用于 Failure情况下，这不会进行类型检查。

您需要 Success 的类型变量case 改为最后一个:

data Validation b a = Success a | Failure b