尝试实现Data.Either

Question

为了帮助我学习应用函数器和函数器，我认为看看Either是如何用类型类Functor和Applicative实现的会很有趣。显然，我可以继续阅读代码，但我发现尝试自己实现一些东西以更好地理解它们会更有用。

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

不管怎样，这就是我目前所知道的

 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)

但每当我尝试用ghci运行它时，我只得到以下错误信息：-

[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

我不太确定为什么会这样，有人能帮上忙吗？

Answer 1

您正在尝试使Functor实例在Success部件上工作，这是正常的做法，但是由于类型参数的顺序，它是在Failure部件中的类型上定义的。

因为您已经将其定义为

data Validation a b = Success a | Failure b

instance Functor (Validation a) where
    ...

这意味着您的fmap实现应该具有(x -> y) -> Validation a x -> Validation a y类型。但由于第二个类型变量用于Failure情况，因此不会进行类型检查。

您希望Success案例的类型变量是最后一个变量：

data Validation b a = Success a | Failure b