具有错误类型参数的函子

Question

我有以下数据类型并已应用于Functor实例：

data Company a b c = 
DeepBlue a c
| Something b
instance Functor (Company a b) where 
fmap f (Something b) = Something b
fmap f (DeepBlue a c) = DeepBlue a (f c)

它没有任何抱怨地被编译。

然后我玩了一下(我知道它不起作用(，因为初学者的好奇心：

data Company a b c = 
DeepBlue a c
| Something b
instance Functor (Company a b) where 
fmap f (Something b) = Something (f b)
fmap f (DeepBlue a c) = DeepBlue a (f c) 

然后编译器抱怨：

D:haskellchapter16srcChapterExercises.hs:18:26: error:
* Couldn't match type `b1' with `b'
`b1' is a rigid type variable bound by
the type signature for:
fmap :: forall a1 b1.
(a1 -> b1) -> Company a b a1 -> Company a b b1
at D:haskellchapter16srcChapterExercises.hs:18:3
`b' is a rigid type variable bound by
the instance declaration
at D:haskellchapter16srcChapterExercises.hs:17:10
Expected type: Company a b b1
Actual type: Company a b1 b1
* In the expression: Something (f b)
In an equation for `fmap': fmap f (Something b) = Something (f b)
In the instance declaration for `Functor (Company a b)'
* Relevant bindings include
b :: b
(bound at D:haskellchapter16srcChapterExercises.hs:18:21)
f :: a1 -> b1
(bound at D:haskellchapter16srcChapterExercises.hs:18:8)
fmap :: (a1 -> b1) -> Company a b a1 -> Company a b b1
(bound at D:haskellchapter16srcChapterExercises.hs:18:3)
D:haskellchapter16srcChapterExercises.hs:18:39: error:
* Couldn't match expected type `a1' with actual type `b'
`b' is a rigid type variable bound by
the instance declaration
at D:haskellchapter16srcChapterExercises.hs:17:10
`a1' is a rigid type variable bound by
the type signature for:
fmap :: forall a1 b1.
(a1 -> b1) -> Company a b a1 -> Company a b b1
at D:haskellchapter16srcChapterExercises.hs:18:3
* In the first argument of `f', namely `b'
In the first argument of `Something', namely `(f b)'
In the expression: Something (f b)
* Relevant bindings include
b :: b
(bound at D:haskellchapter16srcChapterExercises.hs:18:21)
f :: a1 -> b1
(bound at D:haskellchapter16srcChapterExercises.hs:18:8)
fmap :: (a1 -> b1) -> Company a b a1 -> Company a b b1
(bound at D:haskellchapter16srcChapterExercises.hs:18:3)
Failed, modules loaded: none.

编译器尝试说什么？

Answer 1

您已将Company声明为具有三种泛型类型(a、b和c的类型。当你创建一个instance Functor (Company a b)实例时，你已经说过，对于任何a和b，都应该存在一个fmap的函数，其类型为：

(c -> d) -> Company a b c -> Company a b d

函数f的类型为c -> d，因此您不能使用b作为它的输入，因为它没有正确的类型。

Answer 2

你对fmap的定义有类型

fmap :: Company a b c -> (c -> d) -> Company a b d

因此，该函数采用类型为c的值。在

fmap f (Something b) = Something (f b)

您正在尝试将函数c -> d应用于导致错误的b类型的值。消息中的实际类型变量具有不同的名称：a1c，b1d