Haskell 包含单个元素和列表的列表上的Fmap
我有一个这样的数据结构Haskell 包含单个元素和列表的列表上的Fmap,haskell,compiler-errors,functor,type-mismatch,algebraic-data-types,Haskell,Compiler Errors,Functor,Type Mismatch,Algebraic Data Types,我有一个这样的数据结构 data ShoppingList a = Empty | Item a | Listofitems [ShoppingList a] deriving (Show) 我正试着为此编写fmap instance Functor ShoppingList where fmap f Empty = Empty fmap f (Item i) = Item (f i) fmap f (Listofitems [Empty]) = Empty
data ShoppingList a
= Empty
| Item a
| Listofitems [ShoppingList a]
deriving (Show)
我正试着为此编写fmap
instance Functor ShoppingList where
fmap f Empty = Empty
fmap f (Item i) = Item (f i)
fmap f (Listofitems [Empty]) = Empty
fmap f (Listofitems ((Item a):Listofitems [as])) = Listofitems $ (fmap f (Item a)) (fmap f Listofitems [as])
这是我到目前为止写的,但它不是编译的,你能帮我理解这里的问题是什么吗,一个解释会很棒。
我得到的两个错误
src\Ml.hs:19:33: error:
* Couldn't match expected type `[ShoppingList a]'
with actual type `ShoppingList a0'
* In the pattern: Listofitems [as]
In the pattern: (Item a) : Listofitems [as]
In the pattern: Listofitems ((Item a) : Listofitems [as])
* Relevant bindings include
a :: a (bound at src\Ml.hs:19:30)
f :: a -> b (bound at src\Ml.hs:19:8)
fmap :: (a -> b) -> ShoppingList a -> ShoppingList b
(bound at src\Ml.hs:16:3)
|
19 | fmap f (Listofitems ((Item a):Listofitems [as])) = Listofitems $ (fmap f (Item a)) (fmap f Listofitems [as])
| ^^^^^^^^^^^^^^^^
src\Ml.hs:19:68: error:
* Couldn't match expected type `b -> [ShoppingList b]'
with actual type `ShoppingList b'
* The function `fmap' is applied to three arguments,
but its type `(a -> b) -> ShoppingList a -> ShoppingList b'
has only two
In the second argument of `($)', namely
`(fmap f (Item a)) (fmap f Listofitems [as])'
In the expression:
Listofitems $ (fmap f (Item a)) (fmap f Listofitems [as])
* Relevant bindings include
f :: a -> b (bound at src\Ml.hs:19:8)
fmap :: (a -> b) -> ShoppingList a -> ShoppingList b
(bound at src\Ml.hs:16:3)
|
19 | fmap f (Listofitems ((Item a):Listofitems [as])) = Listofitems $ (fmap f (Item a)) (fmap f Listofitems [as])
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
基本上如果我有一个列表=[苹果项目,空,[香蕉项目,空]]
我希望fmap(++M)列表返回[Item AppleM.Empty.[Item BananaM,Empty]]关于数据类型的注释
首先,您的数据类型过于复杂。项目列表实际上应该是一个列表,因此您可以定义data ShoppingList'A=ShoppingList'[A]
,因为您正在使用列表。没有必要将购物清单嵌套起来
你问题的解决办法
但是,如果您需要这样做,这是一个解决方案。注意:我假设您不需要ShoppingList列表,因为您的数据定义中已经有一个列表。所以你可以打电话
--fmap :: (a -> b) -> ShoppingList a -> ShoppingList b
fmap _ Empty = Empty
fmap f (Item i) = Item (f i)
fmap f (Listofitems ls) = Listofitems $ map (fmap f) ls
>>fmap (++ "M") $ Listofitems [Item "Apple", Empty, Listofitems [Item "Banana", Empty]]
Listofitems [Item "AppleM",Empty,Listofitems [Item "BananaM",Empty]]
注:
- 字符串周围需要引号
- 你需要在项目列表中调用它
还要记住,如果需要性能,字符串附加操作对于长字符串是无效的
ShoppingList
看起来已经太复杂了。如果您只需要一个项目列表,请使用newtype ShoppingList a=SL[a]
。无需将空列表和单例列表表示为单独的项目,您的Listofitems
允许您构建项目树,而不仅仅是列表。这样,Functor
实例就变成了fmap f(SL list)=SL(fmap f list)
“但它不是编译的”–那么你把你得到的错误贴出来怎么样?@chepner,我只是想用一个fmap来练习一些困难的问题