Question

我正在尝试构造一个MyData类型的新值，这是我定义的个人数据类型，但是我收到了一个错误。 MyData有一个构造函数List，它接收MyData个值的列表。我试图从MyData类型中提取List的两个元素，并使用Two And构造函数创建一个新元素。解析器工作得很好，我查了一下。

data MyData = Var String | Con Bool | Two TwoOp MyData MyData | List [MyData] 
 deriving Show
data Final= F MyData MyData deriving Show
data TwoOp = And | Or deriving Show

add_and (Right (F (List e1) (List e2))) i1 i2 = do
 a<-e1!!i1
 b<-e1!!i2
 return (Two And a b)

 val (Right (F e1 e2))=add_and (Right (F e1 e2)) 0 1
 parse_value string_expression= val (parse getfinal "" string_expression)

如果不清楚，请询问。我花了很多时间试图解决它，但不知道如何。

以下是我遇到的错误：

 Couldn't match type `MyData' with `m MyData'
    Expected type: [m MyData]
      Actual type: [MyData]
    Relevant bindings include
      add_and :: Either t Final -> Int -> Int -> m MyData
        (bound at D:\PF\final.hs:53:1)
    In the first argument of `(!!)', namely `e1'
    In a stmt of a 'do' block: a <- e1 !! i1

D:\PF\final.hs:55:5:
    Couldn't match type `MyData' with `m MyData'
    Expected type: [m MyData]
      Actual type: [MyData]
    Relevant bindings include
      add_and :: Either t Final -> Int -> Int -> MyData
        (bound at D:\PF\final.hs:53:1)
    In the first argument of `(!!)', namely `e1'
    In a stmt of a 'do' block: b <- e1 !! i2
Failed, modules loaded: none.

Answer 1

您不想使用<-从列表中提取值。

在这种情况下，您只想使用模式匹配：

add_and :: Final -> MyData
add_and (F (List (a:_)) (List (b:_))) = Two And a b
add_and _ = error "I don't know what to do here."

list1 :: MyData
list1 = List [ Var "abc" ]

list2 :: MyData  
list2 = List [ Con False ]

final :: Final
final = F list1 list2

main = print $ add_and final

在ghci中运行时：

*Main> main
Two And (Var "abc") (Con False)

在Haskell中构造我自己的数据类型的值

1 个答案: