Question

我收到了类型错误，因为我试图递归地添加一个Maybe int int函数rankP，如下所示。因为rankP的返回类型是Maybe Int，而rankC的类型是Ints的元组，它告诉我我不能用Maybe Int添加Int。

type Prog = [Cmd]

data Cmd = LD Int
    | ADD
    | MULT
    | DUP
    | INC
    | SWAP
    | POP Int
    deriving Show       
type Stack = [Int]
type D = Stack -> Maybe Stack
type Rank = Int
type CmdRank = (Int,Int)


rankC :: Cmd -> CmdRank
rankC (LD _) = (0,1)
rankC ADD = (2,1)
rankC MULT = (2,1)
rankC DUP = (1,2)
rankC INC = (1,1)
rankC SWAP = (2,2)
rankC (POP x) = (x,0)


rankP :: Prog -> Maybe Rank
rankP [] = Nothing
rankP [x] = Just (snd(rankC x) - fst(rankC x))
rankP (x:xs) = if ((Just (snd(rankC x) - fst(rankC x)) + rankP xs) < 0) then Nothing
                    else Just (snd(rankC x) - fst(rankC x)) + (rankP xs)

以下是我收到的错误：

hw3.hs:43:64:
    Couldn't match expected type `Int' with actual type `Maybe Rank'
    In the return type of a call of `rankP'
    In the first argument of `Just', namely `(rankP xs)'
    In the second argument of `(+)', namely `Just (rankP xs)'
Failed, modules loaded: none.

Answer 1

我的错误略有不同：

No instance for (Num (Maybe Int)) arising from a use of ‘+’
In the first argument of ‘(<)’, namely
  ‘(Just (snd (rankC x) - fst (rankC x)) + rankP xs)’
In the expression:
  ((Just (snd (rankC x) - fst (rankC x)) + rankP xs) < 0)
In the expression:
  if ((Just (snd (rankC x) - fst (rankC x)) + rankP xs) < 0) then
      Nothing
  else
      Just (snd (rankC x) - fst (rankC x)) + (rankP xs)

但请注意

(Just (snd(rankC x) - fst(rankC x)) + rankP xs) < 0

您尝试在+个Maybe Rank对象上使用<，并将结果与Maybe Rank进行比较为0.这些都不起作用。

从您的代码中，您似乎试图“提取”SELECT form_user.email FROM form_instance JOIN form_user ON form_instance.form_user_id = form_user.form_user_id WHERE form_instance.appointment_date >= '2017-04-13' AND <= '2017-05-11' AND form_instance.status = 3的值;请参阅this question。

Answer 2

问题在于，在进行任何数学运算之前，如果确实需要从Maybe Rank中提取Rank，则尝试使用Maybe Rank进行算术运算。一种方法是直接手工完成：

rankP (x:xs) = if y == Nothing
               then Nothing
               else let Just y' = y in
                    if snd(rankC x) - fst(rankC x) + y' < 0
                    then Nothing
                    else Just (snd(rankC x) - fst(rankC x) + y')
    where y = rankP xs

当你开始理解Maybe的monadic属性时，肯定会有更优雅的方法。我还没有在这些方面提供太多帮助。

也许Int类型错误

2 个答案: