在Haskell中添加實例聲明錯誤

Question

我是Haskell的新手。 這是我編寫的代碼：

transition_world world = case world of
   (Head,(x,y)):cs-> (Tail,(x,y)): transition_world cs
   (Tail,(x,y)):cs -> (Conductor, (x,y)): transition_world cs
   (Empty, (x,y)):cs -> (Empty, (x,y)): transition_world cs 
   (Conductor, (x,y)):cs
     | element_occurrence Head == 1 || element_occurrence Head == 2
                  -> (Head, (x,y)): transition_world cs
     | otherwise  -> (Conductor, (x,y)): transition_world cs
   [] -> []

嘗試編譯時，GHCi給了我這個錯誤：

Sources\Transitions\For_List_2D.hs:23:33:
No instance for (Eq (List_2D Cell -> Data.Integer_Subtypes.Nat))
  arising from a use of `=='
Possible fix:
  add an instance declaration for
  (Eq (List_2D Cell -> Data.Integer_Subtypes.Nat))
In the first argument of `(||)', namely
  `element_occurrence Head == 1'
In the expression:
  element_occurrence Head == 1 || element_occurrence Head == 2
In a stmt of a pattern guard for
               a case alternative:
  element_occurrence Head == 1 || element_occurrence Head == 2

Sources\Transitions\For_List_2D.hs:23:36:
No instance for (Num (List_2D Cell -> Data.Integer_Subtypes.Nat))
  arising from the literal `1'
Possible fix:
  add an instance declaration for
  (Num (List_2D Cell -> Data.Integer_Subtypes.Nat))
In the second argument of `(==)', namely `1'
In the first argument of `(||)', namely
  `element_occurrence Head == 1'
In the expression:
  element_occurrence Head == 1 || element_occurrence Head == 2

無法理解我想要什么...我在哪里犯了錯誤？ 我該如何解決？

額外1：

element_occurrence :: Eq e => e -> List_2D e -> Nat
element_occurrence element list = case list of
    (local_element, _): cs
       | local_element == element -> 1 + element_occurrence element cs
       | otherwise                ->     element_occurrence element cs
    [] -> 0

額外2：

local_elements :: Coord -> List_2D e -> List_2D e
local_elements (x, y) list = read_neighbours (x, y) 1 list
  where
    read_neighbours :: Coord -> Distance -> List_2D e -> List_2D e
    read_neighbours (x, y) dist list = case list of
        (element, (x_e, y_e)): cs
            |    abs (x_e - x) <= dist
            && abs (y_e - y) <= dist -> (element, (x_e, y_e)): read_neighbours (x, y) dist cs
            | otherwise              ->                        read_neighbours (x, y) dist cs
        [] -> []

local_elements_list :: Coord -> List_2D e -> [e]
local_elements_list (x, y) list = read_neighbours_list (x, y) 1 list
  where
    read_neighbours_list :: Coord -> Distance -> List_2D e -> [e]
    read_neighbours_list (x, y) dist list = case list of
        (element, (x_e, y_e)): cs
            |  abs (x_e - x) <= dist
            && abs (y_e - y) <= dist -> element: read_neighbours_list (x, y) dist cs
            |  otherwise             ->          read_neighbours_list (x, y) dist cs
        [] -> []

size :: List_2D e -> Nat
size list = case list of
   []    -> 0
   _: xs -> 1 + size xs

Answer 1

“為...添加實例聲明”是GHC最具誤導性的錯誤消息之一。 通常，您應該忽略該建議。

如果它想要實例的類型在其中帶有-> ，這基本上表明您正在嘗試使用只能通過result實現的函數，例如，您已經忘記將該函數應用於其所有參數。 顯然， element_occurence的類型類似於Item -> List_2D Cell -> Nat ，但是您的代碼中只有element_occurrence Head ，這是部分應用的函數 ，它仍然具有List_2D Cell -> Nat類型。 通過將其應用於List_2D Cell ，您將獲得一個Nat ，然后可以將其與數字文字進行比較。

Answer 2

您已將element_occurence定義為2個參數的函數：

element_occurrence :: Eq e => e -> List_2D e -> Nat
element_occurrence element list = -- more stuff

您僅使用一個參數調用了element_occurence：

transition_world world = case world of
{- other cases -}
(Conductor, (x,y)):cs
  | element_occurrence Head == 1 || element_occurrence Head == 2 -> -- stuff

但是，這為您提供了一個類型為“ List_2D Cell-> Nat”的函數類型的值，您嘗試對“ 1”和“ 2”進行相等性測試。

您需要做的是為element_occurence提供兩個參數。

GHC在這里太屈從了，假設您真的打算只用一個參數調用。 （==）的類型為“ Eq a => a-> a-> Bool”，因此，如果您提供了實例“ Eq（List_2D Cell-> Nat）”，則對（==）的調用是可以的，但是可以找不到那個實例。 因此，它將打印第一條錯誤消息。

GHC的數字，你真棒 ，所以你會解決這個錯誤馬上和假設下的收益，它是固定的。 但是現在它在期望“ List_2D單元格-> Nat”（另一邊是==）的位置看到了“ 1”和“ 2”。 現在，數字文字被重載為類型“ Num a => a”，因此如果您提供實例“ Num（List_2D Cell-> Nat）”，則該數字文字可能是“ List_2D Cell-> Nat”。 它找不到該實例，因此它吐出第二條錯誤消息。