哈斯克爾：數據類型

Question

我宣佈我的數據是這樣的：

data Op = Plus | Minus | Mul | Div | Pow 
     deriving (Eq, Show) 

type Name = String 

data Variable a = Variable Name (Expression a) 
      deriving (Eq, Show) 

data Declaration a = Declaration (Variable a) 
      deriving (Eq, Show) 

{- The core symbolic manipulation type -} 
data Expression a = 
      Number a   -- Simple number, such as 5 
     | Expression Op (Expression a) (Expression a) 
      deriving (Eq, Show) 

在GHCI，我想通過鍵入創建宣言的一個實例： Declaration Variable "var1" 2+3 ，但它不工作，我想這是隻是一個錯誤的語法，但我不知道如何。

另外我想知道什麼時候需要使用實例？這是我從一本書上得到了代碼：給

(Declaration Variable "var1" 2) + 3 

instance Num a => Num (Expression a) where 
    a + b = Expression Plus a b 
    a - b = Expression Minus a b 
    a * b = Expression Mul a b 
    negate a = Expression Mul (Number (-1)) a 
    abs a = error "abs is unimplemented" 
    signum _ = error "signum is unimplemented" 
    fromInteger i = Number (fromInteger i) 

Answer 1

Declaration Variable "var1" 2+3 

等同。也就是說，它試圖用3個參數（Variable,"var1",2）調用Declaration，然後將結果添加到3。這沒有意義。

你想

Declaration (Variable "var1" (2+3))