[英]Haskell: Data Type
我聲明我的數據是這樣的:
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中,我想通過鍵入以下內容來創建一個聲明的實例: Declaration Variable "var1" 2+3
但是它不起作用,我猜這只是一個錯誤的語法,但是我不知道怎么做。
我也想知道何時需要使用實例? 這是我從書中得到的代碼:
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)
Declaration Variable "var1" 2+3
相當於
(Declaration Variable "var1" 2) + 3
。 也就是說,它嘗試使用3個參數( Variable
, "var1"
, 2
)調用Declaration
,然后將結果添加到3
。 這是沒有道理的。
你要
Declaration (Variable "var1" (2+3))
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