Haskell:数据类型

Question

我声明我的数据是这样的:

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,我想通过键入来创建一个声明实例: Declaration Variable "var1" 2+3 但它不起作用,我想这只是一个错误的语法,但我无法弄清楚如何.

另外我想知道什么时候需要使用实例？这是我从书中得到的代码:

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

相当于

(Declaration Variable "var1" 2) + 3

.也就是说,它试图打电话Declaration与3个参数(Variable,"var1",2),然后将结果来3.这毫无意义.

你要

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