Question

我正在学习haskell并且在理解错误试图告诉我的内容时遇到一些问题。此代码产生以下错误

data Term = Mul { factor :: Term, factor2 :: Term }
          | Div { dividend :: Term, divisor :: Term } 
          | Add { summand :: Term, summand2 :: Term }
          | Sub { minuend :: Term, subtrahend :: Term }
          | Mon { exponent :: Int }

value :: (Double, Term) -> Double
value x (Mul a b) = (value x a) * (value x b)
value x (Div a b) = (value x a) / (value x b)
value x (Add a b) = (value x a) + (value x b)
value x (Sub a b) = (value x a) - (value x b)
value x (Mon a)   = x^a

错误：

Couldn't match expected type `Term -> (Double, Term)'
            with actual type `Double'
The function `value' is applied to two arguments,
but its type `(Double, Term) -> Double' has only one
In the first argument of `(+)', namely `(value x a)'
In the expression: (value x a) + (value x b)

我做错了什么？

Answer 1

问题在于，您的类型签名和定义并不同意他们如何接受论证。

您需要以咖喱风格（推荐）编写您的类型签名：

value :: Double -> Term -> Double

或以非常规风格（非惯用）编写您的函数：

value (x, Mul a b) = ...

我建议尝试两者，但前者几乎总是在野外完成。

对haskell功能进行故障诊断

1 个答案: