Question

这部分代码应该读取两个或多个数字（省略主要功能），然后用“+”表示总和。使用Rationals是因为稍后我将进行乘法和其他操作。

data Expression =  Number Rational
               | Add (Expression)(Expression)
               deriving(Show,Eq)

solve :: Expression -> Expression
solve (Add (Number x) (Number y)) = Number (x + y)

parse :: [String] -> [Expression] -> Double
parse ("+":s)(a:b:xs) = parse s (solve (Add a b):xs)
parse [] (answer:xs) = fromRational (toRational (read (show answer)::Float))
parse (x:xs) (y) = parse (xs) ((Number (toRational (read x::Float))):y)

（第二个）错误是解析函数无法处理

*Main> parse ["1","2","+"] [Number 3]

*** Exception: Prelude.read: no parse

我已经在Data.Ratio页面和网络上查找了此解决方案，但尚未找到它并希望得到一些帮助。谢谢，

CSJC

Answer 1

第一个等式，

parse ("+":s)(a:b:xs) = parse (s)((solve (Add (Number a) (Number b))):xs)

应该是

parse ("+":s)(a:b:xs) = parse (s)((solve (Add a b)):xs)

因为根据类型签名，a和b已经是Expression s。

或，与第二个和第三个等式一致，将类型更改为

parse :: [String] -> [Rational] -> Double

并将第一个等式改为

parse ("+":s)(a:b:xs) = parse s ((a + b):xs)

修复代码的两种可能方法（存在更多有问题的部分）：

-- Complete solve to handle all cases
solve :: Expression -> Expression
solve expr@(Number _) = expr
solve (Add (Number x) (Number y)) = Number (x + y)
solve (Add x y) = solve (Add (solve x) (solve y))

-- Convert an Expression to Double
toDouble :: Expression -> Double
toDouble (Number x) = fromRational x
toDouble e = toDouble (solve e)

-- parse using a stack of `Expression`s
parse :: [String] -> [Expression] -> Double
parse ("+":s) (a:b:xs) = parse s ((solve (Add a b)):xs)
parse [] (answer:_) = toDouble answer
parse (x:xs) ys = parse xs (Number (toRational (read x :: Double)) : ys)
parse _ _ = 0

-- parse using a stack of `Rational`s
parseR :: [String] -> [Rational] -> Double
parseR ("+":s) (a:b:xs) = parseR s (a+b : xs)
parseR [] (answer:xs) = fromRational answer
parseR (x:xs) y = parseR xs ((toRational (read x::Double)):y)
parseR _ _ = 0

后者是相当谨慎的，因为最终会产生Double，使用Rational来表示堆栈是没有意义的。

在parse的代码中，第三个等式通过Rational构造函数省略了Expression到Number的转换，但在其他方面没有问题。然而，第二个等式包含不同类型的问题：

parse [] (answer:xs) = fromRational (toRational (read (show answer)::Float))

如果answer是Expression或Rational，则show answer无法解析为Float，因此会导致运行时错误，例如你的编辑：

（第二个）错误是解析函数无法处理

*Main> parse ["1","2","+"] [Number 3]
*** Exception: Prelude.read: no parse

在使用第二个等式的位置，堆栈上的第一个元素（answer）为Number (3 % 1)，而show (Number (3 % 1))为"Number (3 % 1)"，这不是String read可以解析为Float。

***异常：Prelude.read：Haskell中没有解析 - 解析，表达式和递归

1 个答案: