2

我真的是 Haskell 的绝对新手，所以我完全不知道如何调试我编写的一些函数。当我打电话时，shuntingYard ["3+4"]我回来了[]，而我想回来[34+]。任何和所有的帮助将不胜感激。

import Char

isOperator :: Char -> Bool
isOperator x = elem x ['+','-','*','/','%','^','!','=','<','>']

associativityOf :: Char -> String
associativityOf x = if elem x ['+','-','*','/','%']
                    then "Left"
                    else "Right"

precedenceOf :: Char -> Int
precedenceOf x
    | elem x "=<>"   = 1 
    | elem x "+-"    = 2
    | elem x "*/%"   = 3
    | elem x "^!"    = 4
    | otherwise      = 0

operatorActions :: [[Char]] -> [[Char]] -> [[Char]]
operatorActions stmt stack
    | ( tokenAssoc == "Left"  && tokenPrecedence <= stackPrecedence ) ||        
      ( tokenAssoc == "Right" && tokenPrecedence <  stackPrecedence ) =
        [stackOper] : _shuntingYard stmt (tail stack)
    | otherwise   = _shuntingYard (tail stmt) ((head stmt) : stack)
    where tokenAssoc       = associativityOf (head (head stmt))
          tokenPrecedence  = precedenceOf    (head (head stmt))
          stackOper        = if (not (null stack))
                           then (head (head stack))
                           else '='
          stackPrecedence  = precedenceOf stackOper

stackOperations :: [[Char]] -> [[Char]]                                
stackOperations stack
    | ((not (null stack)) && (head (head stack)) == '(') = 
      error "Unbalanced parens."
    | null stack = []
    | otherwise  = (head stack) : _shuntingYard [] (tail stack)

_shuntingYard :: [[Char]] -> [[Char]] -> [[Char]]
_shuntingYard stmt stack
    | null stmt          = stackOperations stack
    | all isDigit (head stmt) = (head stmt) : _shuntingYard (tail stmt) stack
    | isOperator  (head (head stmt)) = operatorActions stmt stack
    | (head (head stmt)) == '('=
      _shuntingYard (tail stmt) ((head stmt) : stack)
    | (head (head stmt)) == ')' = if (head (head stack)) == '('
                            then _shuntingYard (tail stmt) (tail stack)
                            else (head stack) : _shuntingYard stmt (tail stack)
    | otherwise = _shuntingYard (tail stmt) stack

shuntingYard :: [[Char]] -> [[Char]]
shuntingYard stmt = _shuntingYard stmt []
4

2 回答 2

5

作为一种通用调试技术，您可以使用 Debug.Trace 模块来找出正在调用哪些函数以及它们的输入是什么。在每一步之后使用查看算法的状态。

import Debug.Trace

-- Show arguments each time _shuntingYard is called
_shuntingYard :: [[Char]] -> [[Char]] -> [[Char]]
_shuntingYard stmt stack = traceShow (stmt, stack) $ __shuntingYard stmt stack

__shuntingYard stmt stack
  | null stmt = stackOperations stack
  {- etcetera -}

这打印：

(["3+4"],[])
([],[])

嗯，第一次通话后你失去了一切。看着 __shuntingYard 的守卫，似乎“否则”的情况被调用了。

也许你想打电话shuntingYard ["3", "+", "4"]？

于 2011-03-20T19:14:09.847 回答
3

好的，让我们来看看你打电话时会发生什么shuntingYard ["3+4"]：

  1. 它调用_shuntingYard ["3+4"] []
  2. 它通过以下守卫_shuntingYard：
    1. null stmt = null ["3+4"] = false
    2. all isDigit (head stmt) = all isDigit "3+4" = false因为+不是数字
    3. isOperator (head (head stmt)) = isOperator '3' = false
    4. 也假为'3' /= '('
    5. 也假为'3' /= ')'
  3. 由于没有一个守卫匹配，所以我们进入默认情况并调用_shuntingYard (tail stmt) stack = _shuntingYard [] []
  4. 这次第一个 guard( null stmt = null []) 匹配，所以我们调用stackOperations []并获取[]。
于 2011-03-20T19:13:08.013 回答