Haskell 函数返回空列表

Question

我确实是 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。当我调用 shuntingYard ["3+4"] 时，我会返回 []，而我想要返回 [34+]。任何和所有的帮助将非常非常感谢。

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 []

Answer 1

作为一种通用的调试技术，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。在每个步骤之后查看算法的状态。

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 -}

打印:

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

嗯，第一次通话后你就失去了一切。看看 __shuntingYard 中的 guard ，似乎“否则”案件被传唤了。

也许您想调用shuntingYard ["3", "+", "4"]？