[英]Haskell Function Returning Empty List
我真的是Haskell的绝对新手,所以我完全不知道如何调试我写的一些函数。 当我打电话给shuntingYard ["3+4"]
我回来[]
,而我想回来[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 []
作为一般调试技术,您可以使用Debug.Trace模块找出正在调用的函数以及它们的输入内容。 使用每个步骤后查看算法的状态。
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 -}
这打印:
(["3+4"],[])
([],[])
嗯,你在第一次通话后失去了一切。 看着__shuntingYard中的警卫,看来“其他”案件被调用了。
也许你想打电话给shuntingYard ["3", "+", "4"]
?
好的,让我们来看看当你打电话给shuntingYard ["3+4"]
时会发生什么:
_shuntingYard ["3+4"] []
_shuntingYard
的守卫:
null stmt = null ["3+4"] = false
all isDigit (head stmt) = all isDigit "3+4" = false
因为+
不是数字 isOperator (head (head stmt)) = isOperator '3' = false
'3' /= '('
'3' /= ')'
_shuntingYard (tail stmt) stack = _shuntingYard [] []
null stmt = null []
)匹配,所以我们调用stackOperations []
并获取[]
。
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