Haskellで数式をパースする

使い方
　:load で読み込んだ後
　myEquation "1+2*3"

import Text.ParserCombinators.Parsec

myDigit :: Parser Char
myDigit = oneOf ['0'..'9']

myNumberString :: Parser String
myNumberString = do
	c<-myDigit
	do
			cs <- myNumberString
			return (c:cs)
		<|> return [c]

myNumber :: Parser Integer
myNumber = do
	s<-myNumberString
	return (read s)

myPlusMinus :: Parser Char
myPlusMinus = oneOf "+-"

myMultiDiv :: Parser Char
myMultiDiv = oneOf "*/"

myCalc '+' left right = (left+right)
myCalc '-' left right = (left-right)
myCalc '*' left right = (left*right)
myCalc '/' _ 0 = 0
myCalc '/' left right = (left `div` right)

myEquation :: Parser Integer
myEquation = do{ left<-myFactor; rest left }
		where
			rest l = do{ op<-myPlusMinus
					;r<-myFactor
					;rest (myCalc op l r)
					}
				<|> return l
{-<|> do{ op<-myPlusMinus
		;right<-myEquation
		;return (myCalc op x right
		}-}

myFactor :: Parser Integer
--myFactor = do{ x<-myTerm; rest x}
myFactor = do {left<-myTerm; rest left }
		where
			rest l = do{ op<-myMultiDiv
					;r<-myTerm
					;rest (myCalc op l r)
					}
				<|> return l
				{-
		left<-myFactor
		op<-myMultiDiv
		right<-myTerm
		return (myCalc op left right)
	<|>	myTerm-}

myTerm :: Parser Integer
myTerm = do
		char '('
		x<-myEquation
		char ')'
		return x
	<|> myNumber

{-

myExpr2List String->[String]
myExpr2List "" = ""
myExpr2List x:xs = if myPlusMinus 

myFactor :: Parser Integer
myFactor =
	do
		char '('
		x<-myExpr
		char ')'
		return x
	<|>	myExpr

myExpr :: Parser Integer
myExpr = do
		left<-myNumber
		do
				op<-myPlusMinus
				right<-myFactor
				return (myCalc op left right)
			<|> return left
	<|> myFactor
-}