情報論理学 10月5日 提出レポート

prefix_to_infix.hs

isBinaryOperator = flip elem ['*','+','>','=']
isUnaryOperator  = flip elem ['-']

prefixToInfix str
    | isBinaryOperator token =
        let left  = prefixToInfix $ tail str
            right = prefixToInfix $ snd left
        in ("(" ++ fst left ++ [token] ++ fst right ++ ")", snd right)
    | isUnaryOperator token =
        let sub = prefixToInfix $ tail str
        in ("(" ++ [token] ++ fst sub ++  ")", snd sub)
    | otherwise = ([token], tail str)
    where token = head str

main = do
    contents <- getContents
    putStr $ unlines $ map (fst . prefixToInfix) $ lines contents