Time for an 2014 update…

Task/Parsing-RPN-to-infix-conversion/Haskell/parsing-rpn-to-infix-conversion.hs

@@ -0,0 +1,39 @@
+data Expression = Const String | Exp Expression String Expression
+
+precedence :: Expression -> Int
+precedence (Const _) = 5
+precedence (Exp _ op _)
+	| op `elem` ["^"]     = 4
+	| op `elem` ["*","/"] = 3
+	| op `elem` ["+","-"] = 2
+	| otherwise = 0
+
+leftAssoc :: Expression -> Bool
+leftAssoc (Const _) = False
+leftAssoc (Exp _ op _) = op `notElem` ["^","*","+"]
+
+rightAssoc :: Expression -> Bool
+rightAssoc (Const _) = False
+rightAssoc (Exp _ op _) = op `elem` ["^"]
+
+instance Show Expression where
+	show (Const x) = x
+	show exp@(Exp l op r) = left++" "++op++" "++right
+		where left  = if leftNeedParen then "( "++(show l)++" )" else show l
+		      right = if rightNeedParen the "( "++(show r)++" )" else show r
+		      leftNeedParen = (leftPrec < opPrec) || ((leftPrec == opPrec) && (rightAssoc exp))
+		      rightNeedParen = (rightPrec < opPrec) || ((rightPrec == opPrec) && (leftAssoc exp))
+		      leftPrec  = precedence l
+		      rightPrec = precedence r
+		      opPrec    = precedence exp
+
+buildExp :: [Expression] -> String -> [Expression]
+buildExp stack x
+	| not.isOp $ x = Const x : stack
+	| otherwise    = Exp l x r : rest
+		where r:l:rest = stack
+		      isOp = (`elem` ["^","*","/","+","-"])
+
+main = do
+	contents <- getContents
+	mapM_ (print.head.(foldl buildExp []).words) (lines contents)