homework.hs

--A simple stack-based RPN interpreter
rpnCalc :: String -> Int
rpnCalc expr = if null res then 0 else head res where
    res = foldl eval [] expr
    eval stack c
        | c `elem` ['0'..'9'] = digitToInt c : stack
        | otherwise           = func c top2 top1 : pop2
        where (top1, _)    = pop 1 stack
              (top2, pop2) = pop 2 stack
    --Char to function mappings
    func '+' = (+)
    func '-' = (-)
    func '*' = (*)
    func '/' = div
    func '^' = (^)
    func _   = error "Invalid RPN expression"
    --Expression validation
    pop n l | lengthGEQ n l = let (x, xs) = splitAt n l in (last x, xs)
            | otherwise     = error "Invalid RPN expression"