--After really struggling alot I just mostly followed what is described in the lecture slides.
parseExpr :: [String] -> (Ast, [String])
--parseExpr [] = (Var "Syntax error", [])
parseExpr [] = error "Not a valid expression"
parseExpr ("+":rs) = let (e1, r1) = parseExpr rs
                         (e2, r2) = parseExpr r1 in (Sum e1 e2, r2)

parseExpr ("*":rs) = let (e1, r1) = parseExpr rs
                         (e2, r2) = parseExpr r1 in (Mult e1 e2, r2)

parseExpr ("-":rs) = let (e1, r1) = parseExpr rs
                         (e2, r2) = parseExpr r1 in (Min e1 e2, r2)

--Changed to guards.
parseExpr (s:rs)
    | (onlyDigits s) = (Tall (read s :: Int), rs)
    | otherwise = error ("Syntax error at: " ++ s)