Hello all, I am experiencing some issues to do my course task in university. I have to write a calculator- function in Haskell. The function argument is a list of strings and also form such list, as each string of the argument made definite action: - If the string has the form of an arithmetic _expression_ - calculate this _expression_. The string result becomes part of the list-result. If the _expression_ contains a variable which is not assigned value, the result is displayed "undefined". - If the string has the form- Name = value calculated from the last _expression_ is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type - If there is not a calculated _expression_ to be assigned to form a string "no value". - If the string is non-blank, but there is a species different from the above two case, form the string "error". - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) and parentheses. Where no brackets, the operations are performed from left to right, but * and / precede the + and -. Implementation of any operation gives integer; in the division rejected the fractional part, if any. Variables have names of one letter - from the Latin small letter. In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2" "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. I say another person have the same task, but he didn't do anything. I started doing this task myself but i get stuck in the middle. Then i started searching for something that could help me and find out you :) The code i have written so far uses the library file "Parsing.lhs" but what i have written is taking those actions that i already described, only for a string. I cannot modify it to work for list of string, and complete the whole task. I'll be glad to finish the task myself, but i am going to need some help. Here is the code i have already written: import Parsing expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t+e) +++ do symbol "-" e <- expr return (t-e) +++ return t term :: Parser Int term = do f <- factor do symbol "*" t <- term return (f * t) +++ do symbol "/" t <- term return (f-t) +++ return f factor :: Parser Int factor = do symbol "(" e <- expr symbol ")" return e +++ natural eval :: String -> Int eval xs = case (parse expr xs) of [(n,[])] -> n [(_,out)] -> error ("undefined") [] -> error "error" Thanks all in advance :) -- Best Wishes Stoyan Peev