[Haskell-cafe] Trying to make a calculator

[Emanuil Boyanov]

Hi guys, I'm new to Haskell, I learn it for couple of months now and we've
learned only simple functions like map, zip etc. But now my assignment is a
lot tougher than I thought so I'll need some guidelines. I've googled my
assignment and I saw a person with the exact assignment so I am gonna copy
the task then I'm gonna show you what I've done so far.
Here it is: Make calculator function.
 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     name = ...
    where the site stands blank corresponding value.     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)
 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"].

=========

So what I've done so far is:

calculator [] = []
calculator [" "] = []
calculator [c] = [c]
calculator (c:" ":cs) = c:calculator cs
calculator (" ":cs) = calculator cs
calculator (c:" ":" ":cs) = calculator (c:" ":cs)
calculator (c:cs)
| (head c `elem` ['a'..'z']) && (head(tail c) `elem` ['+','-','*','/']) &&
(last(take (length c-2)c) `elem` ['1','2','3','4','5','6','7','8','9'])  =
 calculator cs
        | otherwise = "error":calculator cs

I try at first to use pattern matching for the blank strings in the first
list, so that I can get rid off them in the second list. Am I even on the
right way here?
I found the "elem" function and I'm trying to check if the first char of the
string is a letter or a number, then if the second char is one of the
operations and so on. Problem is that I have no idea how can I check if the
variable is defined already or how to save a variable and then give that
variable the new value - is that even possible? :] And another thing `elem`
[1..9] isn't working - how can I make a single char of a string to hold an
integer..

Could you give me some guidelines or at least tell me if I'm on the right
way here so I can move forward and improve my program.
Thanks a lot!
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