2011/7/18 Emanuil Boyanov emanuilboya...@gmail.com:
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!
Hello,
You should work by smaller steps.
For instance, you can begin by abstracting the syntax. At the very
beginning, you get some strings (a list of statements) and you
transform them in something much simpler to use. Here, there is also
an intermediary step to make things easier.
Let's say you have 1 + 2 . You can make things simpler by working
on [1, +, 2]. Here, the problem with blancs is already dealt
with. You just have a list of tokens. Then, to make it simple later on
to differentiate between a number and an operator (and a variable),
you can define your own representation, for instance:
data Token = TNum Int | TOp Op | ...
data Op = Add | Sub | ...
Now the list [1, +, 2] becomes [TNum 1, TOp Add, TNum 2]. Now
you can use pattern matching on the Token and easily know what you
have.
For your variable assignment problem, you have to provide an
environment, i.e. a mapping between variable name and their value. If
you use a recursive function to evaluate your little language, you can
provide that environement as a supplementary argument.
Try to proceed that way, one step at a time, and come back if you have
any problem.
Cheers,
Thu
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