homework help: upn parser
lps at cwru.edu
Mon Dec 15 04:35:34 EST 2003
This is a type definiton, which says that parsing takes one argument, a
String, and gives you back a list of OpTree:
> parsing :: String->[OpTree]
This is a function definition. The part before the = is called the
left hand side, while the part after the = is called the right hand side.
> parsing (a:b:c:xs) t
> |isDigit b = ((parsing xs)) ((Term (read a) c (read b)):t)
The (a:b:c:xs) is an example of a pattern. You are using the list
constructor (:) to break down the list into individual items.
According to your type definition, this argument is of type String,
which is the same thing as [Char]. Thus, a, b, and c are bound to
single characters, while xs is the bound to the rest of the string,
assuming your string has at least three characters. If your string
has less than three characters, then this pattern fails and the next
clause in your function definiton will be tried.
Then, according to your function definition, you then bind the second
argument of the function to the variable "t". However, your type
definition says you only have one argument. Since they disagree, this
is an error.
The right hand side of your function definition contains lots of errors,
as you suggest.
You probably want to rethink your type definition. I'd suggest the
parsing :: String -> [OpTree] -> OpTree
This says that parsing is a function that takes two arguments, a list
of characters and a list of OpTrees, and returns a single OpTree. You
are going to treat the second list argument like a stack. When you run
across a number in your string, you are going to take the number off
the string and push it onto the stack. When you run across an operation
in your string, you are going to take two elements off the stack,
combine the elements from the stack using the operation, and push the
result back on the stack. Thus, your solution would contain this
kind of structure:
parsing ( string_with_number_in_front ) ( stack )
= parsing ( string_without_number ) ( Number (the_number) : stack )
parsing ( string_with_operation_in_front ) (last_element : next_element
= parsing ( string_without_operation ) ( Term last_element
(the_operation) next_element : rest_stack )
Now, you must consider the boundary cases of the function "parsing".
What I mean is, what kinds of clauses do you need for when you are done
with recursion, and what should your first call to this function look
Of course, you need to try out your code and carefully think about what
it's doing. When asking for homework help on haskell-cafe, Haskellers
have been known to introduce small intentional errors into their advice.
Also, it's very impressive that Paul Natorp high school is using
Haskell in it's coursework! In the United States, I've never heard of
a high school that takes functional programming seriously. Sadly, many
American universities don't take functional programming seriously
either. Americans are really behind the game in this regard.
Hope this helps.
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