April 2005 - Haskell-Cafe

Haskell Assistance Question
by Mike Richards 20 Apr '05

20 Apr '05

Hi, Im currently working on a haskell constraint parser for polymorhpic data types for a group of third year programmers, and was wondering if there was a help resource, or board on the internet for haskell programmers to ask questions about code they're working on. Any help or links would be much appreciated. Thanks a lot Mike mike.richards(a)oxygen.ie -- _______________________________________________ Get your free email from http://mail.oxygen.ie Powered by Outblaze

2 1

Bencoding in Haskell
by Tommi Airikka 20 Apr '05

20 Apr '05

Hi! I was just wondering if there are any good ways to represent a bencoded (http://en.wikipedia.org/wiki/Bencoding) message in Haskell? Any suggestions? Thanks, Tommi Airikka

2 6

WASH Unspecified Action
by John Goerzen 19 Apr '05

19 Apr '05

Hi, I've got one particular submit0 button in my WASH program that's causing trouble. Every time a person clicks on it, they get an error message saying "Unspecified Action". What does that mean and how would I fix it? The strange thing is that I have the exact same submit0 button pointing to the exact same function on a different page, and it works fine. Any ideas? Thanks, John

1 0

Re: [Haskell-cafe] Trying to implement this code
by Dmitry Vyal 19 Apr '05

19 Apr '05

Lizz Ross wrote: > Hi im new to Haskell (really just learning). > > Im trying to code a function to take in a List of Strings (e.g.) [apple, orange, > banana, orange, apple, orange] , and create > a binary search tree containing the Strings and integers (where the integers are > the number of occurrences of each word. I think you can use something like this to get statistics about repeats: data (Ord a, Updateable a) => BinTree a = Leaf | Node (BinTree a) a (BinTree a) class Updateable a where update :: a -> a data Word_stat = Word_stat String Int deriving Show instance Eq (Word_stat) where (==) (Word_stat s1 _) (Word_stat s2 _) = s1 == s2 instance Ord (Word_stat) where (Word_stat s1 _) < (Word_stat s2 _) = s1<s2 instance Updateable (Word_stat) where update (Word_stat s i) = Word_stat s (i+1) -- inserts new element in the tree or updates existing one ins_in_tree :: (Ord a, Updateable a) => BinTree a -> a -> BinTree a ins_in_tree Leaf el = Node Leaf el Leaf ins_in_tree (Node left cur right) el | el < cur = Node (ins_in_tree left el) cur right | el == cur = Node left (update cur) right | otherwise = Node left cur (ins_in_tree right el) -- loads list of strings in the tree ins_list :: [String] -> BinTree Word_stat ins_list lst = foldl ins_in_tree Leaf (map wrap lst) where wrap :: String -> Word_stat wrap s = Word_stat s 1 --traverses the tree flat_tree :: (Ord a, Updateable a) => BinTree a -> [a] flat_tree Leaf = [] flat_tree (Node left el right) = (flat_tree left) ++ [el] ++ (flat_tree right) -- function you probably need summary :: [String] -> [Word_stat] summary lst = flat_tree $ ins_list lst --------------- Am not sure about the relevance of this approach as i have very little experience with Haskell and FP. So it would be great if someone offers better solution. Why doesnt translator automatically deduce constraints in type of ins_in_tree and flat_tree functions so i need to explicitly define them?

4 4

Parsec question
by Ketil Malde 19 Apr '05

19 Apr '05

Hi, I've started using Parsec for my parsing needs, and must say I'm very happy with it. There is one thing that I'm struggling with implementing though. Basically, I want to parse a file containing multiple records. I already have a parser for a single record, and of course I could parse the file simply as many record However, this gives me the type Either ParseError [Record] which AFAICT, requires the whole file to be parsed correctly before any result is returned. I think it would be better for my purposes to have a parser of type [Either ParseError Record] so that the list of Records can be processed in an on-line fashion, until an error occurs. Is there an obvious way to achieve this? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

2 1

Re: Trying to implement this code
by Dmitry Vyal 18 Apr '05

18 Apr '05

Lizz Ross wrote: > > This gives me the error message :28 - Syntax error in input (unexpected > symbol "wrap") > Sorry. Looks like my mailer ate tabulation. Just correct the intendation: -- loads list of strings in the tree ins_list :: [String] -> BinTree Word_stat ins_list lst = foldl ins_in_tree Leaf (map wrap lst) where wrap :: String -> Word_stat wrap s = Word_stat s

1 0

Re: [Haskell-cafe] Trying to implement this code
by Scott Turner 18 Apr '05

18 Apr '05

On 2005 April 18 Monday 13:42, Lizz Ross wrote: > Im trying to code a function to [...] create > a binary search tree containing the Strings and integers (where the > integers are the number of occurrences of each word. What have you got so far? Have you declared a data structure for the search tree? Have you seen http://haskell.org/hawiki/HomeworkHelp?

2 1

HTTP and Browser modules
by Atwood, John Wesley 18 Apr '05

18 Apr '05

> 11. HTTP and Browser modules (David Owen) > Message: 11 > Date: Mon, 18 Apr 2005 02:28:17 +0000 > From: "David Owen" <d_w_o(a)hotmail.com> > Subject: [Haskell-cafe] HTTP and Browser modules > To: haskell-cafe(a)haskell.org > Message-ID: <BAY101-F23ED428D824D7DBAF21C54B3290(a)phx.gbl> > Content-Type: text/plain; format=flowed > > Good morning > > I have a query about compiling the original version of the HTTP > implementation in Windows > (http://homepages.paradise.net.nz/warrickg/haskell/http/) > <snip details> > If anybody has seen this problem before or knows how to > correct it I'd be very interested in your assistance :-) David, in HTTP.hs, replace the 2 occurences of "ioError" with "throw". John Atwood

1 0

HTTP and Browser modules
by David Owen 18 Apr '05

18 Apr '05

Good morning I have a query about compiling the original version of the HTTP implementation in Windows (http://homepages.paradise.net.nz/warrickg/haskell/http/) Using the compile commands stipulated a series of similar errors to do with the HTTP module appear. They are to do with a type mismatch, one of these errors for example is: ./HTTP.hs:592: Couldn't match 'IOError' against 'Exception' Expected type: Exception -> IO (Either a [Char]) Inferred type: IOError -> IO (Either ConnError b) In the application 'handleSocketError sk' In the second argument of 'Control.Exception.catch', namely '(handleSocketError sk)' Line 592 is: readBlock sk n = (liftM Right $ fn n) `Exception.catch` (handleSocketError sk) Have hacked about for some time to no avail. I don't know why this occurs because the code appears exactly the same in the updated version (http://www.dtek.chalmers.se/~d00bring/haskell-xml-rpc/http.html) and that compiles, albeit in linux only due to the Cabal constraint. I need it to work in Windows though so must use the original version. (I need it to compile using GHC 6.2.2) If anybody has seen this problem before or knows how to correct it I'd be very interested in your assistance :-) cheers to all david

1 0

Thompson's Exercise 9.13
by Kaoru Hosokawa 17 Apr '05

17 Apr '05

I've been working through Thompson's exercises and got to one I could not solve. It's Exercise 9.13. This is where I need to define init using foldr. init :: [a] -> [a] init "Greggery Peccary" ~> "Greggary Peccar" This is as far as I got: init xs = foldr left [] xs left :: a -> [a] -> [a] left x [] = [] left x1 : (x2 : xs) = x1 : (left x2 xs) But this returns [] and doesn't work. I can't get "left" to know that it is working with the rightmost element. It just throws away every element when its right hand side is empty. I found a solution that works for Strings, but I am looking for a more general solution. This exercise may yet again be one of those that is difficult to solve with my current knowledge of Haskell, but I'm asking anyway. -- Kaoru Hosokawa khosokawa(a)gmail.com

7 15