- Beginners - Haskell.org

Problem building packages
by emmanuel.delaborde 11 Aug '08

11 Aug '08

Hello, I am trying to build HAppS-Util with cabal-install it fails with the following message: could not execute: /Library/Frameworks/GHC.framework/Versions/608/usr/ lib/ghc-6.8.3/ghc-asm What can I do to circumvent this ? Thanks E ----------------------------------------------------------------------------------------------- This e-mail (and any attachments) is confidential and may contain personal views which are not the views of Cimex Media Ltd and any affiliated companies, unless specifically stated. It is intended for the use of the individual or group to whom it is addressed. If you have received it in error, please delete it from your system, do not use, copy or disclose the information in any way nor act in reliance on it and please notify postmaster(a)cimex.com A company registered in England Wales. Company Number 03765711 Registered Office : The Olde Bakehouse, 156 Watling Street East, Towcester, Northants NN12 6DB This email was scanned by Postini, the leading provider in Managed Email Security.

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Factorials using foldx
by Paul Johnston 08 Aug '08

08 Aug '08

Hi (First question so be gentle)! Just started to look at Haskell and I was wondering about various versions of calculating factorials. If I have facr 0 = 1 facr n = foldr (*) 1 [1..n] facl 0 = 1 facl n = foldl (*) 1 [1..n] Is there any difference in efficiency, I remember reading it is better to count down than to count up but left to right or right to left :-) Cheers Paul Paul Johnston Humanities Development Team Room 2.12 Bridgeford Street Building Manchester University Tel 0161 275 1396 Mail Paul.Johnston(a)manchester.ac.uk Web http://web-1.humanities.manchester.ac.uk/prjs/mcasspj/ How to shoot yourself in the foot Unix % ls foot.c foot.h foot.o toe.d toe.o %rm* .o rm:.o no such file or directory

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Representing a subset relationship
by Levi Stephen 04 Aug '08

04 Aug '08

Hi, I was wondering what the best way to represent a 'subset' type relationship would be. The use case I'm looking at is where the subset would be a user selection, so a filter would not be appropriate. For example, say I have something like: type Order = [OrderItem] type OrderItem = OrderItem { qty :: Int, product: Product } Now I'd like the user to be able to select a number of items (e.g., priority shipping, gift wrapping) One possible way I can see is to add extra fields to the OrderItem record, but this seems like it would not be easy to extend. It doesn't seem like using [Product] would be appropriate as if the Product is updated it will need to be updated in both the OrderItem list and in the subset. Another approach would be to use a ProductIdType in place of Product, then look up the actual Product as needed. Are there other (and better) ways to do this? Thanks, Levi

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Pattern Matching
by Alex Watt 03 Aug '08

03 Aug '08

Hi there, The best way I can explain what I'd like to do is show an example, so here goes: same :: a -> a -> Bool same x x = True same _ _ = False Is there any way to do this in Haskell? The actual reason I'd like to do this is something slightly more complicated. I want to match against a list of elements, like so: foo [x,x,x] = ... Thanks for any help. - Alex

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data type design question
by Markus Barenhoff 31 Jul '08

31 Jul '08

Hi, I have written a parsec parser for reading a marshallaled dataformat. The returned data structure is based on the following data type: > data T = TString String > | TInt Integer > | TList [TorrentT] > | TDict [(TorrentT, TorrentT)] I think TString and TInt are clear. The elements of a TList always have the same "type" (same constructor). The TDict is a dictionary where the key is always a TString but the value can be of any of the other "types", even in the same dictionary. F.e the key "foo" may map to a TInt while the key "bar" maps to another TDict. I'am not happy with this declaration, but I'am not sure how to express this better. One haskell data type for each of the four and then using type classes? Maybe something like this? : > type TString = String > type TInt = Integer > type TList = TC t => [t] > type TDict = (TC t) => [(TString, t)] > class TC where ... > instance TC TString > instance TC TInt > instance TC TList > instance TC TDict Thnx for some inseperation! Markus -- Markus Barenhoff - Münster - Germany - Europe - Earth e-mail: alios(a)alios.org - jabber: alios(a)jabber.ccc.de - icq: 27998346

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[Haskell-begin] Looking for cunning ways to update a tree
by Quergle Quergle 29 Jul '08

29 Jul '08

Hello all, This is a really helpful list: I've learned half a dozen new things just by reading this month's traffic. Anyway...I have the following bit of code that updates a tree structure given a route to a leaf: data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Show, Eq) data PathSelector = GoLeft | GoRight deriving (Show, Eq) type Path = [PathSelector] selectChild (Node left _) GoLeft = left selectChild (Node _ right ) GoRight = right updateNode (Node _ right) GoLeft newLeft = Node newLeft right updateNode (Node left _) GoRight newRight = Node left newRight updateLeaf new (Leaf previous) = Leaf new updateTree :: Tree a -> Path -> a -> Tree a updateTree tree path newValue = case path of [] -> updateLeaf newValue tree (p:ps) -> updateNode tree p (updateTree' (selectChild tree p) ps newValue) I wanted to rewrite updateTree without using explicit recursion. Unfortunately, the best I could come up with is: upDownRecurse :: (a -> b -> a) -> (a -> c) -> (a -> b -> c -> c) -> a -> [b] -> c upDownRecurse down bottoming up = upDownRecurse' where upDownRecurse' acc [] = bottoming acc upDownRecurse' acc (x:xs) = up acc x (upDownRecurse' (down acc x) xs) updateTree' :: Tree a -> Path -> a -> Tree a updateTree' tree path newValue = upDownRecurse selectChild (updateLeaf newValue) updateNode tree path So what's the sexier way of doing this? Cheers, -- Matt

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[Haskell-begin] Maximum of a List?
by Steve Klabnik 29 Jul '08

29 Jul '08

Hello everyone- I'm working on some project Euler problems today, and I'm stuck on one. It's not the problem itself that's the problem, it's that finding the maximum of a list makes me run out of heap space! http://projecteuler.net/index.php?section=problems&id=14 my code: import Data.List f :: Int -> Int -> Int f acc x | x == 1 = acc | even x = f (acc + 1) (x `div` 2) | otherwise = f (acc + 1) (3 * x + 1) answer = (foldl' max 0) $ map (f 1) [1 .. 999999] I tried using 'foldl' max ' instead of 'maximum' because I thought that foldl' was supposed to work better than foldl or something...I could be confused on that point. Anyway, here's what I get... / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Prelude> :l ..\test.hs Compiling Main ( ..\test.hs, interpreted ) Ok, modules loaded: Main. *Main> answer GHC's heap exhausted: current limit is 268435456 bytes; Use the `-M<size>' option to increase the total heap size. I also tried writing a simple 'max' function that I thought was tail recursive, and that that would help. Same error. I'm finding it hard to believe that finding a maximum of a list of a million small integers would cause this kind of overflow...is it really that big of a problem? Any help would be appreciated. I have a feeling I'll run into this problem again in the future.

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New List
by Chad Wilson 28 Jul '08

28 Jul '08

And the new list is now begun for people like me, n00bs. -- Chad Wilson

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[Haskell-begin] Cabal-Install Update Problems
by Jeff Wheeler 26 Jul '08

26 Jul '08

Hi, (Note: I also posted this a few days ago to libraries(a)haskell.org without much response, and thought I might have more success here. Please correct me if this is inappropriate.) I'm trying to use cabal-install on two machines, Debian Sid and OS X 10.5, but I am unable to run "update" on either. Most other commands seem to work fine, but the networking for "update" gives an error on both machines, completely independently: Downloading package list from server 'http://hackage.haskell.org/packages/archive' cabal: recv: resource vanished (Connection reset by peer) Running with the -v3 option does not provide much more information: Downloading package list from server 'http://hackage.haskell.org/packages/archive' Sending: GET http://hackage.haskell.org/packages/archive/00-index.tar.gz HTTP/1.1 User-Agent: cabal-install/0.5.1 Creating new connection to hackage.haskell.org cabal: recv: resource vanished (Connection reset by peer) Both machines are using a vanilla Netgear ProSafe VPN Firewall FVS318v3, a very simple prosumer router. Obviously, the router works fine for everything else, and has been running flawlessly for ages. This is using cabal-install from darcs and the cabal 1.4 branch. Thanks a bunch for helping, Jeff Wheeler

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[Haskell-begin] Morphing Endo (ICFP Contest 2007)
by Rafael Gustavo da Cunha Pereira Pinto 25 Jul '08

25 Jul '08

Did any of you tried to do a Haskell implementation of the ICFPC 2007 problem? I was thinking of using it as a learning exercise, but I am afraid of the stack. My approach is: 1) use Data.ByteString.Lazy.Char8 to read the contents of the DNA file 2) create a recursive function process::ByteString -> a that will call itself. I have a few problems: a) the DNA is 8MB long. How can I ensure the stack will hold a recursive call? b) there is an "abnormal ending" function called finish that is called anywhere in the code. Is it a good approach to return Empty to end processing? c) should I go "monadic", keeping the dna on a state monad? Thanks -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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