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Haskell and SSL
by Thomas David Baker 17 Mar '07

17 Mar '07

I'm writing an application that scrapes some data from some websites. I was using Network.Browser from http://haskell.org/http/. So far, so good. However, now I need to scrape some websites that use SSL. Network.Browserdoesn't seem to support this (although for some reason it does work on one of the sites so maybe I'm getting this wrong). Someone on #haskell pointed me at hsgnutls which is an wrapper for parts of GNU TLS. But I actually had more success writing a wrapper around wget. I know that darcs uses curl in a similar way for some stuff but it still feels like I'm doing the Wrong Thing. Does anyone know of a Haskell library that does support scraping SSL enabled web pages? Or would adding it to Network.Browser be terribly difficult? Looking at GNU TLS it looks more complicated than I imagined. Thanks in advance for your help, Tom

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HS-Plugins 1.0 chokes on simple test, WinXP GHC-6.6
by Alistair Bayley 17 Mar '07

17 Mar '07

Does anyone have hs-plugins working on WinXP with ghc-6.6? When I run the simple test below I get this error: Main: c:/ghc/ghc-6.6/HSbase.o: unknown symbol `_free' Main: user error (Dynamic loader returned: user error (resolvedObjs failed.)) Am I doing something obviously dumb? Alistair module Test1 where test1 = putStrLn "test1" module Main where import Prelude hiding (catch) import Control.Exception import Data.List import System.Environment import System.Plugins instance Show (LoadStatus a) where show (LoadFailure errors) = "LoadFailure - " ++ (concat (intersperse "\n" errors)) show (LoadSuccess m p) = "LoadSuccess" main = do a <- getArgs let modName = case a of (n:_) -> n _ -> "Test1" let modPath = "./" ++ modName ++ ".o" let method = "test1" fc <- catch (load modPath [""] [] method) (\e -> return (LoadFailure ["Dynamic loader returned: " ++ show e])) case fc of LoadFailure errors -> do fail (concat (intersperse "\n" errors)) LoadSuccess modul proc -> do let p :: IO (); p = proc proc

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How do I avoid stack overflows?
by oleg＠pobox.com 17 Mar '07

17 Mar '07

DavidA wrote: > I'm trying to write some code which involves lots of matrix multiplications, > but whenever I do too many, I get stack overflows (in both GHCi 6.4.2, and > Hugs May 2006). By placing a couple of strictness annotations, your test' gives the expected answer (given some time) on Hugs. GHCi unfortunately runs into some kind of bug (it says so itself), an unimplemented opcode. The test'' below gives that bug instantly... import List (transpose) -- not needed here -- foldl' f z [] = z -- foldl' f z (h:t) = (foldl' f $! f h z) t -- sum' l = foldl' (+) 0 l map' f [] = [] map' f (h:t) = scons (f h) (map f t) -- strict cons. Could be associated with an infix op, e.g., :$ scons :: a -> [a] -> [a] scons x l | x `seq` l `seq` False = undefined scons x l = x:l u <.> v = sum $ zipWith (*) u v a <<*>> b = multMx a (transpose b) where multMx [] _ = [] multMx (u:us) bT = scons (map' (u <.>) bT) (multMx us bT) id3 = [[1,0,0],[0,1,0],[0,0,1]] -- test = iterate (<<*>> id3) id3 !! 1000000 iterate' f x = x : seq x' (iterate' f x') where x' = f x test' = iterate' (<<*>> id3) id3 !! 1000000 test'' = head $ drop 1000000 $ iterate' (<<*>> id3) id3

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Creating static libraries using GHC
by Tom Hawkins 16 Mar '07

16 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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Newbie question: ghc search path for .c files
by Seth J. Fogarty 16 Mar '07

16 Mar '07

Hi, I am having difficulty compiling a program that uses a foreign library. The readme says to first compile a C file: :> ghc mipFFI.c -c mipFFI.c:1:0: lpkit.h: No such file or directory Alright, that's not uncommon. gcc confirms :> gcc mipFFI.c mipFFI.c:1: lpkit.h: No such file or directory mipFFI.c:2: HsFFI.h: No such file or directory So I then include the directory and gcc finds the lpkit.h file. :> gcc -I /home/sfogarty/lib/lp_solve_5.5 mipFFI.c mipFFI.c:2: HsFFI.h: No such file or directory However ghc does not seem to accept this :> ghc -i/home/sfogarty/lib/lp_solve_5.5 mipFFI.c mipFFI.c:1:0: lpkit.h: No such file or directory I've tried every variation of spacing, capitalization, copying the files to the local directory, simply listing the files ahead. Nothing seems to work! I am horribly confused! And another question while I am here: -package posix and -package lang don't seem to be included on my distribution of ghc. I am having minimal success, as yet, finding where to download them. If I could get a pointer, I'd be very appreciative. If not, I'm sure I'll find it eventually. -- Seth Fogarty sfogarty@[gmail.com|rice.edu|livejournal] Neep-neep at large AIM: Sorrath "Let us not seek to satisfy our thirst for freedom by drinking from the cup of bitterness and hatred." - Martin Luther King, Jr.

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Lack of expressiveness in kinds?
by Andrew Wagner 16 Mar '07

16 Mar '07

Ok, so I'm inching closer to understanding monads, and this question popped up today. Consider the following 2 declarations: data Foo a = Bar a data (Ord a) => Baz a = Bah a Note that both of these have kind * -> *. However, Baz could never be an instance of monad, because there is a restriction on the types it can operate on. Foo, however, is completely polymorphic, without limitation. It seems to me that there ought to be a way to express the difference between the two in the type/kind system. For example, you can almost, but not quite, say that in the declaration "class Monad m where..", m must be of kind *->*, but that's not quite enough to say, because of this example. Am I just missing something, or is there a reason the kind of Baz shouldn't be something other than *->*?

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ANNOUNCE: Google Summer of Code - open for business
by Malcolm Wallace 16 Mar '07

16 Mar '07

We are pleased to announce that haskell.org has been officially accepted as a mentoring organisation for this year's Summer of Code. Student applications are now open! http://code.google.com/summerofcode.html http://hackage.haskell.org/trac/summer-of-code/ Students: want to earn $4500 for 3 months work hacking in Haskell? Go to our ideas page, browse around, and write up a proposal for something you would like to work on. You must be available to work basically full-time for the months of June to August (inclusive). Proposals are competitive against other students. Last year, nine were funded. Your deadline for application is 24th March. Anyone else: Got any good ideas for coding projects, but no time to work on them yourself? Go to our wiki and add it in to the database! Or do you feel you could be either a primary or backup mentor for a student project in an area of your interest, even if it isn't a project you defined yourself? We need willing members of the community to foster the new student talent, so sign up on the wiki's 'people' page if you would like to be involved. Obviously you will need Haskell coding experience, but also be able to advise on time management, strategies for gaining a userbase and keeping them happy, ways of interacting with the wider Haskell community, and so on. Regards, Malcolm

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Typical?
by Hans van Thiel 16 Mar '07

16 Mar '07

Hello All, Us who're learning Haskell have a lot in common... that is, if Amazon has it right... ------------ We recommend: STAR TREK-USS ENTERPRISE D by Corgi Classics Recommended because you purchased or rated: * The Haskell School of Expression: Learning Functional Programming through Multimedia . . . We recommend: Cooking Entrees With the Micheff Sisters: A Vegan Vegetarian Cookbook by Brenda Micheff Walsh Recommended because you purchased or rated: * Purely Functional Data Structures ------------ To be fair, other recommendations were more traditional ...:) Hans van Thiel

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Generalised merge
by Paul Johnson 16 Mar '07

16 Mar '07

The most common kind of primitive recursive function I find myself writing these days is a variation on the theme of merging two sorted lists. You can see some examples in my Ranged Sets library at http://ranged-sets.sourceforge.net/. For instance: -- | Set union for ranged sets. Infix precedence is left 6. rSetUnion, (-\/-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v -- Implementation note: rSetUnion merges the two lists into a single -- sorted list and then calls normalise to combine overlapping ranges. rSetUnion (RSet ls1) (RSet ls2) = RSet $ normalise $ merge ls1 ls2 where merge ls1 [] = ls1 merge [] ls2 = ls2 merge ls1@(h1:t1) ls2@(h2:t2) = if h1 < h2 then h1 : merge t1 ls2 else h2 : merge ls1 t2 -- | Set intersection for ranged sets. Infix precedence is left 7. rSetIntersection, (-/\-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v rSetIntersection (RSet ls1) (RSet ls2) = RSet $ filter (not . rangeIsEmpty) $ merge ls1 ls2 where merge ls1@(h1:t1) ls2@(h2:t2) = rangeIntersection h1 h2 : if rangeUpper h1 < rangeUpper h2 then merge t1 ls2 else merge ls1 t2 merge _ _ = [] Union also has its own merge function. The worst case I've come across was at work. I can't talk about the details, but it involved manipulating two functions of time represented by lists of samples. So I had a type TimeFunc = [(Value, Time)], and the job was to compare two TimeFuncs with samples at different times. Step 1 was to interpolate each TimeFunc with the values for the times in the other TimeFunc, giving a result of type [(Value, Value, Time)] for the union of all the times in both original TimeFuncs. I wrote a truly hairy zipTimeFunc function with guards to match each possible case. It worked, but it must have been 100 lines if you include the comments to explain each case and demonstrate totality. So I'm wondering if anyone has a more general pattern. Unfold? Some variation on a theme of zip? I once tried writing a generalised merge, but it needed half a dozen functions as arguments to handle all the various cases. It was kludgier than just rolling a new merge routine every time. And I don't think it would have handled the TimeFunc case. Paul.

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Haskell beginner
by arnuld 15 Mar '07

15 Mar '07

i want to start learning Haskell and willing to master it :-) i have done some Common Lisp from http://www.gigamonkeys.com/book/. so i know what are functions, variables, etc. BUT i have never done any real life programmming or any kind of software development. in my country no Haskell books are available :-( , hence i can only use online Tutorials. my main purpose is to learn real-life programming by contributing some "Haskell coding" to a GPLed Software written using Haskell. i have found "Yet Another Haskell Tutorial" after searching the archives. is it really a good idea to learn Haskell using only Online material ? (when one does not have any offline resources) -- http://arnuld.blogspot.com/

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