- Beginners - Haskell.org

help with error
by Tyler Hayes 21 Feb '10

21 Feb '10

I'm getting an error and I do not know why... This is what I have: class Finite a where elements' :: [a] instance (Finite a, Show a, Show b) => Show (a -> b) where show f = concat (map show [ a | a <- elements'::[a] ]) and this is what ghci gives me: Could not deduce (Finite a1) from the context () arising from a use of `elements'' at Hw06.lhs:119:42-50 Possible fix: add (Finite a1) to the context of an expression type signature In the expression: elements' :: [a] In a stmt of a list comprehension: a <- elements' :: [a] In the second argument of `map', namely `[a | a <- elements' :: [a]]' What's going on here? Thanks for the help! - Tyler

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Moment of enlightenment for lazy evaluation
by Tom Tobin 21 Feb '10

21 Feb '10

Instead of a question, I thought I'd share a moment of lazy-evaluation enlightenment I had last night. I have some code that recursively descends a directory, gets the SHA1 hashes for all the files, and builds a map of which file paths share the same SHA1 hash. The code that actually generates the hash looked like this: sha1file :: FilePath -> IO String sha1file fn = do bs <- expandPath fn >>= BSL.readFile return $ PureSHA.showDigest $ PureSHA.sha1 bs Everything worked fine on paths without many files in them, but choked on paths with many files: "Exception: getCurrentDirectory: resource exhausted (Too many open files)" This was driving me crazy; ByteString.Lazy.readFile is supposed to close the file once it's done. I kept going over my code, wondering what was at fault, until it finally clicked: *the hashes weren't being generated until I actually tried to view them*, and thus all the files were being held open until that point! I made a single change to my "sha1file" function: sha1file :: FilePath -> IO String sha1file fn = do bs <- expandPath fn >>= BSL.readFile return $ PureSHA.showDigest $! PureSHA.sha1 bs (the "$!") ... and everything worked perfectly. The code now finished processing each file before opening the next one, and I was happy. :-)

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Decent Haskell REPL?
by Jon Harrop 20 Feb '10

20 Feb '10

GHCI seems to require type definitions to be described in a separate file and then loaded into the REPL, which is seriously tedious. Are there any Haskell REPLs that don't have such limitations and, preferably, use JIT compilation instead of an interpreter? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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list of integers to list of nats
by kane96＠gmx.de 20 Feb '10

20 Feb '10

Hi, I have to write a function which maps a list of integers to a list of the corresponding nats. The following code is already there: data Nat = Z | S Nat deriving (Eq,Ord,Show) instance Enum Nat where toEnum i | i < 0 = error "Enum_Nat.toEnum: Negative integer" | i == 0 = Z | otherwise = S (toEnum (i-1)) the function should be: mapIntsToNats :: [Int] -> Maybe [Nat] so for example: [2,0,1,3] should make: [S (S Z), Z, S Z, S (S (S Z))] how can I do that? -- NEU: Mit GMX DSL über 1000,- ¿ sparen! http://portal.gmx.net/de/go/dsl02

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ffi woes
by Jose A. Ortega Ruiz 20 Feb '10

20 Feb '10

Hi, Show below is a short program using the FFI to access libiw and provide, given an interface name such as "wlan0" the associated ESSID. One uses iw_get_basic_config to fill a struct wireless_config (dynamically allocated), and extract from that the desired string, which is a field of type char[MAX_ESSID_LEN] in said struct type. When i run this program (or call getWirelessInfo inside ghci) with an existing network interface, it segfaults (calling it with a non-existent one returns the expected empty string, so the problem is after the first 'if' in getWirelessInfo): can anybody spot what am i doing wrong? Or perhaps give me any hint on how to debug the issue? TIA! jao {-# LANGUAGE CPP, ForeignFunctionInterface #-} module Main (main) where import System.Environment import Foreign import Foreign.C.Types import Foreign.C.String #include <iwlib.h> foreign import ccall "iwlib.h iw_sockets_open" c_iw_open :: IO CInt foreign import ccall "unistd.h close" c_iw_close :: CInt -> IO () -- the 3rd argument is a struct wireless_config*, an input parameter foreign import ccall "iwlib.h iw_get_basic_config" c_iw_basic_config :: CInt -> CString -> Ptr () -> IO CInt -- Given a network interface name, return its ESSID getWirelessInfo :: String -> IO String getWirelessInfo iface = allocaBytes (#size struct wireless_config) $ \wc -> withCString iface $ \istr -> do i <- c_iw_open r <- c_iw_basic_config i istr wc c_iw_close i if (r < 0) then return "" else do hase <- (#peek struct wireless_config, has_essid) wc :: IO CInt eon <- (#peek struct wireless_config, essid_on) wc :: IO CInt if hase > 0 && eon > 0 then do l <- (#peek struct wireless_config, essid_len) wc e <- (#peek struct wireless_config, essid) wc -- wireless_config.essid is a char[MAX_LEN] peekCStringLen (e, fromIntegral (l :: CInt)) else return "" main = (fmap head getArgs) >>= getWirelessInfo >>= print

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Linux install walkthrough
by Adrian Adshead 20 Feb '10

20 Feb '10

What I am looking for is a HOWTO for installing haskell on linux, specifically aGHC 6.10.4 haskell-platform-2.0.2 on Centos 5.4 using Leksah as the IDE, and developinga wx application for running on linux and windows. If anyone could point me towardssomething like that showing libraries needed and hoops to jump through then that would be great. But if not here's a rant about the problems:- I read an article recently (from 2006 I think, so rather outdated) which was talking aboutwhy there were more problems with haskell on windows than with haskell on linux andthe conclusion said that it is because more people develop haskell on linux than onwindows. I thought great, I'd rather use linux anyway so I'll give it a try. What I found was a hugeheadache and not much to help out with any problems. My linux of choice is centos (I want stability over features) and I use the first CD onlyof the latest 5.4 to install a minimal system, then use yum to put on only what I need.That formula has worked well for many other systems I have set up, so that is whatI want to do again. I do that so all my systems start from the same place and I canscript the necessary install steps to get to a working system for whatever the purpose. I found the whole need to jump through so many hoops frustrating and rather a wasteof my time. The developers of the libraries are the people who know what you need torun their libraries so a little help from them wouldn't be missed. Comparing my headacheto the installation of the latest GHC 6.12.1 and cabal-install was a breeze mainly becauseof the bootstrap.sh which does the heavy lifting for you. That would be nice in every library. If each library developer could include something like that which would install necessaryexternal libraries, preferably which could be called as a prerequisite in the Cabal installprocess, then it really would make a huge difference. So far I am finding haskell to be a wonderful language and am really enjoying learningit, but my experience compared to when I was looking at Erlang is that the benefits ofthe language are diminished by the hassle of the infrastructure around the platform.

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Re: [Haskell-beginners] Translating imperative algorithms to Haskell
by Mikhail Novikov 19 Feb '10

19 Feb '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello! Thanks, I'll take a look. I was going to buy Okasaki's book for a very long time, probably I should finally do it :) Best regards, Mikhail On 02/19/2010 11:07 PM, Marc Dominik Migge wrote: > Hi, > > I recommend you read John Hughe's article "Why Functional Programming > Matters"[1]. As one of his examples he implements alpha-beta-pruning. I'm > not really a Haskell expert, but I think the main difference will be in the > kind of data structures you use to implement the algorithms. Many people > have recommended Chris Okasaki's PhD thesis [2] to me for an in-depth > treatment of the topic. > > [1]: http://www.cs.chalmers.se/~rjmh/Papers/whyfp.html > [2]: http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf > > Best regards, > Marc > > > On Fri, Feb 19, 2010 at 9:07 PM, Mikhail Novikov <freiksenet(a)gmail.com>wrote: > > Hello! > > Four days ago I decided to try to learn Haskell by participating in Google > AI Challenge (http://csclub.uwaterloo.ca/contest/) I went with LYAH and > Real-World Haskell and I had no problems with functional core of Haskell, as > I have lispy background. The problems begun when I tried to move my code to > use different kind of decision algorithms, like negascout or flood fill. > > I have found a ready made solution for negascout ( > http://hackage.haskell.org/packages/archive/game-tree/0.1.0.0/doc/html/src/…) > but I was quite shocked by it's complexity, especially considering the > relative easy of imperative algorithm.( > http://en.wikipedia.org/wiki/Negascout) IMHO it is just TOO extremely > complex and hard to read :/ > > In flood fill situation is similar - I need to track all the colored > squares among 4 lines of recursion and I couldn't find a reasonable way to > do it without going to infinite recursion or having lots of duplicates. > > So basically, I wonder if there is some generic way to transfer imperative > algorithms with local state to functional style without making them overly > complex. > > Thanks a lot, Mikhail _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners >> -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkt+/6YACgkQPHyh4sfuKrkrdgCfaH4/z1RrQX33dtoO1QXWQshl LBUAn18H9/MdMjbGzeTJZ9+bxVYs5tfJ =9ZhZ -----END PGP SIGNATURE-----

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consing an element to a list inside a file
by kane96＠gmx.de 19 Feb '10

19 Feb '10

Hi, I want to add an element to a list inside a haskell file: mylist = [1,2,3,4] 0:mylist in Prelude it works fine, but when I do it in a file I get the error: parse error (possibly incorrect indentation) Failed, modules loaded: none. in a line after this two that doesn't exist -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/atbrowser

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defining functions in ghci
by Joe Van Dyk 19 Feb '10

19 Feb '10

This messes me up: $ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help double x =Loading package base ... linking ... done. Prelude> double x = x + x <interactive>:1:9: parse error on input `=' Why does that happen? -- Joe Van Dyk http://fixieconsulting.com

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[OT] Haskell-inspired functions for BASH
by Patrick LeBoutillier 19 Feb '10

19 Feb '10

Hi all, I've been studying Haskell for about a year now, and I've really come to like it. In my daily work I write a lot of BASH shell scripts and I thought I'd try add some of the haskell features and constructs to BASH to make my scripting life a bit easier. So I've been working on a small BASH function library that implements some basic functional programming building blocks. Note: There is no actual Haskell code involved here. All this is very prototypical, but here is an example of some of the stuff I've got so far (map, filter, foldr): $ ls data 1.txt 2.txt # basic map, argument goes on the command line $ ls -d data/* | map basename 1.txt 2.txt # map with lambda expression $ ls -d data/* | map '\f -> basename $f .txt' 1 2 # simple filter, also works with lambda $ ls -d data/* | map basename | filter 'test 1.txt =' 1.txt # sum $ ls -d data/* | map '\f -> basename $f .txt' | foldr '\x acc -> echo $(($x + $acc))' 0 3 Before I continue with this project, I'm looking for a bit of feedback/info: - Does anyone know if there are already similar projets out there? - Does anyone find this interesting? - Any other comment/suggestion/feedback Thanks a lot, Patrick LeBoutillier -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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