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Re: [Haskell-cafe] RE: [Announce] primes
by michael rice 16 Apr '09

16 Apr '09

Just curious, what kind of super-cooled processor is this guy running on? I got the same answer but it took almost three minutes (2:43:15). Michael --- On Thu, 4/16/09, Geoffrey Irving <irving(a)naml.us> wrote: From: Geoffrey Irving <irving(a)naml.us> Subject: Re: [Haskell-cafe] RE: [Announce] primes To: "Daniel Fischer" <daniel.is.fischer(a)web.de> Cc: haskell-cafe(a)haskell.org Date: Thursday, April 16, 2009, 12:57 PM >> > The implementation is reasonably efficient. The query >> > >> > > primes !! 1000000 >> > >> > 15485867 >> > >> > answers after a few seconds. >> >

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How the following works
by Tsunkiet Man 16 Apr '09

16 Apr '09

Hello, I can hardly imagine how the following code works: cinits :: [a] -> [[a]] cinits [] = [[]] cinits (x:xs) = [] : map (x:) (cinits xs) can someone give me a good explaination? (I understand it a bit, but it's really hard for me to figure out how a map in a map function works.) Thank you for your time, Tsunkiet

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google-like "do you mean?" feature
by Michael Mossey 16 Apr '09

16 Apr '09

I'm thinking of writing a parser to load files that my customers have created. I'm a software requirements engineer; the data consists of the customers' thoughts in response to the latest release of the requirements doc. In fact, the files will probably be copies of the requirements doc itself, into which customers have entered their notes and made changes. The original requirements doc will have a format that can be parsed; probably something simple like lines marked with codes like //customer={customer name goes here} //requirement= {requirement text goes here} When I parse the documents that come back from the customers, they are likely to contain some errors. Field names may be mangled or misspelled. Customer names may be entered in unrecognizable variants (e.g. someone named "Michael" is indicated as "Mike") and so forth. I was thinking that it might be useful to have a Google-like "do you mean this?" feature. If the field name is //customer=, then the parser might recognize a huge list of variants like //ustomer=, //customor=, etc... that is, recognize them well enough to continue parsing and give a decent error message in context. Any ideas how to go about this? I don't think I would create a parser language that includes every variant, but instead the field names would be tokens that could be passed to another routine. The variants could be generated ahead of time. I would limit the number of variants to something manageable, like 10,000 for each field name. Thanks, Mike

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Re: [Haskell-cafe] Issues with running Ghci from emacs
by Patai Gergely 16 Apr '09

16 Apr '09

> I end up doing > > :set -i../Documents/haskell/SOE/src > > To set the search directory so that ghci can find the source. > > I've not been how to tailor that 'cd' which is run at start > up (but I've not looked to hard) It is looking for the closest ancestor directory with a file ending in .cabal, and that's where cd goes. Probably the simplest way to trick it without chaniging any setting is to create an empty dummy.cabal in the directory of your program. Gergely -- http://www.fastmail.fm - Send your email first class

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[Announce] primes
by Sebastian Fischer 16 Apr '09

16 Apr '09

I am pleased to announce the package 'primes' that implements lazy wheel sieves for efficient, purely functional generation of prime numbers in Haskell. Following the current discussion about primes in Haskell, I packaged up an implementation inspired by the papers "Lazy wheel sieves and spirals of primes" by Colin Runciman and "The Genuine Sieve of Eratosthenes" by Melissa O'Neil. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/primes The package does not implement all that was wished for in the mentioned discussion. It only exports two operations: The global constant primes :: [Integer] is an infinite list of primes and wheelSieve :: Int -> [Integer] is the function used to build this list. If you are worried about the memory requirements of using a global constant, you can use `wheelSieve 6` instead of `primes`. The implementation is reasonably efficient. The query > primes !! 1000000 15485867 answers after a few seconds. Feel free to contribute more functionality to this package. The sources are on Github: http://github.com/sebfisch/primes If you fork my project, I'll be happy to merge your changes. Cheers, Sebastian

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understanding generics
by Anatoly Yakovenko 16 Apr '09

16 Apr '09

i am trying to understand how data.data works. How would i traverse the subterms of one type and build another type out of them. something like this: > gmapQ (\a -> Right a) (Just "hello") <interactive>:1:0: Inferred type is less polymorphic than expected Quantified type variable `d' escapes In the first argument of `gmapQ', namely `(\ a -> Right a)' In the expression: gmapQ (\ a -> Right a) (Just "hello") In the definition of `it': it = gmapQ (\ a -> Right a) (Just "hello") Thanks, Anatoly

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Re: Looking for the fastest Haskell primes
by Niemeijer, R.A. 16 Apr '09

16 Apr '09

Heinrich Apfelmus wrote: > +1 except that exporting the potentially infinite list of primes is > problematic in that it may become a memory leak. > > I'd suggest to export two versions > > primes :: [Integer] > primes' :: () -> [Integer] > > for casual (i.e. throwaway program to solve a Project Euler problem) and > for memory aware use respectively. I'm afraid I don't quite follow. Would you mind explaining what the first parameter is for and how it would solve the memory leak?

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Data.ByteString woes
by David Carter 16 Apr '09

16 Apr '09

Hi, I am battling with Data.ByteString.Lazy.Char8, and currently losing. Any help gratefully received. Here's my code: import qualified Data.ByteString.Lazy.Char8 as B import Text.Regex.Posix ((=~)) test = B.pack "abc" =~ B.pack "b" :: Bool This works fine without the ".Lazy", and is also fine under GHC 6.8.1, but with GHC 6.10.1, it fails with: No instances for (Text.Regex.Base.RegexLike.RegexLike Text.Regex.Posix.Wrap.Regex B.ByteString, Text.Regex.Base.RegexLike.RegexMaker Text.Regex.Posix.Wrap.Regex Text.Regex.Posix.Wrap.CompOption Text.Regex.Posix.Wrap.ExecOption B.ByteString) I then thought I might work around the problem by converting lazy ByteStrings to strict ones in order to do the regex match. I discovered toChunks, which is advertised in the documentation as type ByteString -> [ByteString], but it actually appears to be of type toChunks :: ByteString -> [Data.ByteString.Internal.ByteString] and I can't see any way of converting a Data.ByteString.Internal.ByteString to a (strict) ByteString. Does anyone know whether this is (a) bug(s) in GHC, Data.ByteString, Text.Regex.Posix or (maybe most likely) my own limited understanding? And any ideas for fixes or workarounds (other than the obvious one of downgrading to GHC 6.8)? Thanks in advance David

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how to upgrade?
by Johannes Waldmann 16 Apr '09

16 Apr '09

Is there a nice way of upgrading ghc: I mean does cabal-upgrade know to install exactly the packages that I had with the previous ghc version? - J.W.

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Issues with running Ghci from emacs
by Daryoush Mehrtash 16 Apr '09

16 Apr '09

I am having problem running GHCI with Haskell files that include imports. I am running emacs22 on Ubuntu, with haskell-mode-2.4 extensions. I load my file (Fal.lhs in this case) in emacs. Then try to run the Ghci by doing C-c C-l. The result is shown below. Ghci fails to find the Draw.lhs which is also on the same directory as the Fal.lhs. Note: If I go to the directory where Fal.lhs is and run Ghci directly it all works fine. Any idea how I can get the interepreter to work form emacs? GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. *Prelude> :cd ~/.cabal/ Prelude> :load "../Documents/haskell/SOE/src/Fal.lhs" * ../Documents/haskell/SOE/src/Fal.lhs:76:9: Could not find module `Draw': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> Thanks, Daryoush

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