June 2009 - Haskell-Cafe

containers dependency on base
by Louis Wasserman 04 Jun '09

04 Jun '09

I have been unable to compile the stable version of containers-0.2.0.1, having to add the constraint base>=4.0.0.0. Can anybody else duplicate this problem? Should I patch it? (I'm working on another modification to containers and encountered this difficulty when attempting to build the whole package.) Louis Wasserman wasserman.louis(a)gmail.com

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Re: [Haskell-cafe] Cabal/primes
by michael rice 04 Jun '09

04 Jun '09

Got it working. I downloaded two packages, primes and Numbers. Since Numbers has the three functions I want to use, primes, isPrime and isProbablyPrime, how do I uninstall the primes package so there won't be a conflict? Michael ================= [michael@localhost hackage.haskell.org]$ ghc-pkg hide primes Writing new package config file... done. [michael@localhost hackage.haskell.org]$ isPrime 7 bash: isPrime: command not found [michael@localhost hackage.haskell.org]$ ghci 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> :m + Data.Numbers.Primes Prelude Data.Numbers.Primes> isPrime 7 Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package Numbers-0.2.1 ... linking ... done. True Prelude Data.Numbers.Primes> take 10 primes [2,3,5,7,11,13,17,19,23,29] Prelude Data.Numbers.Primes> --- On Wed, 6/3/09, michael rice <nowgate(a)yahoo.com> wrote: From: michael rice <nowgate(a)yahoo.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "brian" <briand(a)aracnet.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 3:46 PM This is working differently than it did last night, when at least import was working. Michael [michael@localhost ~]$ ghci 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> import Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> :m + Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> Data.Numbers.Primes> :browse <interactive>:1:21: parse error on input `:' Prelude> isPrime 3525266 <interactive>:1:0: Not in scope: `isPrime' Prelude> ========== [michael@localhost ~]$ cd .cabal [michael@localhost .cabal]$ ls -l total 20 -rw-rw-r-- 1 michael michael 1729 2009-06-02 18:59 config drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 lib drwxrwxr-x 2 michael michael 4096 2009-06-02 19:05 logs drwxrwxr-x 3 michael michael 4096 2009-06-02 18:59 packages drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 share [michael@localhost .cabal]$ cd packages [michael@localhost packages]$ ls -l total 4 drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 hackage.haskell.org [michael@localhost packages]$ cd hackage.haskell.org [michael@localhost hackage.haskell.org]$ ls -l total 12464 -rw------- 1 michael michael 11735040 2009-06-02 22:14 00-index.tar -rw------- 1 michael michael 992994 2009-06-02 22:14 00-index.tar.gz -rw-rw-r-- 1 michael michael 418 2009-06-02 22:14 build-reports.log drwxrwxr-x 3 michael michael 4096 2009-06-02 22:14 Numbers drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 primes [michael@localhost hackage.haskell.org]$ [michael@localhost hackage.haskell.org]$ pwd /home/michael/.cabal/packages/hackage.haskell.org [michael@localhost hackage.haskell.org]$ ============= --- On Wed, 6/3/09, brian <briand(a)aracnet.com> wrote: From: brian <briand(a)aracnet.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 10:46 AM Prelude> :m + Data.Numbers.Primes Prelude Data.Numbers.Primes> :browse isPrime :: Integer -> Bool isProbablyPrime :: (System.Random.RandomGen g) => Integer -> g -> (Bool, g) primes :: [Integer] Prelude Data.Numbers.Primes> isPrime 3525266 Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package Numbers-0.2.1 ... linking ... done. False Prelude Data.Numbers.Primes> On Jun 3, 2009, at 6:57 AM, michael rice wrote: > OK, I downloaded Numbers. Still missing the three functions I wanted, which are primes, isPrime, and isProbablyPrime. How do I get these? > > Michael > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANN: wp-archivebot 0.1 - archive Wikipedia's external links in WebCite
by Gwern Branwen 04 Jun '09

04 Jun '09

I'd like to announce wp-archivebot. # What wp-archivebot is a relatively simple little script which follows all the links in a RSS feed, combs the destination for http:// links, and submits them to WebCite. WebCite https://secure.wikimedia.org/wikipedia/en/wiki/WebCite is an organization much like the more famous Internet Archive. Unlike the Wayback Machine, however, WebCite will archive pages on-demand.* # Why This is good, since link-rot and 404 errors are a fact of life on Wikipedia. Links go stale, fall dead, get banned, edited, censored, etc. If those links are being used as a reference for some important fact or detail, then there is a very big problem. Even the hit-or-miss Internet Archive has proven to be very useful for editors**, so a more reliable way of archiving links would be even better. # Limitations The WebCite FAQhttp://webcitation.org/faq mentions that a good project would be to > develop a wikipedia bot which scans new wikipedia articles for cited URLs, submits an archiving request to WebCite®, and then adds a link to the archived URL behind the cited URL Adding a link would be both quite difficult and require community approval; further, although I have thought about this for years, there's no obvious good way to add a link. Any method is either visually awkward, possibly otiose (if [[Google]] links to google.com as the homepage in its infobox, there's no purpose to have an archived version of google.com!) and certainly will bloat up the markup - even if there's any way to insert links without bolloxing templates and other such constructs. So I'm satisfied to just archive the link. WebCite is searchable, after all. If enough people run bots like this and achieve enough coverage, then perhaps editors can be educated to always check in WebCite as well. # Download & Install As ever, wp-archivebot is Free and is available from Hackage at: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wp-archivebot You can install with ease by a simple 'cabal install wp-archivebot', or download the tarball and compile it yourself with the usual 'runhaskell Setup configure && runhaskell Setup build && runhaskell Setup install' dance. # Usage wp-archivebot takes one mandatory argument, an email address; WebCite needs to have somewhere to send notices of archival success/failure. wp-archivebot takes a second, optional, argument. This is a RSS feed to use. It defaults to Special:NewPages on the English Wikipedia, but one could just as well follow, say, RecentChanges. Here's an example: > wp-archivebot gwern0(a)gmail.com 'http://en.wikipedia.org/w/index.php?title=Special:RecentChanges&feed=rss' (This sets my email address as the recipient, and follows RecentChanges. This may not be a good idea as RecentChanges is *much* busier than NewPages.) ## Example Here's an example session's output: [12:35 PM] 829Mb$ wp-archivebot gwern0(a)gmail.com "http://www.webcitation.org/archive?url=http://en.wikisource.org/wiki/Berkel…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/pl…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/pl…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/mi…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/mi…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.erniestires.net/&email=gw…" "http://www.webcitation.org/archive?url=http://www.erniestires.net/&email=gw…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://www.leighrayment.com/commons/…" "http://www.webcitation.org/archive?url=http://www.leighrayment.com/commons/…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://www.esec.edu&email=gwern0@gma…" "http://www.webcitation.org/archive?url=http://www.esec.edu&email=gwern0@gma…" ... # Related The development version (HEAD) of the Gitit wiki has plugin support; one of those plugins, WebArchiver.hs, will on every page-save comb through for off-wiki links and submit them to WebCite in the same way as this bot. It's nice to know that if those links ever disapear, you can retrieve them from WebCite and 'see' the revision with the same set of external links as when the revision was created. * Technically, the Internet Archive will archive on demand as well - but you need to pay them. ** In many more ways than one might expect. For example, not infrequently someone will visit an article and claim it is plagiarizing some other webpage. With the IA, it's easy to go back to the first version of that webpage and crosscheck against the article's history - quite often it is the other website that plagiarized us! -- gwern

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Nested Lists
by Paul Keir 04 Jun '09

04 Jun '09

Hi all, If I have a list, and I'd like to convert it to a list of lists, each of length n, I can use a function like bunch: bunch _ [] = [] bunch n as = let (c,cs) = splitAt n as in c:bunch n cs > bunch 8 [1..16] [[1,2,3,4,5,6,7,8],[9,10,11,12,13,14,15,16]] If I now want to do the same for the nested lists, I can compose an application involving both map and bunch: > map (bunch 4) . bunch 8 $ [1..16] [[[1,2,3,4],[5,6,7,8]],[[9,10,11,12],[13,14,15,16]]] and I can "bunch" the new length 4 lists again: > map (map (bunch 2)) . map (bunch 4) . bunch 8 $ [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] Clearly there is a pattern here involving the bunch function and latterly, three Int parameters; 2, 4 and 8. My question is, can I create a function that will take such parameters as a list, and give the same result, for example: > f [2,4,8] [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] or perhaps: > f [bunch 2, bunch 4, bunch 8] [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] I think it may not be possible because the type signature of f would depend on the length of its list parameter; but I'm not sure. -Paul

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Re: Strange type error with associated type synonyms
by John Lato 04 Jun '09

04 Jun '09

Hello, Apologies for the late contribution; I've been away recently. I have a very strong opinion on this, though. > From: Bulat Ziganshin <bulat.ziganshin(a)gmail.com> > > Thursday, May 28, 2009, 2:06:36 AM, HT wrote: > > Prelude>> let a = 'a'; b = "b" in a==b > >> <interactive>:1:27: >> Couldn't match expected type `Char' against inferred type `[Char]' >> .... > >> Is the type of 'a' wrong or that of 'b'? > > it is not important, well, at least we can live with it. Compiler > should say: > > First argument of == should be of type String > while a is of type Char I find this to be a somewhat important issue. If there weren't so many polymorphic functions it wouldn't be as big of a problem. The only reason the first argument of == should be a String is because the second argument is (or alternatively 2nd should be Char, etc.). For polymorphic functions, in which the type of one argument is fixed by another, I think the error message should ideally point out how the compiler is getting expected/inferred types. E.g. Couldn't match expected type `Char' against inferred type `[Char]' Type of [first argument] to `==' must match type of [second argument] where [first|second argument] is filled in with the appropriate expressions. I've become somewhat adept at figuring this out, but I think it would be helpful for those who haven't experienced it before. A related issue is with multiple function definitions. Here's a short example: foo n = ... where bar (Just a) = return $ somefunc a bar (Nothing) = somefunc n a = otherfunc assume that 'a' and 'n' have the same type. Now the type checker won't allow bar, because the two definitions have differing types. GHC's error message would ideally state that the definitions for 'bar' have differing types, but instead we get a message about mismatched types within one of the definitions. I know this example is rather contrived, but I don't have any real code to hand because I've fixed it already. You get a better message if you give a type signature for 'bar', but that requires lexical scoping (at least in my case), which means the code would no longer be Haskell98 (which it otherwise is IIRC). Cheers, John Lato

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Scary type inference for monadic function definitions
by Ahn, Ki Yung 04 Jun '09

04 Jun '09

Scary type inference for monadic function definitions (or, why you'd want to annotate types for monadic function definitions) This is a real example that I've experienced. I defined the following function. > checkOneVerseByLineWith readLine v = > do mg <- readLine > case mg of > Just g -> return Just (v==g) > Nothing -> return Nothing My intention was to use it something like this: checkOneVerseByLineWith (readline "% ") where readline is the library function from System.Console.Readline. As you can see, there is an obvious mistake which I forgot to group (Just (v==g)) in parenthesis. However, GHC or any other Haskell 98 compliant implementation will infer a type for you and this will type check! Try it yourself if in doubt. Of course, checkOneVerseByLineWith (readline "% ") won't type check because checkOneVerseByLineWith has strange type. The reason why the above definition type checks is because ((->) r) is an instance of Monad.

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cabalizing "legacy' source
by Vasili I. Galchin 04 Jun '09

04 Jun '09

Hello, I have cabalizing some "legacy" Haskell source. By "legacy", I totally don't mean in a pejorative sense ... code has been designed and written well. I am making good progress ... I have succeeded in building an archive file(library) on a POSIX platform, i.e. Linux. The original author intended to build several executables from this .a file. In addition there are many pairings of .hs/.lhs along with "main" test files. Currently I have only one .cabal file. This is inadequate for the author's intent: 1) I need to build "n" test executables. 2) I need to build 1/2 CLI executables. I need advice on the cabal front. E.g. should I first run the library .cabal and then run several subsidary .cabal files, e.g. "n"test .cabals and also 1/2 CLI executable .cabals??? I am looking for advice so that I don't have a clumsy cabal build process/tree! Thanks, Vasili

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type checking that I can't figure out ....
by Vasili I. Galchin 04 Jun '09

04 Jun '09

Hello Haskellers, I isolated to a not so small piece: > {-# OPTIONS -fglasgow-exts #-} > {-# LANGUAGE UndecidableInstances #-} > import Control.Monad.Identity > import Control.Monad.Reader > import Control.Monad.State > import qualified Data.List as L > import qualified Data.Map as M > import Data.Array import IOExts The type of a regular expression. > data Re t > = ReOr [Re t] > | ReCat [Re t] > | ReStar (Re t) > | RePlus (Re t) > | ReOpt (Re t) > | ReTerm [t] > deriving (Show) The internal type of a regular expression. > type SimplRe t = Int > data SimplRe' t > = SReOr (SimplRe t) (SimplRe t) > | SReCat (SimplRe t) (SimplRe t) > | SReStar (SimplRe t) > | SReLambda > | SReNullSet > | SReTerm t > deriving (Eq, Ord, Show) The regular expression builder monad. > data (Ord t) => ReRead t > = ReRead { > rerNullSet :: SimplRe t, > rerLambda :: SimplRe t > } > data (Ord t) => ReState t > = ReState { > resFwdMap :: M.Map (SimplRe t) (ReInfo t), > resBwdMap :: M.Map (SimplRe' t) (SimplRe t), > resNext :: Int, > resQueue :: ([SimplRe t], [SimplRe t]), > resStatesDone :: [SimplRe t] > } > type ReM m t a = StateT (ReState t) (ReaderT (ReRead t) m) a TEMP WNH Dfa construction > data ReDfaState t > = ReDfaState { > dfaFinal :: Bool, > dfaTrans :: [(t, SimplRe t)] > } > deriving (Show) TEMP WNH The ReInfo type > data ReInfo t > = ReInfo { > reiSRE :: SimplRe' t, > reiNullable :: Bool, > reiDfa :: Maybe (ReDfaState t) > } > deriving (Show) TEMP WNH > class (Monad m, Ord t) => ReVars m t where { } > instance (Monad m, Ord t) => ReVars m t where { } TEMP WNH > remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t) > remLookupFwd re > = do fwd <- gets resFwdMap > -- let { Just reinfo = M.lookup fwd re } -- PROBLEM > reinfo <- M.lookup fwd re -- PROBLEM > return reinfo When I "compile" with ghci I get: Dfa_exp.lhs:91:32: Couldn't match expected type `M.Map (M.Map (SimplRe t) (ReInfo t)) t1' against inferred type `SimplRe t2' In the second argument of `M.lookup', namely `re' In a 'do' expression: reinfo <- M.lookup fwd re In the expression: do fwd <- gets resFwdMap reinfo <- M.lookup fwd re return reinfo I trimmed the original code down a lot! But still can't why I am getting type check errors!!! Help! Kind regards, Vasili

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ANN: memscript-0.0.0.2 (Command line utility for memorizing scriptures or any other text)
by Ahn, Ki Yung 04 Jun '09

04 Jun '09

memscript: Command line utility for memorizing scriptures or any other text http://hackage.haskell.org/cgi-bin/hackage-scripts/package/memscript "memscript <filename>" Run memscript with a UTF-8 (or ASCII since ASCII is a subset of UTF8) plain text file. Try to exactly guess the text line by line. If your guess is wrong it will show you a diff like output comparing your guess and the original verse. This will repeat until you get all the verses right. For the test data I included four beloved Psalms (1,23,121,127) from the Old Testament, each in Revised Korean Version (RKV) and New International Version (NIV), which I happened to have had to memorize. You can use it for any other text you'd want to memorize, such as your favorite poems, quotes, or whatsoever. No craft or ticks, really simple and straightforward utility but serves well for the purpose. I used readline because that was the only sane way I know of handling multibyte inputs. ==== an example usage ==== kyagrd@kyagrd:~/cscs/memscript$ memscript 001en.txt % Blessed is the man who does not walk in the counsel of the wicked or stand in the way of sinners or sit in the seat of mockers. % But his delight is in the law of the LORD, and on his law he meditates night and day. === < But his delight is in the law of the LORD, and on his law he meditates day and night. --- > But his delight is in the law of the LORD, and on his law he meditates night and day. === % But his delight is in the law of the LORD, and on his law he meditates day and night. % He is like a tree planted by streams of water, which yields its fruit in season and whose leaf does not wither. Whatever he does prospers. % Not so the wicked! They are like chaff that the wind blows away. % The wicked will not stand in the judgment, nor sinners in the assembly of the righteous. === < Therefore the wicked will not stand in the judgment, nor sinners in the assembly of the righteous. --- > The wicked will not stand in the judgment, nor sinners in the assembly of the righteous. === % Therefore the wicked will not stand in the judgment, nor sinners in the assembly of the righteous. % For the LORD watches over the way of the righteous, but the way of the wicked will perish. kyagrd@kyagrd:~/cscs/memscript$

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measure a function in ghci
by Nico Rolle 04 Jun '09

04 Jun '09

hi there is there a quick way to check which function is doing the job quicker? i have 2 functions which i want to compare. both give the same output but they do it in different manner. i want to test which one is faster. i use ghci. thanks regards

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