May 2010 - Haskell-Cafe

Resource module
by Arie Peterson 02 Jun '10

02 Jun '10

Dear list, For some time, I have maintained a small private module centred around the following type of "resource": > newtype Resource cap m = Resource { with :: forall a. (cap -> m a) -> m a } Interpretation: @Resource cap m@ is a resource, providing a "capability" of type @cap@, which does administration (like opening and closing handles) in the monad @m@. I would like to use a standard module instead of this howe-grown one, so I can publish things that depend on it. Does anyone know a package that provides this? Things I tried: - The type is suspiciously similar to the continuation monad transformer; in fact @Resource cap m@ is isomorphic to @forall t. ContT t m cap@. However, I can't use this latter type directly, because I would like functions on Resources like the instance > instance (Monoid cap) => Monoid (Resource cap m) , which is not possible without a newtype wrapper. - I tried using the "regions" package, since it has a very similar purpose, but this seems impossible: some resources I could not express in the form required for its class 'Resource' (methods 'open' and 'close'). If something like this is not yet around, I'll upload my version, but I'd like to reuse, if possible. Regards, Arie

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TagSoup 0.9
by Ralph Hodgson 02 Jun '10

02 Jun '10

Hello Neil , I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* -> *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. where line 29 is the type declaration for 'bundle' in the following code: > module TQ.TagSoup.TagSoupExtensions where > import TQ.Common.TextAndListHandling > import Text.HTML.TagSoup > import Text.HTML.Download > import Control.Monad > import Data.List > import Data.Char > type Bundle = [Tag] [snip] > tagsOnPage :: String -> IO(String) > tagsOnPage url = do > tags <- liftM parseTags $ openURL url > let results = unlines $ map(show) $ tags > return (results) > extractTags :: Tag -> Tag -> [Tag] -> [Tag] > extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags > extractTagsBetween :: Tag -> [Tag] -> [Tag] > extractTagsBetween _ [] = [] > extractTagsBetween markerTag tags = if startTags == [] > then [] > else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail startTags) > where > startTags = dropWhile (~/= markerTag ) tags I need to repair this code quickly. I am hoping you can quickly help me resolve this. Thanks. Ralph Hodgson, @ralphtq <http://twitter.com/ralphtq>

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Haskell + CouchDB, anyone?
by Clint Moore 01 Jun '10

01 Jun '10

Hi there, I'm trying to use any version of couchdb with Haskell and Database.CouchDB and I'm not having a ton of luck. I've talked to others in the past that have used it without a problem so I'm wondering what it is that I'm doing wrong. I can connect, and create new databases without a problem - do any of you couchdb-using haskell people see a problem with this line? runCouchDB' $ newDoc (db "testify") $ JSObject $ toJSObject [("name",JSString $ toJSString $ "value")] I figured I would start small and just try to put something basic into a document. The rest of this email is also at a gist at http://gist.github.com/417442 Here is a list of the revisions of couchdb and the different errors I'm getting back as well as some of the code I'm using to try to test this.. Hopefully it will help. {- CouchDB 0.11.0: [error] [<0.95.0>] attempted upload of invalid JSON undefined [info] [<0.95.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 400 CouchDB Trunk as of 5/28 - 10:26 PST: [info] [<0.95.0>] 127.0.0.1 - - 'PUT' /honks? 201 [debug] [<0.114.0>] 'PUT' /honks/cmoore? {1,1} Headers: [{'Connection',"keep-alive"}, {'Content-Type',"application/json"}, {'Host',"localhost:5984"}, {'User-Agent',"hs-HTTP-4000.0.5"}] [debug] [<0.114.0>] OAuth Params: [] [error] [<0.114.0>] attempted upload of invalid JSON undefined [info] [<0.114.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 400 [debug] [<0.114.0>] httpd 400 error response: {"error":"bad_request","reason":"invalid UTF-8 JSON"} CouchDB rev 818083: [debug] [<0.107.0>] 'PUT' /honks/cmoore? {1,1} Headers: [{'Connection',"keep-alive"}, {'Content-Type',"application/json"}, {'Host',"localhost:5984"}, {'User-Agent',"hs-HTTP-4000.0.5"}] [debug] [<0.107.0>] OAuth Params: [] [debug] [<0.107.0>] Minor error in HTTP request: {invalid_json,undefined} [debug] [<0.107.0>] Stacktrace: [{mochijson2,decode,1}, {couch_httpd_db,db_doc_req,3}, {couch_httpd_db,do_db_req,2}, {couch_httpd,handle_request,5}, {mochiweb_http,headers,5}, {proc_lib,init_p_do_apply,3}] [info] [<0.107.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 500 [debug] [<0.107.0>] httpd 500 error response: {"error":"invalid_json","reason":"undefined"} CouchDB 0.10.1: [error] [<0.278.0>] attempted upload of invalid JSON undefined [info] [<0.278.0>] 127.0.0.1 - - 'POST' /testify? 400 -} module Main where import Text.JSON import Database.CouchDB import Database.CouchDB.JSON data Person = Person { name :: String, email :: String } deriving (Eq,Show) instance JSON Person where showJSON (Person nm em) = JSObject $ toJSObject [("name", showJSON nm),("email", showJSON em)] readJSON val = do obj <- jsonObject val nm <- jsonField "name" obj em <- jsonField "email" obj return (Person nm em) create = runCouchDB "localhost" 5984 $ createDB "honks" main :: IO () main = do _ <- runCouchDB "localhost" 5984 $ newNamedDoc (db "honks") (doc "cmoore") $ Person "cmoore" "cmoore(a)w.com" putStrLn "Honk." --- Later, against 0.10.1: cmoore@cathedral:~/tmp$ ghci GHCi, version 6.12.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :m +Database.CouchDB Prelude Database.CouchDB> :m +Text.JSON Prelude Database.CouchDB Text.JSON> :set prompt "> " > runCouchDB' $ createDB "testify" Loading package array-0.3.0.0 ... linking ... done. Loading package syb-0.1.0.2 ... linking ... done. Loading package base-3.0.3.2 ... linking ... done. Loading package bytestring-0.9.1.6 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.1.5 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package old-time-1.0.0.4 ... linking ... done. Loading package HTTP-4000.0.8 ... linking ... done. Loading package containers-0.3.0.0 ... linking ... done. Loading package pretty-1.0.1.1 ... linking ... done. Loading package json-0.4.4 ... linking ... done. Loading package CouchDB-0.10.2 ... linking ... done. > runCouchDB' $ newDoc (db "testify") $ JSObject $ toJSObject [("name",JSString $ toJSString $ "value")] *** Exception: HTTP/1.1 400 Bad Request Server: CouchDB/0.10.1 (Erlang OTP/R13B) Date: Fri, 28 May 2010 18:46:36 GMT Content-Type: text/plain;charset=utf-8 Content-Length: 54 Cache-Control: must-revalidate

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name of this monadic combinator?
by Michael Vanier 31 May '10

31 May '10

I stumbled across this monadic combinator: mcombine :: Monad m => (a -> a -> a) -> m a -> m a -> m a mcombine f mx my = do x <- mx y <- my return (f x y) I used it to chain the outputs of two Parsec String parsers together using this operator: (<++>) :: Monad m => m String -> m String -> m String (<++>) = mcombine (++) mcombine seems like such a basic operation that it should be a library function, but I couldn't find one matching it on hoogle. Is there one? - Mike

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yet another functional reactive programming tutorial :)
by Jinjing Wang 31 May '10

31 May '10

Dear list, As I'm learning frp and reading the wonderful tutorial at http://www.formicite.com/dopage.php?frp/frp.html , I'm putting up some more basic cheatsheet style tutorial for myself. http://github.com/nfjinjing/frp-guide Feel free to take advantage of it. -- jinjing

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LLVM -> Haskell
by Tom Hawkins 31 May '10

31 May '10

Is there any work being done to read LLVM object code into Haskell? I've looked through the llvm library [1], but it appears focused on code generation. -Tom [1] http://hackage.haskell.org/package/llvm

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Third Ghent Functional Programming Group Meeting: Tuesday, June 29 at 19h
by Jeroen Janssen 31 May '10

31 May '10

(apologies if you receive multiple copies) Dear all, We would like to announce that the third Ghent Functional Programming Group Meeting will take place on Tuesday, June 29, in the Technicum Building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University at 19h. If you are interested in giving a talk, please do not hesitate to drop us a note! The final program will be announced shortly. For more information you can follow us at twitter (@ghentfpg) or sign up for our google group (http://groups.google.com/group/ghent-fpg) Hope to see you all then, Bart Coppens (Bart.Coppens(a)elis.ugent.be; Twitter: @bartcopp) Jasper Van der Jeugt (jaspervdj(a)gmail.com; Twitter: @jaspervdj) Jeroen Janssen (jejansse(a)gmail.com; Twitter: @jejansse) Note: if you do not want to receive any more e-mails from Ghent FPG, just send an e-mail to Jeroen Janssen (jejansse(a)gmail.com) and we will remove you from our mailinglist.

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Improving Haskell on Windows by adding it to TakeoffGW
by MH 31 May '10

31 May '10

That's a great idea and I would love to help but I have no idea where and how to start. I would love to work on making Haskell installer work on TakeofGW. Would it be possible to write down some notes describing what needs to be done? Thanks, MH

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Announce: berp, an implementation of Python 3, in Haskell
by Bernie Pope 31 May '10

31 May '10

I'm pleased to announce the first public release of berp, version 0.0.2. Berp is (the beginnings of) an implementation of Python 3, written in Haskell. It provides a compiler and an interpreter. In both cases the input Python program is translated into Haskell code. The compiler turns the Haskell code into machine code. The interpreter runs the Haskell code immediately via the GHCi interpreter. The user interface of the interpreter imitates the one provided by CPython. A cabal package is available: http://hackage.haskell.org/package/berp The source code is hosted on github: http://github.com/bjpop/berp Documentation is available on a github wiki: http://wiki.github.com/bjpop/berp/ Note: berp is known to work with GHC 6.10.4, but it may not work with 6.12.x, due to issues with GHC's memory usage when compiling the language-python package (ticket #3972 on the GHC trac). As you can see by the very low version number (0.0.2), berp is still young; there is lots of room for improvement. However, it does support a fairly wide variety of Python's features, and some extensions such as callCC and tail call optimisation. Read more about it on the github wiki mentioned above. Cheers, Bernie.

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ANNOUNCE: MonadCatchIO-foreign
by Antoine Latter 31 May '10

31 May '10

Hello Haskell, I'd like to announce a very small library in two flavors. The problem I'm trying to solve is that we have some capabilities for writing functions which are polymorphic over monad but still use IO capabilities - liftIO :: (IO a -> m a) from the packages transformers and mtl. The packages[1,2] MonadCatchIO offer similar polymorphism for the exception capabilities of the IO monad. The package MonadCatchIO-foreign offers similar polymorphism for the following functions: alloca, allocaBytes, allocaArray, allocaArray0 and withForeignPtr. It's a small thing, but I don't see why it should be written twice. Links on hackage: http://hackage.haskell.org/package/MonadCatchIO-mtl-foreign http://hackage.haskell.org/package/MonadCatchIO-transformers-foreign GitHub link: http://github.com/aslatter/MonadCatchIO-foreign Feedback, praise and adulation welcome. Antoine

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