- Haskell-Cafe - Haskell.org

language proposal: ad-hoc overloading
by Ryan Ingram 07 Sep '08

07 Sep '08

The point of having a strongly typed language is so the compiler can do more work for you. But right now I do a lot of typing (pun not intended) to appease the compiler. Let me give you an example: module Prob where import qualified Data.Map as M ... newtype Prob p a = Prob { runProb :: [(a,p)] } combine :: (Num p, Ord a) => Prob p a -> Prob p a combine m = Prob $ M.assocs $ foldl' (flip $ uncurry $ M.insertWith (+)) M.empty $ runProb m Do you see it? All those "M." just seem dirty to me, especially because the compiler should be able to deduce them from the types of the arguments. My proposal is to allow "ad-hoc" overloading of names; if a name is ambiguous in a scope, attempt to type-check the expression against each name. It is only an error if type-checking against all names fails. If type-checking succeeds for more than one then the expression is ambiguous and this is also an error. Pros: shorter code, less busywork to please the compiler Cons: potentially exponential compile time? Any thoughts? -- ryan

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HXT from schema to data model
by Pierre-Edouard Portier 07 Sep '08

07 Sep '08

Hi! Is there a way to generate a data model and a set of picklers from an XML (or RelaxNG) Schema using the HXT tool box? Thank You, Pierre-Edouard

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Online Real World Haskell, problem with Sqlite3 chapters
by david48 07 Sep '08

07 Sep '08

In the online version of Real world Haskell, there's a problem with examples in ghci when the module Database.HDBC.Sqlite3 is imported. It goes on like this for all of chapter 21 and 22. Example : ( note: this is not me trying to reproduce the examples, it's an actual copy & paste from the site url http://book.realworldhaskell.org/read/using-databases.html ) ghci> :module Database.HDBC Database.HDBC.Sqlite3 Could not find module `Database.HDBC.Sqlite3': Use -v to see a list of the files searched for. ghci> conn <- connectSqlite3 "test1.db" <interactive>:1:8: Not in scope: `connectSqlite3' ghci> :type conn <interactive>:1:0: Not in scope: `conn' ghci> disconnect conn <interactive>:1:0: Not in scope: `disconnect' <interactive>:1:11: Not in scope: `conn'

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A problem with nested regions and higher-order functions
by Mario Blažević 07 Sep '08

07 Sep '08

I forgot the subject, sorry for reposting... I'm trying to apply the nested regions (as in Lightweight Monadic Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the proper term, in hope to gain a bit more type safety in this little library I'm working on (Streaming Component Combinators, available on Hackage). I guess the problem is that I'm getting too much type safety now, because I can't get the thing to compile. Most of the existing code works, the only exceptions seem to be various higher-order functions. I've reduced the problem to several lines of Literate Haskell code below, can anybody find a solution or confirm there isn't one? > {-# LANGUAGE EmptyDataDecls, Rank2Types #-} > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, IncoherentInstances #-} > module Main where > main = undefined I'll call the main type, originally a monad transformer, simply Region. I'm leaving out the Monad and MonadTransformer instances, because they don't contribute to the problem. The parameter r is the phantom region type. > newtype Region r a = Region a The Ancestor class is used to denote relationship between two regions where one is nested in another. > data Child r > class Ancestor r r' > instance Ancestor r (Child r) > instance Ancestor r1 r2 => Ancestor r1 (Child r2) Handle is a simple wrapper around a value. It carries information about the region that originates the value. > data Handle r x = Handle x A typical calculation in the Region monad will take a bunch of Handles inherited from an Ancestor region and do something with them. The Ancestor constraint is there to ensure that the handles are not fake but genuinely inherited. > type SingleHandler x y = forall r1s rs. Ancestor r1s rs => > Handle r1s x -> Region rs y > type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => > Handle r1d x -> Handle r2d y -> Region rd z And now I get to the problem. The following higher-order function doesn't type-check: > mapD :: (SingleHandler x z -> SingleHandler y z) > -> DoubleHandler x w z -> DoubleHandler y w z > mapD f d = \y w-> f (\x-> d x w) y I get the same error from GHC 6.8.2 and 6.8.2: Test.lhs:36:28: Could not deduce (Ancestor r2d rs) from the context (Ancestor r1s rs) arising from a use of `d' at Test.lhs:36:28-32 Possible fix: add (Ancestor r2d rs) to the context of the polymorphic type `forall r1s rs. (Ancestor r1s rs) => Handle r1s x -> Region rs z' In the expression: d x w In the first argument of `f', namely `(\ x -> d x w)' In the expression: f (\ x -> d x w) y The same code compiles just fine if all the Ancestor constraints are removed. I don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC recommends. I think it ought to be able to figure things out based on the exisisting constraints, but I may be wrong: perhaps higher-order functions pose a fundamental problem for type-level programming. Can anybody shed any light on this?

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(no subject)
by Mario Blažević 07 Sep '08

07 Sep '08

Hello. I'm trying to apply the nested regions (as in Lightweight Monadic Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the proper term. I was hoping to gain a bit more type safety in this little library I'm working on -- Streaming Component Combinators, available on Hackage. I guess the problem is that I'm getting too much type safety now, because I can't get the thing to compile. Most of the existing code works, the only exceptions seem to be various higher-order functions. I've reduced the problem to several lines of Literate Haskell code below, can anybody think of a solution or a reason there can't be one? > {-# LANGUAGE MultiParamTypeClasses, EmptyDataDecls, Rank2Types #-} > {-# LANGUAGE FunctionalDependencies, FlexibleInstances, IncoherentInstances #-} > module Main where > main = undefined I'll call the main type, originally a monad transformer, simply Region. I'm leaving out the Monad and MonadTransformer instances, because they don't contribute to the problem. The parameter r is the phantom region type. > newtype Region r a = Region a The Ancestor class is used to denote relationship between two regions where one is nested in another. > data Child r > class Ancestor r r' > instance Ancestor r (Child r) > instance Ancestor r1 r2 => Ancestor r1 (Child r2) Handle is a simple wrapper around a value. It carries information about the region that originates the value. > data Handle r x = Handle x A typical calculation in the Region monad will take a bunch of Handles inherited from an Ancestor region and do something with them. The Ancestor constraint is there to ensure that the handles are not fake but genuinely inherited. > type SingleHandler x y = forall r1s rs. Ancestor r1s rs => > Handle r1s x -> Region rs y > type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => > Handle r1d x -> Handle r2d y -> Region rd z And now I'm getting to the problem. The following higher-order function doesn't type-check: > mapD :: (SingleHandler x z -> SingleHandler y z) > -> DoubleHandler x w z -> DoubleHandler y w z > mapD f d = \y w-> f (\x-> d x w) y I get the same error from GHC 6.8.2 and 6.8.2: Test.lhs:36:28: Could not deduce (Ancestor r2d rs) from the context (Ancestor r1s rs) arising from a use of `d' at Test.lhs:36:28-32 Possible fix: add (Ancestor r2d rs) to the context of the polymorphic type `forall r1s rs. (Ancestor r1s rs) => Handle r1s x -> Region rs z' In the expression: d x w In the first argument of `f', namely `(\ x -> d x w)' In the expression: f (\ x -> d x w) y The same code compiles just fine if all the Ancestor constraints are removed. I don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC recommends. I think it ought to be able to figure things out based on the exisisting constraints, but I may be wrong: perhaps higher-order functions pose an insurmountable problem for type-level programming in Haskell. Can anybody shed any light on this?

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Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
by Don Stewart 06 Sep '08

06 Sep '08

nomeata: > Hi, > > Am Freitag, den 05.09.2008, 18:38 -0700 schrieb Don Stewart: > > xmonad packages are available in the package systems of at least: > > > > Debian, Gentoo, Arch, Ubuntu, OpenBSD, > > NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware > > > > and 0.8 packages will appear in coming days (some are already available). > > Debian xmonad and xmonad-contrib package uploaded (although > xmonad-contrib is in the two-day delayed queue, to give autobuilders a > chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! -- Don

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GADTs and instances
by Tony Hannan 06 Sep '08

06 Sep '08

Hello haskellers, Anyone know the trick for making a Binary instance of a GADT. See sample code below followed by the type error reported by ghc version 6.8.3 Thanks, Tony ---- {-# LANGUAGE GADTs #-} module GADTTest where import Data.Binary import Control.Monad (liftM) data Query a where Lookup :: String -> Query (Maybe Int) Fetch :: [String] -> Query [Int] instance (Binary a) => Binary (Query a) where put (Lookup x) = putWord8 0 >> put x put (Fetch x) = putWord8 1 >> put x get = getWord8 >>= \tag -> case tag of 0 -> liftM Lookup get 1 -> liftM Fetch get ----- GADTTest.hs:12:0: Couldn't match expected type `Maybe Int' against inferred type `[Int]' When trying to generalise the type inferred for `get' Signature type: forall a. (Binary a) => Get (Query a) Type to generalise: Get (Query a) In the instance declaration for `Binary (Query a)'

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Haskell Weekly News: Issue 84 - September 6, 2008
by Brent Yorgey 06 Sep '08

06 Sep '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080906 Issue 84 - September 06, 2008 --------------------------------------------------------------------------- Welcome to issue 84 of HWN, a newsletter covering developments in the [1]Haskell community. This is the "This issue is not late since the HWN will henceforth be published on Saturday now that I have real work to do" edition. Featured this week: darcs hacking sprint plans solidify, xmonad 0.8 released, typed sprintf and sscanf, and tons of discussion about everything from functional references to splitting up the base library to the future direction of Haskell. Announcements unicode-properties 3.2.0.0, unicode-names 3.2.0.0. Ashley Yakeley [2]announced the release of the [3]unicode-properties 3.2.0.0 and [4]unicode-names 3.2.0.0 packages, which are representations in Haskell of various data in the Unicode 3.2.0 Character Database. experimental static blog engine in Haskell. jinjing [5]announced the initial release of [6]Panda, an experimental static blog engine written in Haskell. darcs hacking sprint, venues confirmed! (25-26 October). Eric Y. Kow [7]announced that two venues (Brighton, UK and Portland, Oregon, USA) have been confirmed for the [8]darcs hacking sprint on 25-26 October. darcs weekly news #2. Eric Y. Kow The second weekly issue of the [9]darcs weekly news has been published. xmonad 0.8 released!. Don Stewart [10]announced the release of [11]xmonad 0.8, featuring a general purpose "gaps" replacement, locale support, the ability to create your own configuration parsers, and various other [12]enhancements and fixes. POPL logo design contest. Don Stewart [13]forwarded a message announcing a logo design contest for POPL 2009. Dust off your magic markers/photoshop skills and get designing! ICFP09 Announcement. Matthew Fluet [14]announced [15]ICFP 2009, to be held 31st August to 2nd September 2009 in Edinburgh. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Fast parallel binary-trees for the shootout: Control.Parallel.Strategies FTW!. Don Stewart [16]announced that the Computer Language Shootout recently got a quad core 64 bit machine, and outlined a plan and some initial results for porting the Haskell entries to take advantage of the available parallelism. The initial view on typed sprintf and sscanf. oleg [17]announced an implementation of typed sprintf and sscanf functions sharing the same formatting specifications, which also led to some interesting discussion and alternative proposals. Discussion Generalize groupBy in a useful way?. Bart Massey [18]proposed changing the implementation of groupBy to extend its usefulness for predicates which are not equivalence relations. Splitting SYB from the base package in GHC 6.10. JosÃ© Pedro MagalhÃ£es [19]initiated a discussion regarding splitting the SYB libraries out of the base package for GHC 6.10. The base library and GHC 6.10. Ian Lynagh initiated a [20]discussion on further splitting up the base package for GHC 6.10. Functional references. Tim Newsham began a [21]discussion on functional references and the possibility of merging the existing four or five implementations into something more standard. Types and Trees. Matt Morrow [22]wrote something about types and type representations, involving some commutative diagrams and some code. I haven't read it yet but it looks neat! Research language vs. professional language. Ryan Ingram started an interesting [23]discussion on the future direction(s) of the Haskell language. language proposal: ad-hoc overloading. Ryan Ingram [24]proposed adding ad-hoc name overloading to Haskell, prompting quite a bit of discussion. Top Level <-. Ashley Yakeley originally [25]asked whether there is any interest in implementing a top level "<-" to run monadic code. This set off a cascade of discussion the likes of which have been rarely seen on the Cafe. I would tell you what the discussion has been about but I must confess that I haven't read it. Blog noise [26]Haskell news from the [27]blogosphere. * Eric Kow (kowey): [28]darcs hacking sprint (25-26 October 2008). * Magnus Therning: [29]Confusion with HaskellDB. * Xmonad: [30]xmonad 0.8 released!. * Real-World Haskell: [31]Speaking in Silicon Valley next week. * Luke Palmer: [32]Composable Input for Fruit. * Luke Palmer: [33]Slicing open the belly of the IO monad in an alternate universe. * "FP Lunch": [34]Modular Monad Transformers. * Douglas M. Auclair (geophf): [35]Fuzzy unification parser in Haskell. * Bjoern Edstroem: [36]Speeding up Haskell with C -- a very short introduction. * Eric Kow (kowey): [37]darcs weekly news #2. * Roman Cheplyaka: [38]Physics and polyhedra. * Well-Typed.Com: [39]The new haskell.org community SPARC server is online. * Douglas M. Auclair (geophf): [40]A stream of primes as Comonad. * Eric Kidd: [41]Ubiquitous Hoogle. Eric shows how to integrate a [42]Hoogle search command into [43]Ubiquity. * Dan Piponi (sigfpe): [44]Untangling with Continued Fractions: Part 3. The vector space monad and rational tangles. Quotes of the Week * dons: maybe we should do a "recommend RWH to a java programmer" campaign :) * gwern: we fill fight them in the registers, we will fight them in the caches; we shall fight them in the core, and even in the backing store. And we shall never surrender! About the Haskell Weekly News New editions are posted to [45]the Haskell mailing list as well as to [46]the Haskell Sequence and [47]Planet Haskell. [48]RSS is also available, and headlines appear on [49]haskell.org. To help create new editions of this newsletter, please see the information on [50]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [51]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9973 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unicode-properti… 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unicode-names 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44181 6. http://jinjing.blog.easymic.com/ 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44164 8. http://wiki.darcs.net/index.html/Sprints 9. http://code.haskell.org/darcs/darcs-news 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44157 11. http://xmonad.org/ 12. http://haskell.org/haskellwiki/Xmonad/Notable_changes_since_0.7 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44069 14. http://article.gmane.org/gmane.comp.lang.haskell.general/16413 15. http://www.cs.nott.ac.uk/~gmh/icfp09.html 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43968 17. http://article.gmane.org/gmane.comp.lang.haskell.general/16405 18. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/10016 19. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9962 20. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9929 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44098 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44028 23. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/43933 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/43897 25. http://thread.gmane.org/gmane.comp.lang.haskell.general/16393 26. http://planet.haskell.org/ 27. http://haskell.org/haskellwiki/Blog_articles 28. http://koweycode.blogspot.com/2008/09/darcs-hacking-sprint-25-26-october-20… 29. http://therning.org/magnus/archives/375 30. http://xmonad.wordpress.com/2008/09/05/xmonad-08-released/ 31. http://www.realworldhaskell.org/blog/2008/09/05/speaking-in-silicon-valley-… 32. http://luqui.org/blog/archives/2008/08/09/composable-input-for-fruit/ 33. http://luqui.org/blog/archives/2008/08/14/slicing-open-the-belly-of-the-io-… 34. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=111 35. http://logicaltypes.blogspot.com/2008/09/fuzzy-unification-parser-in-haskel… 36. http://blog.bjrn.se/2008/09/speeding-up-haskell-with-c-very-short.html 37. http://koweycode.blogspot.com/2008/09/darcs-weekly-news-2.html 38. http://physics-dph.blogspot.com/2008/08/physics-and-polyhedra.html 39. http://blog.well-typed.com/2008/09/the-new-haskellorg-community-sparc-serve… 40. http://logicaltypes.blogspot.com/2008/09/stream-of-primes-as-comonad.html 41. http://www.randomhacks.net/articles/2008/09/01/ubiquitous-hoogle 42. http://www.haskell.org/hoogle/ 43. https://wiki.mozilla.org/Labs/Ubiquity 44. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_31.h… 45. http://www.haskell.org/mailman/listinfo/haskell 46. http://sequence.complete.org/ 47. http://planet.haskell.org/ 48. http://sequence.complete.org/node/feed 49. http://haskell.org/ 50. http://haskell.org/haskellwiki/HWN 51. http://code.haskell.org/~byorgey/code/hwn/

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What's the best way to give up?
by David F. Place 06 Sep '08

06 Sep '08

Hi, all. Say I have a function solve which is a constraint solver. It reconfigures its input to be a solution. If there is no solution, it returns the input. solve :: a -> Either a a solve input output = maybe (Left input) Right $ solve' input If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds. I can think of several ways of doing that. I could keep a tally of the number of variable assignments and give up when it reaches an impossibly huge number. I could change the type to a -> IO (Either a a ) and use getCPUTime. Is there a standard way to do this? Can you think of another way to do it? Cheers, David

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Monad and kinds
by Ramin 06 Sep '08

06 Sep '08

Hello, I'm new here, but in the short time I have known Haskell, I can already say it's my favorite computer language. Except for monads, and no matter how many tutorials I read, I find the only kind of monad I can write is the monad that I copied and pasted from the tutorial, i.e. I still don't get it even though I thought I understood the tutorial, and I'm stuck using monads others have already written. My project is this: I am writing a mini-database of sorts. I thought I would make a query a monad, so I could chain multiple queries together using the "do" notation, and run more complex queries on the list in one shot. The query itself is stateful because it contains information that changes as the database is traversed. The query may also make updates to the records. I got the program to work perfectly using purely functional techniques, but complex queries resulted in that stair-step looking code. So I thought this would be the perfect opportunity to try my hand at monads. The query monad I wrote looks something like this (much simplified): data Query state rec = Query !(state, rec) Where "state" is the type of state, so a query can contain any information relevant to the search and can be updated as the search progresses. Then, "rec" is the type of records in the database, which isn't determined inside the database module, so I can't just declare "rec" to be of any type I choose. But I just cannot write the instance declaration for my Query data type no matter what I try. If I write: instance Monad Query where return (initState, someRecord) = Query (initState, someRecord) {- code for (>>=) -} GHC gives an error, "Expected kind `* -> *', but `Scanlist_ctrl' has kind `* -> * -> *' ". If I try this: instance Monad (Query state) where return (initState, someRecord) = Query (initState, someRecord) {- code for (>>=) -} GHC give an error, "Occurs check: cannot construct the infinite type: a = (s, a) when trying to generalise the type inferred for `return' ". I get the sense I am trying to shove a square peg into a round hole. I was thinking of trying some other things, like implementing the monad in a higher-level module where I knew the type of the records I would be using, but I don't like being told where to implement things. I also thought of trying to re-write my query algorithm to somehow use "Control.Monad.State.Strict" instead of my own query type, but then I wouldn't get to write my own monad! Though the information given in this e-mail is limited, is there anyone who can clearly see there is something about monads that I just don't get and tell me what it is? Anyone who took the time to read this, I am very appreciative. Ramin Honary

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