- Haskell-Cafe - Haskell.org

"New FGL" naming survey
by Ivan Lazar Miljenovic 24 Jul '10

24 Jul '10

Whether or not the new FGL that Thomas Bereknyei and I are working on should keep the name was a semi-hot issue when we first mentioned the fact that we were working on a new version about a month ago. As such, I've created a survey here to try and find out what the Haskell community overall thinks we should call it: https://spreadsheets.google.com/viewform?formkey=dGpzMmFnUWY3Uktodk5wdHlLQk… More info can be found on the actual survey page. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] FGL problem: cannot acces data constructor NodeMap
by Ivan Lazar Miljenovic 24 Jul '10

24 Jul '10

Once again cc-ing -cafe; Immanuel, can you please do "reply to all" rather than just "reply" in GMail? :p Immanuel Normann <immanuel.normann(a)googlemail.com> writes: > 2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> > >> CC-ing haskell-cafe again: >> >> Immanuel Normann <immanuel.normann(a)googlemail.com> writes: >> >> > 2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> >> >> > Some of my lnodes in the graph have by construction unique labels and I >> want >> > to lookup the lnode from a graph given only the label of that lnode. >> >> Well, from my understanding NodeMap doesn't do what you want, since it >> assumes that _all_ of the node labels are unique > > > I wouldn't bother when NodeMap maps also non-unique labels to single nodes, > because I only lookup nodes with unique labels, e.g. assume my label are of > the form > data Label = Unique String | Multi Label I'm of the suspicion that using NodeMaps in this fashion will have problem when considering nodes with the same label (even if you don't explicitly use it for those nodes), as NodeMap seems to assume that node labels are unique. For example, mkNode assumes node labels are unique. >> Furthemore, the way of using it seems to be analogous of how one would use a >> State monad >> rather than for actual interactive use throughout. >> > > Could you elaborate this further, please. http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph… and http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph… If you just want a mapping from labels to nodes, you may be better off using your own custom Map (and maybe even defining a new graph type that has such a Map as part of it). The successor to FGL-5.x (whatever we'll end up calling it) will allow you to have custom node types rather than using Ints. However, this will not work for you in this situation. The node type is just an _index_ type, analogous to the actual hash value in a hash{map,array} (as opposed to the value which has the hashing function applied upon it). It's there to provide a unique identifier for a particular node. Arguably, an abstract type would be even better as an Int as it will prevent you from doing silly things like adding node indices together, but that's beside the point. If you wish to find a node with a specific label, try using something like gsel: http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] FGL problem: cannot acces data constructor NodeMap
by Ivan Lazar Miljenovic 24 Jul '10

24 Jul '10

CC-ing haskell-cafe again: Immanuel Normann <immanuel.normann(a)googlemail.com> writes: > 2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> > >> Immanuel Normann <immanuel.normann(a)googlemail.com> writes: >> > Afterall, my purpose is to get access to the map in a NodeMap and finally >> to >> > apply lookup to it. But I don't know how to access the map from a NodeMap >> > (the map selector isn't accessible either). >> >> You're not meant to; the point of NodeMap is to serve as a wrapper >> around the normal graph types, not for you to use directly (exactly the >> same as why the constructor for Map isn't exported). >> >> May I ask, however, why you want to use Data.Graph.Inductive.NodeMap? >> > > Some of my lnodes in the graph have by construction unique labels and I want > to lookup the lnode from a graph given only the label of that lnode. Well, from my understanding NodeMap doesn't do what you want, since it assumes that _all_ of the node labels are unique. Furthemore, the way of using it seems to be analogous of how one would use a State monad rather than for actual interactive use throughout. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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FGL problem: cannot acces data constructor NodeMap
by Immanuel Normann 24 Jul '10

24 Jul '10

Hi, I have a problem with the data constructor NodeMap Data.Graph.Inductive.NodeMap of the graph library fgl-5.4.2.3 (also fgl-5.4.2.2): I cannot access the data constructor NodeMap, as the ghci session shows: Prelude> :m Data.Graph.Inductive.NodeMap Prelude Data.Graph.Inductive.NodeMap> :t NodeMap <interactive>:1:0: Not in scope: data constructor `NodeMap' However, when I load the source directly it works: Prelude> :l Data/Graph/Inductive/NodeMap.hs [1 of 3] Compiling Data.Graph.Inductive.Graph ( Data/Graph/Inductive/Graph.hs, interpreted ) [2 of 3] Compiling Data.Graph.Inductive.Internal.FiniteMap ( Data/Graph/Inductive/Internal/FiniteMap.hs, interpreted ) [3 of 3] Compiling Data.Graph.Inductive.NodeMap ( Data/Graph/Inductive/NodeMap.hs, interpreted ) Ok, modules loaded: Data.Graph.Inductive.Internal.FiniteMap, Data.Graph.Inductive.Graph, Data.Graph.Inductive.NodeMap. *Data.Graph.Inductive.NodeMap> :t NodeMap NodeMap :: (Ord a) => FiniteMap a Node -> Int -> NodeMap a Why is that so? Afterall, my purpose is to get access to the map in a NodeMap and finally to apply lookup to it. But I don't know how to access the map from a NodeMap (the map selector isn't accessible either). Immanuel

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Weird behavior with arrow commands
by Ronald Guida 24 Jul '10

24 Jul '10

I am trying to figure out how to use GHC's arrow commands, and I found some extremely weird behavior. In GHC's manual, there is a description of arrow commands, which I don't really understand. http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html… (Primitive Constructs) I have two questions: 1. What are arrow commands supposed to do? 2. What is this code supposed to do? -- start of code -- {-# LANGUAGE Arrows #-} module Main where import Control.Arrow foo :: (b -> String) -> ((((b, Int), Float), Double) -> String) -> (b -> String) foo f g b = f b ++ " " ++ g (((b, 8), 1.0), 6.0) bar :: (t -> String) -> ((Double, Int) -> String) -> t -> String bar f g = proc x -> do (f -< x) `foo` \n m -> g -< (n) main = do putStrLn $ foo show show 17 putStrLn $ bar show show 17 putStrLn $ foo show show 42 putStrLn $ bar show show 42 -- end of code -- Output from GHCi: 17 (((17,8),1.0),6.0) 17 (6.730326920298707e-306,0) 42 (((42,8),1.0),6.0) 42 (6.730326920298707e-306,0) Output after compiling with GHC: 17 (((17,8),1.0),6.0) 17 (5.858736684536801e-270,0) 42 (((42,8),1.0),6.0) 42 (5.858736684536801e-270,0) GHC Version: The Glorious Glasgow Haskell Compilation System, version 6.12.3 Thank you -- Ron

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ANNOUNCE: Haskell Platform 2010.2.0.0
by Don Stewart 24 Jul '10

24 Jul '10

We're pleased to announce the fifth release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2010.2.0.0: http://hackage.haskell.org.nyud.net/platform/ (Caching server). The specification, along with installers (including Windows, Apple and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team

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[Haskell-cafe] a patch for base.Foreign.C.String, the current locale
by Roman Beslik 24 Jul '10

24 Jul '10

Hi. I have composed a patch that implements Foreign.C.String according to Foreign Function Interface Addendum, i.e. C strings are treated as they are encoded with the current locale. Search "Locale.hs" in the patch for further directions. Ask if you do not know how to build a patched version of the "base" library. http://www.beroal.in.ua/prg/haskell/locale.patch -- Best regards, Roman Beslik.

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Heavy lift-ing
by michael rice 24 Jul '10

24 Jul '10

Hi, I don't understand what's taking place here. >From Hoogle: ================= liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Promote a function to a monad, scanning the monadic arguments from left to right. For example, liftM2 (+) [0,1] [0,2] = [0,2,1,3] liftM2 (+) (Just 1) Nothing = Nothing ================= What does it mean to "promote a function to a monad?" It would seem that the monad values must understand the function that's being promoted, like Ints understand (+). Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) Just 2 But how does one add [0,1] and [0,2] to get [0,2,1,3]? Michael

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Re: [Haskell-cafe] Heavy lift-ing
by michael rice 24 Jul '10

24 Jul '10

Thanks all, Wild, at least up to the "optional" part, which I haven't dug into yet. So the (+) for the Maybe monad and the (+) for the List monad are one in the same, the magic springs from the monads. Why is it called "lift"-ing? Michael --- On Fri, 7/23/10, Jürgen Doser <jurgen.doser(a)gmail.com> wrote: From: Jürgen Doser <jurgen.doser(a)gmail.com> Subject: Re: [Haskell-cafe] Heavy lift-ing To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Friday, July 23, 2010, 4:50 PM El vie, 23-07-2010 a las 15:05 -0400, Nick Bowler escribió: > On 11:43 Fri 23 Jul , michael rice wrote: > [...] > > But how does one add [0,1] and [0,2] to get [0,2,1,3]? > > liftM2 (+) [0,1] [0,2] gives the list > > [0+0, 0+2, 1+0, 1+2] which one could have found out by asking ghci: Prelude Control.Monad> let f a b = show a ++ " + " ++ show b Prelude Control.Monad> liftM2 f [0,1] [0,2] ["0 + 0","0 + 2","1 + 0","1 + 2"] or simpler: Prelude Control.Monad> liftM2 (,) [0,1] [2,3] [(0,2),(0,3),(1,2),(1,3)] i.e., the in the list monad, you pair each element of one list with each element of the other(s). > (recall that (>>=) in the list monad is concatMap). (>>=) = flip concatMap, to be precise. Or, concatMap = (=<<) Now let's have some fun with equational reasoning to see what liftM2 does exactly: (Only read this if you really want to!) liftM2 f a b = { definition of liftM2 } do {x <- a; y <- b; return (f x y)} = { simplified translation of do-notation } a >>= \x -> (b >>= \y -> return (f x y)) = { change (>>=) to (=<<) and flip arguments } (\x -> ((\y -> return (f x y)) =<< b)) =<< a = { specialized to the list monad } (\x -> ((\y -> [f x y])) `concatMap` b)) `concatMap` a = { change concatMap to prefix application } concatMap (\x -> concatMap (\y -> [x+y]) b) a and indeed: Prelude> concatMap (\x -> concatMap (\y -> [x+y]) [0,2]) [0,1] [0,2,1,3] with some effort, I think one can understand what happens here. It should also be clear how this is generalized to liftM3, liftM4, etc. Oh, btw, what about liftM1? Obviously, this should be the following: liftM1 f a = { definition } do { x <- a ; return f a } = { same changes as above } concatMap (\x -> [f x]) a = { definition of concatMap } concat (map (\x -> [f x]) a = { concating singletons can be simplified } map (\x -> f x) a = { eta-reduction } map f a i.e., liftM1 = map, which is indeed just fmap for lists, as already pointed out. You can use this to simplify the last line of the concatMap derivation above: concatMap (\x -> concatMap (\y -> [x+y]) b) a = { see above } concatMap (\x -> map (\y -> x+y) b) a = { use operator section } concatMap (\x -> map (x+) b) a which is about as clear as possible a definition for liftM2 (+) Jürgen

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HaTeX build failure
by Daniel Díaz 23 Jul '10

23 Jul '10

Hi, I uploaded a package, named HaTeX, to Hackage, but it gets a build failure: cabal: Couldn't read cabal file "./HaTeX/1.0.0/HaTeX.cabal It makes reference to 1.0.0 version, even when building version 1.0.1. I get the same error when I use cabal install. However, using configure>build>register I don't have any problem. I would like this package works well, but I don't know how to solve this problem. In fact, I don't know what's the problem. Here a link to the HaTeX package: http://hackage.haskell.org/package/HaTeX Any idea?

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cabal, Setup.lhs example
by Tom Hawkins 23 Jul '10

23 Jul '10

I have a script I'm using to generate some Haskell code for a library. How do I specify this flow in the cabal setup file? Would someone point me to a relevant library I can reference as an example? -Tom

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src/Text/XML/HaXml/Lex.hs:(156, 0)-(160, 22): Non-exhaustive patterns in function white
by Alexander Kotelnikov 23 Jul '10

23 Jul '10

Hello. I decided to run a couple of test XML-RPC applications using haxr. Everything worked fine while responces from my (actually, other people's) server were something relatively small (a number, a line), but with larger responces (starting around couple of kilobytes) my applications began to fail with the error I put into subject. Sounds like XML parsing problem. Though server gives a proper responce (I used tcpdump to see what is returned, looks like a valid XML with methodResponce). Any idea what is wrong and how this can be fixed? Thanks, Alexander

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Page rank and GHC docs directory organization
by Ryan Newton 23 Jul '10

23 Jul '10

GHC docs seem to have the problem that newer versions only gradually overtake older ones in page rank, resulting in the effect that if one uses Google to find library documentation, they may accidentally look at an old version. For example, if I google "Data.Data Haskell" the first link brings me to: http://www.haskell.org/ghc/docs/6.10.2/html/libraries/base/Data-Data.html Oops, version 6.10.2! I'm no web expert, but I think the problem is that the "latest" directory isn't used consistently by others and/or the fact that latest/ redirects to a concrete version number. Thus if you go to: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Data.html It redirects immediately to 6.12.2: http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base/Data-Data.html So is the 6.12.2 target accruing pagerank rather than the latest one? Even if someone links the /latest/ URL? If that's the problem, would it fix things just to make latest/ a full directory structure in its own right (a clone rather than redirect)? Cheers, -Ryan

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Hackage survey
by Sylvain Le Gall 23 Jul '10

23 Jul '10

Hello dear haskellers, I am working on OASIS-DB, a tool similar to Hackage and would like to have more information on how Haskellers use Hackage. I have setup a small poll (10 questions only) with the help of John Goerzen and Don Stewart. I will be very thankful that you answer this poll: http://polldaddy.com/s/2C49D15023CB88C6 I will share the data with the Haskell community, so that we will be able to make progress on Hackage and OASIS-DB. Cheers Sylvain Le Gall

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bounded ranges
by Chad Scherrer 23 Jul '10

23 Jul '10

Hello cafe, I'm trying to do some things with bounded indices so I can carry around arrays (well, Vectors, really) without needing to refer to the bounds. For example, if I know my indices are Bool values, I can do > rangeSize (minBound, maxBound :: Bool) 2 I'd like to be able to do this in general, but... > :t rangeSize (minBound, maxBound) <interactive>:1:11: Ambiguous type variable `a' in the constraints: `Bounded a' arising from a use of `minBound' at <interactive>:1:11-18 `Ix a' arising from a use of `rangeSize' at <interactive>:1:0-29 Probable fix: add a type signature that fixes these type variable(s) I thought it might help to put it into a module and do a better job with the type, like this: bdRangeSize :: (Ix i, Bounded i) => i -> Int bdRangeSize _ = rangeSize (minBound, maxBound :: i) but I still have problems: MyArray.hs:22:36: Could not deduce (Bounded i1) from the context () arising from a use of `maxBound' at MyArray.hs:22:36-43 Possible fix: add (Bounded i1) to the context of an expression type signature In the expression: maxBound :: i In the first argument of `rangeSize', namely `(minBound, maxBound :: i)' In the expression: rangeSize (minBound, maxBound :: i) I thought maybe it's an existential types problem or something, but I don't understand why it would be coming up here. Any thoughts? Oh yes, and I'm using GHC version 6.12.1. Thanks, Chad

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gd on Win32
by Jason Felice 23 Jul '10

23 Jul '10

Howdy, The listed maintainer for gd is no longer maintaining it. I'm wondering if anyone else is, if there is a version control URL, and how I can send patches. I need to get it working for Win32. Thanks

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Regular Expression to Determinate Finite Automata translator
by Aaron Gray 23 Jul '10

23 Jul '10

Hi, I am a Haskell newbie. I have coded a Regular Expression to Determinate Finite Automata translator. Algorithm from the Dragon Book. Would someone eyeball the code and give me suggestions please. I have not done anything on character classes yet though. And the parsing is a bit of a hack. What I am not sure about is having to have multiple versions of similar datatype, each with variations in order to enumerate and generate followPos set. Is there a better way of implementing this ? Many thanks in advance, Aaron

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music-related problem
by Michael Mossey 22 Jul '10

22 Jul '10

Wondering if I could get some suggestions for coding this problem. A musical document (or "score") consists primarily of a list of measures. A measure consists primarily of lists of "items". We'll consider only one kind of item: a note. Items have a location within the measure. A note's location indicates both where it goes on the page (i.e. a visual representation of the score) and what moment in time it begins sounding (i.e. rendering the score in sound). My concern here is sound. data Doc = [Measure] data Loc = ... (represents a location within the musical document including measure number) data Measure = Measure [(Loc,Item)] -- In the Meausre, we can assume (Loc,Item) are in -- ascending order Notes also have an end, when indicates when in time they stop sounding. See the 'end' field below. Also note the 'soundedEnd' 'tieStart' and 'tieStop' fields which I will explain. data Item = Note { pitch :: Pitch , end :: Loc , soundedEnd :: Maybe Loc , tieNext :: Bool , tiePrior :: Bool } There is a concept of "tied notes". When two notes are tied together, their durations are summed and they are sounded continuously as if one note. Ties have several uses, but one important one is to make a sound that begins in one measure and ends in a later measure, by tying notes across measures. The 'tieNext' field indicates if a note is tied to the following note (that is, the next note of the same pitch). 'tiePrior' indicates if tied to immediately prior note of same pitch. A chain of notes can be tied. Notes in the middle with have both tieNext and tiePrior set. In the event a note is within a chain of ties, its 'soundedEnd' field needs to be computed as Just e where e is the end of the last note in the chain. This information is useful when rendering the document as sound. My problem is: - given a Doc in which all fields have been set EXCEPT soundedEnd (all soundedEnd's are given a default value of Nothing) - update those notes in the Doc which need to have soundedEnd set. This involves chasing down the chain of ties. I can solve a simpler problem which is -- Given a note with tieNext set, and a list of notes, find -- the end Loc of the last note in the chain. Only notes -- with the same pitch as 'firstNote' are considered when looking -- for the chain of notes. computeSoundedEnd :: Item -> [Item] -> Loc computeSoundedEnd firstNote notes = compSndEnd (pitch firstNote) notes compSndEnd :: Pitch -> [Item] -> Loc compSndEnd _ [] = error "tie chain doesn't come to completion" compSndEnd p (n:ns) = if pitch n == p then if tieNext n then if tiePrior n then compSndEnd p ns else error "illegal tie chain" else if tiePrior n then end n else error "illegal tie chain" else compSndEnd p ns The thing that is hard for me to understand is how, in a functional paradigm, to update the entire Doc by chasing down every tie and making all necessary updates. Thanks, Mike

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Re: [Haskell] ANNOUNCE: vorbiscomment 0.0.1
by Antoine Latter 22 Jul '10

22 Jul '10

On Wed, Jul 21, 2010 at 10:16 AM, Chris Wagner <christopher.t.wagner(a)gmail.com> wrote: > I'm happy to announce that I have packaged up my first Haskell library, > vorbiscomment, which allows for reading of "Vorbis comments" from Ogg > Vorbis files. The code is rugged and surely buggy, but any feedback, > patches, etc. are more than welcome. > > I do have a specific question in releasing this, and you can respond to > my personal email address if this mailing list is not appropriate for > discussion, but I'm wondering where my library's functionality should > live within the module hierarchy. At present, I have simply placed it > in a top-level "Vorbis" module, but this is surely not the proper place > for it. > Hi Chris, I have 5 packages which populate under the Codec.* hierarchy, so I would try to stake something out underneath Codec.Audio.*. Are these comments unique to Vorbis, or are they for anything contained in Ogg? Antoine (also, moving discussion to -cafe)

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Problem with reloading modules in GHC API
by Hongmin Fan 22 Jul '10

22 Jul '10

Hi, I'm using GHC API to dynamically load some module, and evaluate it; and later change the content of the module, and re-evaluate it. But I found unless I delete the object file created by previous compilation, the module seems not reloaded. I have set ghcLink = LinkInMemory as an older post suggested To illustrate what I'm saying, here is a piece of code (sorry for any naivety in the code, new to Haskell too) import System.IO (IOMode(..),hClose,hPutStr,openFile) import Directory (removeFile) import GHC import GHC.Paths import DynFlags import Unsafe.Coerce src_file = "Target.hs" obj_file = "Target.o" main = do writeTarget "arg" func0 <- compileTarget putStrLn $ show $ func0 2 writeTarget "arg*2" func1 <- compileTarget putStrLn $ show $ func1 2 writeTarget input = do -- removeFile obj_file `catch` (const $ return ()) -- uncomment this line to have correct results h <- openFile src_file WriteMode hPutStr h "module Target (Target.target) where\n" hPutStr h "target::Double -> Double\n" hPutStr h "target arg = \n " hPutStr h input hClose h compileTarget = defaultErrorHandler defaultDynFlags $ do func <- runGhc (Just libdir) $ do -- setup dynflags dflags <- getSessionDynFlags setSessionDynFlags dflags { ghcLink = LinkInMemory } -- load target module target <- guessTarget src_file Nothing setTargets [target] r <- load LoadAllTargets case r of Failed -> error "Compilation failed" Succeeded -> do m <- findModule (mkModuleName "Target") Nothing -- set context and compile setContext [] [m] value <- compileExpr ("Target.target") do let value' = (unsafeCoerce value) :: Double -> Double return value' return func The code basically write to a Haskell source file twice with different content, and hoping to get different results, but unless I uncomment the line with removeFile, the output of 2 runs are the same; using 'touch' to touch the source file being written between 2 runs also gives the correct results. So maybe caused by some caching mechanism? I'm using GHC 6.12.1 in Ubuntu 10.04. I have this workaround of deleting the obj file, but I'm wondering the "correct" way of doing it. Did some search on GHC API, but never got something relevant. Thanks, Hongmin

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Re: [Haskell] ANNOUNCE: vorbiscomment 0.0.1
by Henk-Jan van Tuyl 22 Jul '10

22 Jul '10

Redirecting to Haskell-café On Wed, 21 Jul 2010 19:16:16 +0200, Chris Wagner <christopher.t.wagner(a)gmail.com> wrote: > I'm happy to announce that I have packaged up my first Haskell library, > vorbiscomment, which allows for reading of "Vorbis comments" from Ogg > Vorbis files. The code is rugged and surely buggy, but any feedback, > patches, etc. are more than welcome. > > I do have a specific question in releasing this, and you can respond to > my personal email address if this mailing list is not appropriate for > discussion, but I'm wondering where my library's functionality should > live within the module hierarchy. The "Hierarchical module names" page [1] on the HaskellWiki gives some guidelines; I suppose the name Sound.Vorbis.Comment would be a good one. Regards, Henk-Jan van Tuyl [1] http://www.haskell.org/haskellwiki/Hierarchical_module_names -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Data types a la carte
by Victor Nazarov 22 Jul '10

22 Jul '10

I've been experimenting with type level programming and open recursion. My aim this far is to implement very modular interpreter for lambda calculi and combinatory logic powerful enough to demonstrate steps of some term's reduction to normal form. I'd like to be able to setup the set of combinators and reduction rules and to express the set in type signature. I'd like to achieve maximum modularity of the code by all possible means. Below is the beginning of this effort. Show instances are defined completely modular. Show instance for Lambda and show instance for Combinator reuse the very same code of Show instance of ApplicativeF. However, there are problems with this code, and I'd like to receive some suggestions to cope with them. 1) UndecidableInstances are required for this code to compile. Is it possible to avoid this language extension? What is the reason for it to be required? 2) app, var, abs functions. app function is defined differently for every example. Is it possible to perform some automatic lifting so that app will work for any combination of ApplicativeF? Can I use Functor class or should I define my own type-class for this purpose? Is it possible at all? 3) The code below uses technique similar to Data Type a la Carte. Is it possible to achieve the same modularity using Finally Tagless technique? > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE UndecidableInstances #-} Some type level combinators first > newtype (:.) f g a = O (unO :: f (g a)) > newtype (:<*>) f g a = S {unS :: f a (g a)} > newtype Flip f x y = Flip {unFlip :: f y x} > newtype Mu f = Mu {unMu :: f (Mu f)} Their show instances doesn't affect representation of underlying type > instance Show (f (Mu f)) => Show (Mu f) > where showsPrec d (Mu f) = showsPrec d f > instance Show (f (g a)) => Show ((f :. g) a) > where showsPrec d (O f) = showsPrec d f > instance Show (f a (g a)) => Show ((f :<*> g) a) > where showsPrec d (S f) = showsPrec d f > instance Show (f y x) => Show (Flip f x y) > where showsPrec d (Flip f) = showsPrec d f Applicative is some structure that permits application of some objects. We use open recursion here. ApplicativeF o is a functor that represent this structure. And Mu (Flip ApplicativeF o) is a fix-point of this functor. > data ApplicativeF a o = App a a | Obj o > type Applicative o = Mu (Flip ApplicativeF o) Pareses are left associative in applicative structure > instance (Show o, Show a) => Show (ApplicativeF o a) > where showsPrec d (Obj o) = showsPrec d o > showsPrec d (App a b) = > showParen (d > 4) $ showsPrec 4 a . (' ':) . showsPrec 5 b Here is an example of applicative structure of strings > test1 :: Applicative String > test1 = obj "x" `app` obj "y" `app` obj "z" `app` (obj "w" `app` obj "v") > where obj = Mu . Obj > app a b = Mu $ App a b Lambda calculi is a calculi of some objects with applicative structure. This object may represent a variable or an abstraction of variable. LambdaObjF encodes this fact. We use all combinators defined above to define objects of lambda calculi. > data LambdaObjF name a = Var Name | Abs Name a > type Lambda name = Mu (ApplicativeF :<*> LambdaObjF name) > instance (Show a) => Show (LambdaObjF String a) > where showsPrec d (Var s) = showParen (d > 5) (showString s) > showsPrec d (Abs s a) = > showParen (d > 3) $ > ('\\':) . showString s . showString ". " . showsPrec 3 a Lets try to build an object of lambda calculi > type Name = String > test2 :: Lambda Name > -- test2 = (\x. \y. x) a b > test2 = (abs "x" (abs "y" (var "x")) `app` var "a") `app` var "b" > where var = Mu . S . Flip . Obj . Var > abs x b = Mu . S . Flip . Obj $ Abs x b > app a b = Mu . S . Flip $ App a b Combinatory logic defines some basic combinators > data CombinatorObjF a = CK | CS > type Cominator = Mu (ApplicativeF :<*> CombinatorObjF) > instance Show (CombinatorObjF a) > where showsPrec d CK = showString "K" > showsPrec d CS = showString "S" > test3 = s `app` k `app` k `app` (k `app` s) > where s = Mu . S . Flip . Obj $ CS > k = Mu . S . Flip . Obj $ CK > app a b = Mu . S . Flip $ App a b -- Victor Nazarov

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[Ann] New version of parser combinator library uu-parsinglib-2.4.0
by S. Doaitse Swierstra 22 Jul '10

22 Jul '10

I have uploaded a new version of the package uu-parsinglib, te collection of online, error-correcting parser combinators, with both an applicative and a monadic interface. * The new version behaves better when more than three corrective steps have to be taken at a specific position. In order ro do so we have added a form of abstract interpretation which computes the minimal length of each alternative, so the shortest one can be chosen (and not one of the recursive alternatives!). * Furthermore we compute for each parser whether it can recognise the empty string. This is used to provide error messages when such parsers are used in list-like constructs, on of the most commonly made mistakes by innocent parser writers. * a combinator <?> was added which can be used to label productions, and which label is used to report what was expected * some further overloadings of pSym have been added which function at a lower level, thus leading to much faster parsers, especially when using the combinator to construct scanner like parsers: e.g. spaces = pMunch (==' ') makes sure that all spaces are skipped in a single go, instead of dealing with each recognised ' ' by itself. I hope you like the extensions, and would be happy to hear from you, both if you use them successfully, or if you have any reason for not using them. Doaitse Swierstra

1 0

The where-clause and guards
by Eitan Goldshtrom 21 Jul '10

21 Jul '10

I'm trying to fit a where clause to some guards I'm using. I have the following f a b | c > 1 = ... | c < 1 = ... | otherwise = ... where c = a+b yet I'm getting a parsing error. Is this not the correct way to combine "where" with "guards"? -Eitan

2 5

List manager and duplicate copies of messsages
by José Romildo Malaquias 21 Jul '10

21 Jul '10

Hello. I have noticed that I do not receive duplicate copies of messages from haskell-cafe, although "Avoid duplicate copies of messages?" is set to "No" in the mailing list membership configuration. I want the copies because I archive all the mailing list messages with procmail based on the "X-BeenThere" header line. Has anybody else seen this behaviour, or am I missing anything? Is there any problems with the mailing list manager? Regards, Romildo -- Computer Science Department Universidade Federal de Ouro Preto, Brasil

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Re: [Haskell-cafe] Test. Ignore.
by "José Romildo Malaquias" 21 Jul '10

21 Jul '10

> ----- Original Message ----- > From: José Romildo Malaquias <j.romildo(a)gmail.com> > To: José Romildo Malaquias <romildo(a)operamail.com> > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Test. Ignore. > Date: Wed, 21 Jul 2010 13:33:34 -0300 > > > On Wed, Jul 21, 2010 at 01:25:19PM -0300, "José Romildo Malaquias" wrote: > > > ----- Original Message ----- > > > From: "José Romildo Malaquias" <romildo(a)operamail.com> > > > To: "José Romildo Malaquias" <j.romildo(a)gmail.com> > > > Cc: haskell-cafe(a)haskell.org > > > Subject: Re: [Haskell-cafe] Test. Ignore. > > > Date: Wed, 21 Jul 2010 13:09:16 -0300 > > > > > > ----- Original Message ----- > > > > From: José Romildo Malaquias <j.romildo(a)gmail.com> > > > > To: José Romildo Malaquias <romildo(a)operamail.com> > > > > Cc: haskell-cafe(a)haskell.org > > > > Subject: Re: [Haskell-cafe] Test. Ignore. > > > > Date: Wed, 21 Jul 2010 13:01:03 -0300 > > > > > > > > > > > > On Wed, Jul 21, 2010 at 12:53:53PM -0300, "José Romildo Malaquias" wrote: > > > > > Please, ignore this message. > > > > > > > > Replying to the list and to the sender, to see if he gets both messages > > > > using a non gmail account. > > > > I confirm that I received only one copy of the above message, > > that > was sent directly to my email address at operamail.com. I > > did not > receive any copy from the mailing list manager. > > > > Romildo (at opera.com) > > > > That was expected, as by default the "Avoid duplicate copies of > > messages?" option is set to "Yes" in the mailing list membership > > configuration. Now I have just set it to "No", and I expect to receive > > both copies next time someone from the mailing list "replies to all" > > to one of my messages. > > > > Romildo at operamail.com > > Replying to all. > > Romildo at gmail.com Well, romildo at operamail.com received both copies of the message. So the list manager worked as expected. Romildo at operamail.com -- _______________________________________________ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com

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Re: [Haskell-cafe] Test. Ignore.
by "José Romildo Malaquias" 21 Jul '10

21 Jul '10

> ----- Original Message ----- > From: "José Romildo Malaquias" <romildo(a)operamail.com> > To: "José Romildo Malaquias" <j.romildo(a)gmail.com> > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Test. Ignore. > Date: Wed, 21 Jul 2010 13:09:16 -0300 > > > > ----- Original Message ----- > > From: José Romildo Malaquias <j.romildo(a)gmail.com> > > To: José Romildo Malaquias <romildo(a)operamail.com> > > Cc: haskell-cafe(a)haskell.org > > Subject: Re: [Haskell-cafe] Test. Ignore. > > Date: Wed, 21 Jul 2010 13:01:03 -0300 > > > > > > On Wed, Jul 21, 2010 at 12:53:53PM -0300, "José Romildo Malaquias" wrote: > > > Please, ignore this message. > > > > Replying to the list and to the sender, to see if he gets both messages > > using a non gmail account. > > I confirm that I received only one copy of the above message, that > was sent directly to my email address at operamail.com. I did not > receive any copy from the mailing list manager. > > Romildo (at opera.com) That was expected, as by default the "Avoid duplicate copies of messages?" option is set to "Yes" in the mailing list membership configuration. Now I have just set it to "No", and I expect to receive both copies next time someone from the mailing list "replies to all" to one of my messages. Romildo at operamail.com -- _______________________________________________ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com

3 2

Re: [Haskell-cafe] Test. Ignore.
by "José Romildo Malaquias" 21 Jul '10

21 Jul '10

> ----- Original Message ----- > From: José Romildo Malaquias <j.romildo(a)gmail.com> > To: José Romildo Malaquias <romildo(a)operamail.com> > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Test. Ignore. > Date: Wed, 21 Jul 2010 13:01:03 -0300 > > > On Wed, Jul 21, 2010 at 12:53:53PM -0300, "José Romildo Malaquias" wrote: > > Please, ignore this message. > > Replying to the list and to the sender, to see if he gets both messages > using a non gmail account. I confirm that I received only one copy of the above message, that was sent directly to my email address at operamail.com. I did not receive any copy from the mailing list manager. Romildo (at opera.com) -- _______________________________________________ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com

1 0

Test. Ignore.
by "José Romildo Malaquias" 21 Jul '10

21 Jul '10

Please, ignore this message. Romildo -- _______________________________________________ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com

2 1

Finding zipper for custom tree
by Sergey Mironov 21 Jul '10

21 Jul '10

Hello list! I am trying to understand zipper concept using papers like [1] and [2]. Though main idea looks clear, I still have a problem in applying it for custom data types. Please help me with deriving Zipper-type from > data DTree a = P | D [(a, DTree)] Looking in [1] ('Zippers via Differentiation' chapter) I tried to do the following: 1. Find a DTreeF type which is a pattern functor ([2], chapter 2.1) of my DTree 2. Write DTreeF in 'algebraic' form (using '+' and '*') 3. Find DTreeF' - "derivative" of DTreeF 4. Define my zipper type using list of DTreeF' Step 1 likely ends with > data DTreeF a x = P | D [(a,x)] [2] says that using this pattern functor one could build a fixed-point version of DTree: > data Fix f = In {out :: (f (Fix f))} > data DTreeFP = Fix DTreeF but seems that I have nothing to do with it right now. Step 2 is my main question: In [1] authors did it for binary tree: data Tree a = Leaf a | Bin (Tree a) (Tree a) data TreeF a x = Leaf a | Bin x x and thus TreeF = a + x * x TreeF' = x + x My DTree has inner list of tuples. How should I rewrite it in terms of '+' and '*' ? Any comments are welcome! [1] - http://en.wikibooks.org/wiki/Haskell/Zippers [2] - http://people.cs.uu.nl/andres/Rec/MutualRec.pdf -- Thanks, Sergey

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Merge hsql and HDBC -- there can only be one!
by Christopher Done 21 Jul '10

21 Jul '10

One thing that would be nice is a unification of the general database libraries hsql and HDBC. What is the difference between them? Why are there two, and why are there sets of drivers for both (duplication of effort?)? I've used both in the past but I can't discern a real big difference (I used the hsql-sqlite library and the HDBC-postgresql library, whichever worked...). It seems the best thing to do is either actively merge them together and encourage the community to move from one to the other -- judging from what I've read HDBC is more up to date and newer than hsql -- or have some documentation with damn good reasons to choose one or the other, because currently this is a needless source of confusion and possible duplication of effort for Haskell's database libraries. I wasn't going to post until I'd actually researched the difference myself properly but I didn't get chance to have a look over the weekend, but I thought I'd pose the question. Do people actually care about this?

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Re: Tiger compiler in Haskell: annotating abstract syntax tree
by oleg＠okmij.org 21 Jul '10

21 Jul '10

Jose' Romildo Malaquias wrote: > I am writing here to ask suggestions on how to annotate an ast with > types (or any other information that would be relevant in a compiler > phase) in Haskell. There is also a general way of annotating AST post factum, described in http://okmij.org/ftp/Algorithms.html#tree-annot The method lets one attach arbitrarily many annotations to an already built AST, *without* the need to change the definition of the datatype. One does not even have to anticipate annotations! The method would work with your AST > data Exp > = IntExp Integer > | VarExp Symbol > | AssignExp Symbol Exp as _it is_, without any modifications -- neither to the data type definition, nor to the tree. The method was demonstrated when writing a compiler of sorts: annotating an AST with an inferred type for each node. If the type inference fails, we can still print out the inferred types for the good subexpressions.

2 3

lambda terms in which the variables are positions in lambda terms
by Greg Meredith 21 Jul '10

21 Jul '10

Dear Haskellians, The following code may provide some amusement. i offer a free copy of Barwise's Vicious Circles to the first person to derive deBruijn indices from this presentation. Best wishes, --greg -- -*- mode: Haskell;-*- -- Filename: Term.hs -- Authors: lgm -- Creation: Tue Jul 20 16:37:37 2010 -- Copyright: Not supplied -- Description: -- ------------------------------------------------------------------------ module Generators( Term , MuTerm , DoTerm , ReflectiveTerm , TermLocation , ClosedTermLocation , ClosedReflectiveTerm , unfoldLocation , unfoldTerm , makeMention , makeAbstraction , makeApplication , generateTerms ) where -- M[V,A] = 1 + V + VxA + AxA data Term v a = Divergence | Mention v | Abstraction v a | Application a a deriving (Eq, Show) -- \mu M data MuTerm v = MuTerm (Term v (MuTerm v)) deriving (Eq, Show) -- \partial \mu M data DoTerm v = Hole | DoAbstraction v (DoTerm v) | DoLeftApplication (DoTerm v) (MuTerm v) | DoRightApplication (MuTerm v) (DoTerm v) deriving (Eq, Show) -- first trampoline data ReflectiveTerm v = ReflectiveTerm (MuTerm (DoTerm v, ReflectiveTerm v)) deriving (Eq, Show) -- second trampoline data TermLocation v = TermLocation ((DoTerm v), (ReflectiveTerm v)) deriving (Eq, Show) -- first bounce data ClosedTermLocation = ClosedTermLocation (TermLocation ClosedTermLocation) deriving (Eq, Show) -- second bounce data ClosedReflectiveTerm = ClosedReflectiveTerm (ReflectiveTerm ClosedTermLocation) deriving (Eq, Show) -- the isomorphisms implied by the trampolines unfoldLocation :: ClosedTermLocation -> ((DoTerm ClosedTermLocation), (ReflectiveTerm ClosedTermLocation)) unfoldLocation (ClosedTermLocation (TermLocation (k, t))) = (k, t) unfoldTerm :: ClosedReflectiveTerm -> (MuTerm ((DoTerm ClosedTermLocation), (ReflectiveTerm ClosedTermLocation))) unfoldTerm (ClosedReflectiveTerm (ReflectiveTerm muT)) = muT -- variable mention ctor makeMention :: ClosedTermLocation -> ClosedReflectiveTerm makeMention ctl = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Mention (unfoldLocation ctl))))) -- abstraction ctor makeAbstraction :: ClosedTermLocation -> ClosedReflectiveTerm -> ClosedReflectiveTerm makeAbstraction ctl crt = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Abstraction (unfoldLocation ctl) (unfoldTerm crt))))) -- application ctor makeApplication :: ClosedReflectiveTerm -> ClosedReflectiveTerm -> ClosedReflectiveTerm makeApplication crtApplicad crtApplicand = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Application (unfoldTerm crtApplicad) (unfoldTerm crtApplicand))))) -- a simple test generateTerms :: () -> [ClosedReflectiveTerm] generateTerms () = -- x [(makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence))))), -- \x -> x (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))), -- ((\x -> x)(\x -> x)) (makeApplication (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))) (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))))] -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

1 0

[SOLVED] Default Curl call fails with CurlSSLConnectError
by aditya siram 21 Jul '10

21 Jul '10

Hi all, Just wanted to document this in case others stumble across it. On Ubuntu Lucid "curlGetString "https://<some url>" []" can sometimes fail with (CurlSSLConnectError,""). Setting SSL to version 3 manually fixes the problem: "curlGetString "https://<some-url>" [CurlSSLVersion 3]". This might also be of interest to the library maintainer, and Don Stewart whose download-curl package, because it is a wrapper around the curl package, exhibits the same behavior. thanks ... -deech

1 0

HTTP Redirect With HTTPS Rejected
by aditya siram 21 Jul '10

21 Jul '10

The problem is that any page that that responds with 301.302,303 or 307 and redirects to an "https" is rejected by the library. The code that does the rejection is in "Network.Browser" on line 865. It only accepts "http:". Why was this design decision made? It makes it impossible for me to use this library for my project. -deech On Tue, Jul 20, 2010 at 3:30 PM, aditya siram <aditya.siram(a)gmail.com> wrote: > Hi all, > I am trying to simply download a web page using the HTTP library. It > works fine with "http://www.google.com" but a redirecting page like > "http://gmail.com" creates an error. Code is at > http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27864#a27864. > > Thanks , > -deech >

2 2

Regex multiple matches
by Omari Norman 21 Jul '10

21 Jul '10

Hi, How do I use Text.Regex.PCRE to get information on multiple matches? For instance, in ghci I get this error message: Prelude Text.Regex.PCRE> "foo" =~ "o" :: [(Int,Int)] <interactive>:1:0: No instance for (RegexContext Regex [Char] [(Int, Int)]) arising from a use of `=~' at <interactive>:1:0-11 Possible fix: add an instance declaration for (RegexContext Regex [Char] [(Int, Int)]) In the expression: "foo" =~ "o" :: [(Int, Int)] In the definition of `it': it = "foo" =~ "o" :: [(Int, Int)] "Real World Haskell"'s text suggests this should work, but the ghci output is littered with errors. http://book.realworldhaskell.org/read/efficient-file-processing-regular-exp… Thanks for any help. Omari

2 3

HTTP Redirect Error
by aditya siram 21 Jul '10

21 Jul '10

Hi all, I am trying to simply download a web page using the HTTP library. It works fine with "http://www.google.com" but a redirecting page like "http://gmail.com" creates an error. Code is at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27864#a27864. Thanks , -deech

1 0

seeking guideline re modules and stacks of State Monad Transformers
by Ali Razavi 20 Jul '10

20 Jul '10

Hi, I am working on a project that features a few modules with inter-dependent states. It is expected that some of the modules import and use some of the stateful functions from other modules, and simultaneously, need to manage the state of their own. I need to make a design choice as to whether I should add the new modules' state as separate layesr by transforming the monad transformer stack in each module, or create a separate module to handle the amalgamation of all states with a single datatype. Both solutions have salient issues. The latter, namely stacking more layers, can quickly become unwieldy, whereas the former breaches encapsulation. Any better ideas? To give it more of a concrete context, imagine the following: module A where type AState = State Int.... someAFunc :: a -> AState a --------------- module B where type BState = StateT Int AState ... someBFunc :: b -> BState b -------------- -- now I want module C to only have access to B's part of BState, but not the A-related part module C where type CState = StateT Int BState Thanks, Ali

1 0

Talking to Java from Haskell?
by Daniel Cook 20 Jul '10

20 Jul '10

Hi, Someone has written a large Java library (QuickFIX/J) which speaks a gnarled, ugly protocol (FIX). There don't appear to be any FIX protocol libraries in Hackage. I need my Haskell program to talk to a 3rd-party system that only speaks FIX. Should I: a) Reimplement the protocol directly Haskell? (This appears to be non-trivial) b) Wrap the Java library with some code to use a lightweight message queue (zeromq) to send messages to my Haskell program? (This would require essentially re-implementing an abstracted subset of the the protocol into 0MQ messages) c) Find a way for Haskell to interact directly with Java? (the various JNI bridges seem very unmaintained...) To me, (b) seems like the best approach, but I'd like to hear what the cafe thinks.. Cheers, - Dan

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Re: [Haskell] ANNOUNCE: Haskell 2010 Report (final)
by Roman Cheplyaka 20 Jul '10

20 Jul '10

* Simon Marlow <marlowsd(a)gmail.com> [2010-07-06 13:00:36+0100] > The Haskell 2010 report is done! I've uploaded it to > www.haskell.org, and linked it from the main Haskell wiki: > > http://www.haskell.org/haskellwiki/Language_and_library_specification > > online HTML version: > > http://www.haskell.org/onlinereport/haskell2010 > > PDF: > > http://www.haskell.org/definition/haskell2010.pdf Greetings! Now, can different compiler/interpreter authors make statements about support of this standard in their compilers please? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

4 13

Design for 2010.2.x series Haskell Platform site
by Don Stewart 20 Jul '10

20 Jul '10

Hey all, As you might know, the next major release of the Haskell Platform is coming up next week. We've had the current download site design for a while now: http://haskell.org/platform/ However, I'm thinking it would be nice to have themed release designs. Examples: http://www.gnome.org/ GNOME 2.30 Ubuntu, http://www.ubuntu.com/ and so on. If anyone is interested in a 2010.2 series design for the HP site, the repository containing the stylesheet is here: http://code.haskell.org/haskell-platform/ And you can find the index.html and style.css in the download-website/ directory. We worked faily hard on the content, but feel free to play with the style. -- Don

20 42

ANNOUNCE: AngloHaskell 2010 - CALL FOR TALKS
by Derek Wright 20 Jul '10

20 Jul '10

AngloHaskell 2010 - ANNOUNCEMENT and CALL FOR TALKS AngloHaskell 2010 - 5th Annual Haskell Meeting in England Friday September 10th and Saturday September 11th, 2010 Microsoft Research, Cambridge, UK http://www.haskell.org/haskellwiki/AngloHaskell/2010 [Please note the change of date from previous years and earlier discussions] AngloHaskell is a free gathering of all people Haskell-related from beginners to seasoned hackers, both academic and industrial. AngloHaskell 2010 is generously hosted by Microsoft Research. As a result no charge is made for attending or speaking. Friday will be a day of talks and discussion at MSR followed by a more informal day of activities on Saturday somewhere else in Cambridge. AngloHaskell welcomes talks on all Haskell related subjects. From short comments (Functional Grit) to full presentations, all are welcome. Please add your name and the title of your talk to the wiki page (or email me and I will add it for you). If you want to attend (even if you are only a possible at the moment) please add your name to the wiki page, or let me know by email. Organisational discussions are on held on the #anglohaskell channel on the Freenode IRC network, feel free to drop in with questions or comments.

1 0

Re: [Haskell-cafe] Tiger compiler in Haskell: annotating abstract syntax tree
by Job Vranish 20 Jul '10

20 Jul '10

Ah, I found the attachment on your other email. I would recommend using the Fix and Ann types, instead of the AnnFix type. I modified your code a bit (and fixed the Show instances etc...) and put it here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27823#a27823 Let me know if you have questions about it. - Job <http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27823#a27823> 2010/7/19 Job Vranish <job.vranish(a)gmail.com> > I didn't get any attachments from you, but haskell-cafe might filter them > out (I'm not sure). > > But, the usual derived instances for Show should work fine for your > expression and annotation types. > For the Fix type you can use: > > instance (Show (f (Fix f))) => Show (Fix f) where > show (Fix a) = show "Fix " ++ show a > > hmmm, but you'll probably need: > > {-# LANGUAGE FlexibleContexts, UndecidableInstances #-} > > > - Job > > > > 2010/7/19 José Romildo Malaquias <j.romildo(a)gmail.com> > >> On Mon, Jul 19, 2010 at 01:51:57PM -0400, Job Vranish wrote: >> > Martijn van Steenbergen has a good blog post that describes the method I >> > generally use: >> > >> http://martijn.van.steenbergen.nl/journal/2010/06/24/generically-adding-pos… >> > >> > In his example he annotates the expression tree with position >> information, >> > but you can use the same method to add type annotations, or whatever you >> > want. >> >> After a quick read at Martijn blog article I've written the attached >> test program, which works. >> >> But I am not succeeding in deriving Show for the data types. Any help? >> >> Romildo >> >> > 2010/7/19 José Romildo Malaquias <j.romildo(a)gmail.com> >> > >> > > Hello. >> > > >> > > In his book "Modern Compilder Implementation in ML", Appel presents a >> > > compiler project for the Tiger programming language where type >> checking >> > > and intermediate code generation are intrinsically coupled. >> > > >> > > There is a function >> > > >> > > transExp :: Absyn.Exp -> (Tree.Exp,Types.Type) >> > > >> > > that do semantic analysis, translating an expression to the Tree >> > > intermediate representation language and also do type checking, >> > > calculating the type of the expression. >> > > >> > > Maybe the compiler can be made more didatic if these phases are >> separate >> > > phases of compilation. >> > > >> > > The type checker would annotate the abstract syntax tree (ast) with >> type >> > > annotations, that could be used later by the translater to >> intermediate >> > > representation. >> > > >> > > In an imperative language probably each relevant ast node would have a >> > > field for the type annotation, and the type checker would assign the >> > > type of the node to this field after computing it. >> > > >> > > I am writing here to ask suggestions on how to annotate an ast with >> > > types (or any other information that would be relevant in a compiler >> > > phase) in Haskell. >> > > >> > > As an example, consider the simplified ast types: >> > > >> > > data Exp >> > > = IntExp Integer >> > > | VarExp Symbol >> > > | AssignExp Symbol Exp >> > > | IfExp Exp Exp (Maybe Exp) >> > > | CallExp Symbol [Exp] >> > > | LetExp [Dec] Exp >> > > >> > > data Dec >> > > = TypeDec Symbol Ty >> > > | FunctionDec Symbol [(Symbol,Symbol)] (Mybe Symbol) Exp >> > > | VarDec Symbol (Maybe Symbol) Exp >> > > >> > > Expressions can have type annotations, but declarations can not. >> > > >> > > Comments? >> > >

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RE: Design for 2010.2.x series Haskell Platform site (Don Stewart)
by Niemeijer, R.A. 20 Jul '10

20 Jul '10

> Is it just me, or does aligning [OSX,Win,Linux] `zip` [Comprehensive, > Robust, CuttingEdge] send the wrong message... Yeah, I noticed that too when designing it, but at the time it didn't bother me too much. I do see how it could confuse some people though, so I've made a design alternative that solves this problem. I think it also makes for a better landing page for people who don't yet know anything about Haskell, since the current design doesn't explain what Haskell actually is. I'll let you guys be the judge though. New alternative: http://dl.dropbox.com/u/623671/haskell_platform_redesign/index2.htm

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[romildo@malaquias.DHCP-GERAL: Re: [Haskell-cafe] Tiger compiler in Haskell: annotating abstract syntax tree]
by José Romildo Malaquias 20 Jul '10

20 Jul '10

Forgot the attachment. Romildo

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possible bug in default module lookup scheme / or invalid haskell?
by Carter Schonwald 19 Jul '10

19 Jul '10

Hello All, I'm not sure if this either a bug in how ghc does path/module lookup or it simply is invalid haskell: consider modules A, A.B and A.B.C where A imports A.B, and A.B imports A.B.C with the following file system layout A.hs A/B.hs A/B/C.hs minimal file examples: module A where import A.B testA = "will it really really work? ------------ module A.B where import A.B.C testB = "will it work ----------------- module A.B.C where testC = "will this work?" ---------- if i run ghci A.hs everything's fine but if in directory B i rune ghci B.hs, i get A/B.hs:2:8: Could not find module `A.B.C': Use -v to see a list of the files searched for. ----------- it seems to me that if the default search path for ghc(i) includes the current directory (which according to docs it does), this shouldn't be happening. (or is there some why this is good Behavior?) thanks! -Carter

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qtHaskell
by Ali Razavi 19 Jul '10

19 Jul '10

Greetings, I have only used the wxHaskell library before, but I am looking into trying one of these more 'advanced' frameworks. To serve my proclivity for QT, I would like to know how its Haskell binding, qtHaskell, compares to that of Gtk. Regards, Ali Razavi

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MonadLib usage
by Emil Melnikov 19 Jul '10

19 Jul '10

Recently I discovered for myself "monadLib" as an alternative for mtl-like packages. It seems that "monadLib" has better interface, but I'm quite confused with it's type classes: they have the monad type variable before the effect type variable (e.g. "ReaderM m i" instead of "ReaderM i m"). AFAIU, this ordering makes impossible to use these type classes in conjunction with GHC's cunning newtype deriving extension. Of course one can easily derive all instances using functionality provided by "MonadLib.Derive" module, but this is a boilerplate. Is there any way to avoid this boilerplate? -- Emil.

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[Newbie] What to improve in my code
by Frank1981 19 Jul '10

19 Jul '10

First of all: I'm not sure if this question is allowed here. If not, I apologize I'm trying to solve the following problem: For each word in a text find the number of occurences for each unique word in the text. i've come up with the following steps to solve this: * remove all punctuation except for whitespace and make the text lowercase * find all unique words in the text * for each unique word, count the number of occurences. This has resulted in the following code: removePunctuation :: [Char] -> [Char] removePunctuation str = filter (\c -> elem c (['a'..'z'] ++ ['A'..'Z'] ++ ['\t', ' ', '\n'])) str process :: [Char] -> [String] process str = words (map toLower (removePunctuation str)) unique :: (Eq a) => [a] -> [a] unique [] = [] unique (x:xs) = [x] ++ unique (filter (\s -> x /= s) xs) occurenceCount :: (Eq a) => a -> [a] -> Int occurenceCount _ [] = 0 occurenceCount x (y:ys) | x == y = 1 + occurenceCount x ys | otherwise = occurenceCount x ys occurenceCount' :: [String] -> [String] -> [(String, Int)] occurenceCount' [] _ = [("", 0)] occurenceCount' (u:us) xs = [(u, occurenceCount u xs)] ++ occurenceCount' us xs Please remember i've only been playing with Haskell for three afternoons now and i'm happy that the above code is working correctly. However i've got three questions: 1) occurenceCount' [] _ = [("", 0)] is plain ugly and also adds a useless tuple to the end result. Is there a better way to solve this? 2) I'm forcing elements into a singleton list on two occasions, both in my unique function and in my occurenceCount' function. Once again this seems ugly and I'm wondering if there is a better solution. 3) The whole process as i'm doing it now feels pretty imperatively (been working for years as a Java / PHP programmer). I've got this feeling that the occurenceCount' function could be implemented using a mapping function. What ways are there to make this more "functional"? -- View this message in context: http://old.nabble.com/-Newbie--What-to-improve-in-my-code-tp29156025p291560… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Running tests in Cabal
by Magnus Therning 19 Jul '10

19 Jul '10

In my Cabal file I have defined a flag that controls whether tests are built or not. Now I'd like to hook it up a bit more so that './Setup.hs test' actually runs the tests. I haven't found a way to access that configuration flag in my hook though. Is it not available in some way? Currently I'm settling for calling 'withExeLBI' and triggering the running based on the exe name. That feels somewhat circuitous though. Is there some better approach? Cheers, M

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