- Haskell-Cafe - Haskell.org

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

04 Jun '09

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

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

04 Jun '09

Ahn, Ki Yung 쓴 글: > Scary type inference for monadic function definitions > (or, why you'd want to annotate types for monadic function definitions) > > This is a real example that I've experienced. > > I defined the following function. > >> checkOneVerseByLineWith readLine v = >> do mg <- readLine >> case mg of >> Just g -> return Just (v==g) >> Nothing -> return Nothing > > My intention was to use it something like this: > > checkOneVerseByLineWith (readline "% ") > > where readline is the library function from System.Console.Readline. > > As you can see, there is an obvious mistake which I forgot to > group (Just (v==g)) in parenthesis. However, GHC or any other > Haskell 98 compliant implementation will infer a type for you > and this will type check! Try it yourself if in doubt. > > Of course, checkOneVerseByLineWith (readline "% ") won't type check > because checkOneVerseByLineWith has strange type. The reason why > the above definition type checks is because ((->) r) is an instance > of Monad. Oh, I happened to be importing Control.Monad.Trans somehow, it just doesn't work with Prelude import itself. And, there were already some discussions on this last year: [Haskell-cafe] The danger of Monad ((->) r) http://www.mail-archive.com/haskell-cafe@haskell.org/msg23680.html Tomasz Zielonka Tue, 15 May 2007 03:05:30 -0700

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ANN: HSH 2.0.0
by John Goerzen 04 Jun '09

04 Jun '09

Hi, I'm pleased to announce the release of version 2.0.0 of HSH, the Haskell shell scripting library. This version features a complete rewrite of the core. Since System.Process has finally become capable enough to be the low-level of HSH, HSH has been reimplemented in terms of it. This has produced a dramatic reduction in code size and complexity. It also gains cross-platform support; HSH now works on Windows. Finally, the API is both simpler and more flexible. The 180-test suite for HSH continues to run and pass with only minor tweaks for the new API. Optional support for setting environment variables on commands to be run has been added as well. It will stay out of your way unless you need it, in which case it is easy. As a reminder, HSH lets you easily set up shell command pipelines. You can also pipe data between Haskell functions and shell commands with complete ease. The package also provides Haskell versions of a number of familiar shell primitives to help you get up and running quickly. HSH 2.0.0 has been uploaded to Hackage. -- John

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

04 Jun '09

Excellent! Thanks. Michael --- On Wed, 6/3/09, Don Stewart <dons(a)galois.com> wrote: From: Don Stewart <dons(a)galois.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "michael rice" <nowgate(a)yahoo.com> Cc: "brian" <briand(a)aracnet.com>, haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 6:40 PM nowgate: > Got it working. > > I downloaded two packages, primes and Numbers. Since Numbers has the three > functions I want to use, primes, isPrime and isProbablyPrime, how do I > uninstall the primes package so there won't be a conflict? Easy! $ ghc-pkg unregister primes -- Don

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ANN: atom-0.0.5
by Tom Hawkins 04 Jun '09

04 Jun '09

A few bug fixes and doc improvements. Thanks to John Van Enk and Brian Lewis for the patches! http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom -Tom

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

04 Jun '09

Hi Andrew (Bromage), I reversed the parameter order to Data.Map.lookup and calling fromJust to pull out value from Maybe wrapper ... all as you suggested: > remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t) > remLookupFwd re > = do fwd <- gets resFwdMap > let { Just reinfo = fromJust(M.lookup re fwd) } -- PROBLEM > return reinfo I am still getting a type mismatch: Swish\HaskellRDF\Dfa\Dfa.lhs:162:29: Couldn't match expected type `Maybe t' against inferred type `ReInfo t1' In the expression: fromJust (M.lookup re fwd) In a pattern binding: Just reinfo = fromJust (M.lookup re fwd) In the expression: do fwd <- gets resFwdMap let Just reinfo = fromJust (M.lookup re fwd) return reinfo Vasili

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Using type families to define runtime representation and evaluation strategy?
by Corey O'Connor 04 Jun '09

04 Jun '09

I'm interested in the feasibility of extending the compiler using a construct similar to type synonym families to determine runtime representation and evaluation strategy for types. Can anybody point me to existing work in this area? I'm not sure of the exact terminology to be using here so my description is going to be fuzzy and probably off the mark on a few points. If anybody could help me clarify what I'm trying to described that would be appreciated. What I imagine is an open construct that be evaluated at compile time to determine: - How the data for a type is represented in memory. Whether values are boxed or unboxed etc. - How an expression should be represented. As a lazy evaluation thunk (weak head normal form?) or evaluated to normal form. - How a expression should be reduced. Since "the best" evaluation strategy depends on the application the equations should be open. For one application a data structure should be evaluated the normal form but not another. This is why the construct should be open much like type synonym families. One application could define an instance that declares the value should be reduced to HNF when defined while the other application could define an instance that declares the value to be reduced to WHNF. Still, the library implementor needs the ability to define reasonable defaults. If the construct was closed I don't think these requirements could be satisfied. This resembles extending the Control.Parallel.Strategies evaluation strategies and adaptive-container's container representation to be represented by type level equations. Control.Parallel.Strategies has a good interface but littering the equations with expressions declaring the evaluation strategy seems awkward. Preferably the evaluation strategy should be definable in parallel to the equations when possible. Strictness annotations already resemble the start of such a construct. And any such construct would have to be able to take the place of strictness annotations. Cheers, Corey O'Connor

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ANN: new version of uu-parsinglib
by S. Doaitse Swierstra 04 Jun '09

04 Jun '09

A new version of the uu-parsinglib has been uploaded to hackage. It is now based on Control.Applicative where possible. Be warned that functions like some and many will be redefined in the future. Doaitse Swierstra

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

04 Jun '09

This is working differently than it did last night, when at least import was working. Michael [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> :m + Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> Data.Numbers.Primes> :browse <interactive>:1:21: parse error on input `:' Prelude> isPrime 3525266 <interactive>:1:0: Not in scope: `isPrime' Prelude> ========== [michael@localhost ~]$ cd .cabal [michael@localhost .cabal]$ ls -l total 20 -rw-rw-r-- 1 michael michael 1729 2009-06-02 18:59 config drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 lib drwxrwxr-x 2 michael michael 4096 2009-06-02 19:05 logs drwxrwxr-x 3 michael michael 4096 2009-06-02 18:59 packages drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 share [michael@localhost .cabal]$ cd packages [michael@localhost packages]$ ls -l total 4 drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 hackage.haskell.org [michael@localhost packages]$ cd hackage.haskell.org [michael@localhost hackage.haskell.org]$ ls -l total 12464 -rw------- 1 michael michael 11735040 2009-06-02 22:14 00-index.tar -rw------- 1 michael michael 992994 2009-06-02 22:14 00-index.tar.gz -rw-rw-r-- 1 michael michael 418 2009-06-02 22:14 build-reports.log drwxrwxr-x 3 michael michael 4096 2009-06-02 22:14 Numbers drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 primes [michael@localhost hackage.haskell.org]$ [michael@localhost hackage.haskell.org]$ pwd /home/michael/.cabal/packages/hackage.haskell.org [michael@localhost hackage.haskell.org]$ ============= --- On Wed, 6/3/09, brian <briand(a)aracnet.com> wrote: From: brian <briand(a)aracnet.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 10:46 AM Prelude> :m + Data.Numbers.Primes Prelude Data.Numbers.Primes> :browse isPrime :: Integer -> Bool isProbablyPrime :: (System.Random.RandomGen g) => Integer -> g -> (Bool, g) primes :: [Integer] Prelude Data.Numbers.Primes> isPrime 3525266 Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package Numbers-0.2.1 ... linking ... done. False Prelude Data.Numbers.Primes> On Jun 3, 2009, at 6:57 AM, michael rice wrote: > OK, I downloaded Numbers. Still missing the three functions I wanted, which are primes, isPrime, and isProbablyPrime. How do I get these? > > Michael > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Linkage errors in scenegraph
by Gregory D. Weber 04 Jun '09

04 Jun '09

I'd like to get the scenegraph package (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/scenegraph) to work, but am encountering linkage errors. I installed scenegraph 0.1.0.1 for ghc-6.10.1 on Fedora 10 Linux. # runghc Setup configure --prefix=/opt # runghc Setup build # runghc Setup install Then I created this little program based on the example at http://www.haskell.org/haskellwiki/SceneGraph (in file SGTest.hs): module Main where import Graphics.SceneGraph import Control.Monad.Identity main :: IO () main = runScene $ osgt $ myScene myScene :: OSGT Identity SceneNode myScene = cube 0.5 `colour` Green `scale` v1x 40 `translate` vy 2 <+> camera `translate` vy (-10) <+> light `translate` vz 10 Then I compiled it and got a long list of linker errors; here are the first few lines: ghc --make SGTest Linking SGTest ... /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__210.o): In function `sdE1_info': (.text+0x8d): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__210.o): In function `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziBasic_translate_srt': (.data+0x0): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__207.o): In function `sdDq_info': (.text+0x7e): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_scaleM_info' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__207.o): In function `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziBasic_scale_srt': (.data+0x0): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_scaleM_closure' /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1/libHSscenegraph-0.1.0.1.a(Basic__491.o):(.text+0x27): undefined reference to `__stginit_scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziUtils_' Also with this command, same result but a lot more detail: ghc --make -v -package scenegraph SGTest With -v, the output includes gcc -v -o SGTest -DDONT_WANT_WIN32_DLL_SUPPORT SGTest.o -L/opt/lib/scenegraph-0.1.0.1/ghc-6.10.1 -L/opt/lib/hmatrix-0.5.1.1/ghc-6.10.1 -L/opt/lib/storable-complex-0.2/ghc-6.10.1 ... -lHSscenegraph-0.1.0.1 -lHShmatrix-0.5.1.1 -lgsl -llapack -lHSstorable-complex-0.2 -lHSQuickCheck-1.2.0.0 ... which seems to show that the proper -L and -l options are included. If I'm decoding the first "undefined reference" symbol -- (.text+0x8d): undefined reference to `scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure' -- correctly, then the missing symbol is in the scenegraph-0.1.0.1 package, in module Graphics.SceneGraph.Matrix, and is associated with the translateM function defined in that module. I looked for this symbol with nm: # pwd /opt/lib/scenegraph-0.1.0.1/ghc-6.10.1 # nm HSscenegraph-0.1.0.1.o | grep translateM U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_info # nm libHSscenegraph-0.1.0.1.a | grep translateM U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_closure U scenegraphzm0zi1zi0zi1_GraphicsziSceneGraphziMatrix_translateM_info but I understand the "U" means that it is referenced by some other function in these files, but is undefined in them. Since the module Graphics.SceneGraph.Matrix does not declare any exports, I thought that might be the reason the symbol was undefined. But looking back at the Haskell 98 language definition, I found that the absence of an export declaration in a module means that all the symbols of the module are exported. Also, I notice that in the cabal file for scenegraph, the list of exposed modules Exposed-Modules: Graphics.SceneGraph, Graphics.SceneGraph.Basic, Graphics.SceneGraph.Vector, Graphics.SceneGraph.Render, Graphics.SceneGraph.SimpleViewport, Graphics.SceneGraph.GraphViz, Graphics.SceneGraph.Library, Graphics.SceneGraph.Dump, Graphics.SceneGraph.Textures does not include Graphics.SceneGraph.Matrix, but that should only mean that I can't call functions of that module directly -- not that the other SceneGraph modules can't call them -- right? So, why is this symbol undefined? And how can I get it to work? Oh by the way: I can actually use this package if I move into its src directory, add a main function to Examples.hs, and compile and run it right there. Just can't use it as an installed library, it seems. -- ___ ___ __ _ / _ \ / _ \| | | | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \____/\_____/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550

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GSoC update: sneak peek at the live heap profiler
by Patai Gergely 04 Jun '09

04 Jun '09

Hello everyone, finally there's a bit of eye candy for anyone interested in the heap profiling project: http://just-bottom.blogspot.com/ Feel encouraged to comment and check out the project page too, where you can already see the beginnings of the core library. Gergely -- http://www.fastmail.fm - The way an email service should be

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ANNOUNCE: The Haskell Platform 2009.2.0.1
by Don Stewart 04 Jun '09

04 Jun '09

We're pleased to announce the second release of the Haskell Platform: a single, standard Haskell distribution for everyone. The specification, along with installers (including Windows and Unix installers for a full Haskell environment) are available. Download the Haskell Platform 2009.2.0.1: http://hackage.haskell.org/platform/ The Haskell Platform is a blessed library and tool suite for Haskell distilled from 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. Distro maintainers that support the Haskell Platform can be confident they're fully supporting Haskell as the developers intend it. Developers targetting the platform can be confident they have a trusted base of code to work with. What you get: http://hackage.haskell.org/platform/contents.html With regular time-based releases, we expect the platform will grow into a rich, indispensable development environment for all Haskell projects. Please note that this is a beta release. We do not expect all the installers to work perfectly, nor every developer need met, and we would appreciate feedback. You can help out by packaging the platform for your distro, or reporting bugs and feature requests, or installing Haskell onto your friends' machines. The process for adding new tools and libraries will be outlined in coming weeks. The Haskell Platform would not have been possible without the hard work of the Cabal development team, the Hackage developers and maintainers, the individual compiler, tool and library authors who contributed to the suite, and the distro maintainers who build and distribute the Haskell Platform. Thanks! -- The Platform Infrastructure Team

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Bool as type class to serve EDSLs.
by Sebastiaan Visser 04 Jun '09

04 Jun '09

Hello, While playing with embedded domain specific languages in Haskell I discovered the Num type class is a really neat tool. Take this simple example embedded language that can embed primitives from the output language and can do function application. >data Expr :: * -> * where > Prim :: String -> Expr a > App :: Expr (a -> b) -> Expr a -> Expr b Take these two dummy types to represent things in the output language. >data MyNum >data MyBool Now it is very easy to create an Num instance for this language: >primPlus :: Expr (MyNum -> MyNum -> MyNum) >primPlus = Prim "prim+" >instance Num (Epxr MyNum) where > a + b = primPlus `App` a `App` b > fromInteger = Prim . show > ... Which allows you to create very beautiful expression for your language embedded inside Haskell. The Haskell expression `10 * 5 + 2' produces a nice and well typed expression in your embedded domain. But unfortunately, not everyone in the Prelude is as tolerant as the Num instance. Take the Eq and the Ord type classes for example, they require you to deliver real Haskell `Bool's. This makes it impossible make your DSL an instance of these two, because there are no `Bool's only `Expr Bool's. Which brings me to the point that, for the sake of embedding other languages, Haskell's Prelude (or an alternative) can greatly benefit from (at least) a Boolean type class like this: class Boolean a where ifthenelse :: a -> b -> b -> b -- Not sure about this representation. ... And one instance: >instance Boolean (Expr MyBool) where > ifthenelse c a b = Prim "if-then-else" `App` c `App` a `App` b Now we can change (for example) the Eq type class to this: >class Eq a where > (==) :: Boolean b => a -> a -> b > (/=) :: Boolean b => a -> a -> b For which we can give an implementation for our domain: >primEq :: Epxr (a -> a -> MyBool) >primEq = Prim "==" >instance Eq (Expr a) where > a == b = primEq `App` a `App` b And now we get all functionality from the Prelude that is based on Eq (like not, &&, ||, etc) for free in our domain specific language! Off course there are many, many more examples of things from the standard libraries that can be generalised in order to serve reuse in EDSLs. Anyone already working on such a generalized Prelude? I can imagine much more domains can benefit from this than my example above. Any interesting thoughts or pointers related to this subject? Gr, -- Sebastiaan Visser

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Re: [Haskell-cafe] Cabal/primes
by michael rice 03 Jun '09

03 Jun '09

OK, I downloaded Numbers. Still missing the three functions I wanted, which are primes, isPrime, and isProbablyPrime. How do I get these? Michael

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ANN: heap-1.0.0
by Stephan Friedrichs 03 Jun '09

03 Jun '09

Hello haskell-cafe, I'm pleased to announce a rewrite of the heap package, heap-1.0.0 [1]. It is not 100% compatible to the version 0.6.0, but provides major improvements: - The HeapPolicy type class hack used to distinguish between min- max-, min-prio- and max-prio-heaps has been replaced by the HeapItem type family (the type family provides functions to convert insertable 'items' to (priority, value) pairs and back. Thus a static min-heap (HeapT) can be used as underlying implementation, but the package can still provide MinHeap, MaxHeap, MinPrioHeap and MaxPrioHeap. - A nice side effect of the above is, that ({Min,Max}PrioHeap p) now is an instance of Functor - Faster {from,to}{Asc,Desc}List conversions - A binary with quickcheck test cases can now be built from cabal (with a "Test" flag) (the old versions already had lots of test cases, but cabal didn't know about them). Regards, Stephan [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-1.0.0 -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Notice for package authors
by Duncan Coutts 03 Jun '09

03 Jun '09

Package authors, We are about to add an additional quality check in the Hackage upload process that will affect many packages. Hackage will require an upper bound on the version of the base package and reject packages that omit it. Lots of packages currently specify: build-depends: base or build-depends: base >= 3 This is bad. There is no way for the tools to discover if you mean base 3 or 4 and yet most packages break when we pick the wrong one. You must specify an upper bound so that your package does not break when the next major version of the base library is released. For example, if you've tested with base 3 and 4 then use: build-depends: base >= 3 && < 5 Or if it needs version 4 then use: build-depends: base == 4.* Background ---------- When base 4 was released with ghc-6.10.x the vast majority of packages would have broken were it not for a compatibility shim that we added to the cabal-install tool. It defaulted to picking base 3 when the version constraints did not otherwise specify the use of base 4. The same trick will not work when base 5 is released because it will likely not be possible to continue to ship base 3. It is also important that we start to move towards making the majority of packages compatible with base 4. cabal-install changes --------------------- We will shortly release a minor update to cabal-install to change the way it picks between base 3 and 4. Currently if it has the choice it picks version 3. However that is not optimal for cases like: build-depends: base >= 3 && < 5 where it is clear that it is supposed to work with both. In this case we should not be pessimistic in going with the old base 3 and should instead pick base 4. More precisely, we will only apply the compatibility shim when the version is unbounded above. When it is bounded above, as Hackage will soon require, the normal preference to use the latest version applies. If you'd like to help us check this feature is working properly then grab the latest darcs version of cabal-install (which needs the latest darcs version the Cabal lib). See also http://hackage.haskell.org/trac/hackage/ticket/485 Duncan

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HXT XmlPicklers - TH Derivation
by Max Cantor 03 Jun '09

03 Jun '09

Hi, I have developed some simple TH code to automatically derive XmlPickler instances for my types and if there is interest, I will clean it up and submit a patch. Its not complete, but is a start. Any interest? Max

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Datastructure with some business-rules
by Mathijs Kwik 03 Jun '09

03 Jun '09

Hi all, I'm building a tree-like structure that somewhat resembles a package/ dependency structure as most packagemanagers/ports-systems have them. It's tree-like (Data.Tree at the moment), but I will probably need to make some structural changes to allow for more complex stuff like circular dependencies and stuff like 'alternatives'. While I'm still on the Tree (nice & simple), I want to explore some business-rules & validation stuff. This is a bit of a contrived example: Let's say that nodes are (NodeType, Int) pairs, where the Nodetype is just a simple algebraïc type. Now certain types of nodes are only allowed to 'grow' on top of some other nodes, but up to 10 levels deep. So certain nodes are only allowed if one of 10 ancestors is of some other type. Or to make this a bit weirder (maybe too contrived): its parent should have a 'brother' of some type. Ofcourse I can make a 'validate' function that checks if a tree plays by the rules, but I would like to put the typesystem to use somehow, since there aren't that many rules. So I would like to have an error thrown any time an invalid structure is used/created instead of having to validate myself every time. I would like this, because in the future I will need to 'morph' trees into each other so inbetween steps that exist must still be valid. First I thought about creating some auto-validate monad that just validates the result after every step, but before going that way I would like to make sure this is the way to go. I understand that Data.Tree (and other functors) just take 1 type 'payload' and they don't mind what's in there, while my 'validation' rules are about the payload _and_ the place in the tree, so I will probably have to change a bit here. Is this at all possible? Is it ok to do or is a 'external' validation/validation monad clearer? What would be the way to go? Thanks for any help! Mathijs

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ANN: Anglohaskell 2009
by Philippa Cowderoy 03 Jun '09

03 Jun '09

Anglohaskell 2009 is go! I'm taking on the mantle of organiser, and Microsoft Research have offered us space for talks in Cambridge again. The event will be held on the 7th and 8th of August. More info at http://www.haskell.org/haskellwiki/AngloHaskell/2009 , planning and discussion in #anglohaskell on freenode. For those not familiar with Anglohaskell, it's a somewhat-informal get-together featuring a mixture of talks, discussion and socialising with topics from the hobbyist to the pragmatic to the theoretical. All are welcome, regardless of experience, and best of all - it's free! If anyone wants to offer a talk, help with running the event, accomodation for haskellers from out of town or some ideas, please feel free to edit the wiki page appropriately and/or give us a yell in #anglohaskell. -- Philippa Cowderoy <flippa(a)flippac.org>

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beginners question about fromMaybe
by Nico Rolle 03 Jun '09

03 Jun '09

hi there heres a code snipped, don't care about the parameters. the thing is i make a lookup on my map "m" and then branch on that return value probePhase is sc [] m = [] probePhase is sc (x:xs) m | val == Nothing = probePhase is sc xs m | otherwise = jr ++ probePhase is sc xs m where jr = joinTuples sc x (fromMaybe [] val) key = getPartialTuple is x val = Map.lookup key m the line "jr = joinTuples sc x (fromMaybe [] val)" is kind of ugly because i know that it is not Nothing. is there a better way to solve this? regards

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Checking a value against a passed-in constructor?
by Dan Cook 03 Jun '09

03 Jun '09

Hi, (Relatively new to Haskell here ..) So I have the following: data MyVal = Atom String | Bool Bool And I want to do something like this check :: (Bool -> MyVal) -> MyVal -> True check f (f x) = True check _ _ = False What that means is I want to pass a MyVal constructor and a MyVal, and return True if the second argument was constructed with the first. More generally, I'd like to be able to do genCheck :: (* -> MyVal) -> MyVal -> True genCheck f (f x) = True genCheck _ _ = False So that I can pass in _any_ MyVal constructor and let the function just check if the second argument was constructed with the first, without caring which constructor it is. What is the preferred way to do this, since neither of those functions compile? Cheers, - Dan

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Cabal/primes
by michael rice 03 Jun '09

03 Jun '09

Finally got adventurous enough to get Cabal working, downloaded the primes package, and got the following error message when trying isPrime. Am I missing something here? Michael ================== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Data.Numbers.Primes Prelude Data.Numbers.Primes> take 10 primes Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package primes-0.1.1 ... linking ... done. [2,3,5,7,11,13,17,19,23,29] Prelude Data.Numbers.Primes> isPrime 7 <interactive>:1:0: Not in scope: `isPrime' Prelude Data.Numbers.Primes>

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Success and one last issue with Data.Binary
by David Leimbach 03 Jun '09

03 Jun '09

I've got the following "printHex" string as a response from a 9P server running on the Inferno Operating System. (thanks to a friendly mailing list contributor who sent a nice example of using Data.Binary) 1300000065ffff000400000600395032303030 This is a little endian encoded ByteString with the following fields in it: Rversion {size :: Word32, mtype :: Word8, tag :: Word16, msize :: Word32, ssize :: Word16, version :: ByteString} But when I try to use the following implementation of "get" to decode this stream, I'm getting the following error: "too few bytes. Failed reading at byte position 20" Unfortunately, I'm only expecting 19 bytes, and in fact never asked for byte 20. (I am just asking for everything up to ssize, and then "getRemainingLazyByteString"). Is this a bug? Is it mine or in Data.Binary? :-) Here's my "get" function: get = do s <- getWord32le mtype <- getWord8 getSpecific s mtype where getSpecific s mt | mt == mtRversion = do t <- getWord16le ms <- getWord32le ss <- getWord16le v <- getRemainingLazyByteString return $ MessageClient $ Rversion {size=s, mtype=mt, tag=t, msize=ms, ssize=ss, version=v} The good news is I'm talking 9P otherwise, correctly, just having some decoding issues. I hope to have a hackage package eventually for this. Dave

3 17

Compiling a windows app - embedding a manifest
by Günther Schmidt 03 Jun '09

03 Jun '09

Hi all, is it possible to make ghc embedd a particular manifest in the .exe during the compilation process? Günther

2 1

code reviewers wanted for hashed-storage (darcs)
by Eric Kow 03 Jun '09

03 Jun '09

Dear Haskellers, Will you have a few spare hours this summer? The Darcs team needs your help! Summary ------- We need two volunteers to help us review the standalone hashed-storage module, which will be used by Darcs in the future. Background ---------- Darcs supports 'hashed' repositories in which each file in the pristine cache is associated with a cryptographic hash. Hashed repositories help Darcs to resist some forms of corruption and also allow for nice features such as a global patch cache and lazy patch fetching. Unfortunately, our implementation can be rather slow. Petr has some nice ideas for making these repositories work a lot faster. He will be producing a library he calls 'hashed-storage' which generalises the idea of storing files associating them with a cryptographic hash and furthermore improves on the current implementation used by Darcs. The hashed-storage library is general purpose and may find a use in other applications that need to manage a large number of files. I'm excited about this project because if it succeeds, we can finally start hammering home the point that yes, Darcs *can* be a fast revision control system. It may not fix all our problems -- we'll still need a Darcs 3 -- but it should alleviate at least some of the practical day to day issues. I want to ensure that Petr's project is a success. One thing in particular is that I would like him to get regular code review from the Haskell community, perhaps from folks who aren't already on the Darcs team. Do you think you can help? Two volunteers needed --------------------- I'm looking for just two volunteers this summer: 1. Do you have one hour per week this summer? We need somebody to track and make comments on changes to hashed-storage over this summer. The current library has only 8 modules, with less than 1400 lines of code, so it should be relatively easy to get started :-) 2. Do you have a spare weekend in August? Towards the end of the summer, it would be nice to have somebody examine the "final" version of hashed-storage and give us their thoughts. Again, no Darcs experience is needed, as this is a standalone module. In fact, a fresh perspective would be very helpful. More information ---------------- You can download hashed-storage from hackage from his darcs repository: darcs get http://repos.mornfall.net/hashed-storage For more information about his project, see http://web.mornfall.net/blog/summer_of_code.html Please get in touch with me if you can help. Thanks! Eric -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Possible Haskell Project
by Tom Hawkins 02 Jun '09

02 Jun '09

My family and I are moving in the coming months. My wife will be attending a new school in the fall. Among the many hassles of moving are locating and transferring medical records to new doctors and clinics. During our time in Minnesota, we've visited several clinics and hospitals, so our medical data is spread across a dozen different networks. Collecting this data is a chore. And reciting it to every new medical provider is frustrating, especially knowing someone else has already typed this stuff into the computer. For obvious reasons, there are many software companies competing in the Electronic Health Record (EHR) industry. I haven't spent much time researching the field, but I have to imagine there is no shortage of bloated standards and next to nil interoperability. With President Obama pushing for electronic health records, this could be an opportunity for the Haskell community to deliver a common framework that the US, and maybe the rest of the world, could build upon -- a longshot I realize. At the core, the fundamental problem is not that complicated. It's just storing and retrieving a person's various health events: checkups, prescriptions, procedures, test results, etc. The main technical challenges are database distribution and patient security. Both are fun problems, and our friends at Galios continue to show how effective Haskell is at building secure systems. Any thoughts? Ideas? -Tom BTW, Anyone looking to rent a house in Eden Prairie, MN? A Haskell employer is just 4 miles down the road (Eaton).

4 4

Compiling a windows app - embedding application icon
by Günther Schmidt 02 Jun '09

02 Jun '09

Hi all, is it possible to make ghc embedd an application icon in the .exe during the compilation process? Günther

2 1

Spam apology
by Daniel Cook 02 Jun '09

02 Jun '09

Sorry for all the repeated messages, my e-mail client exploded.

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HaL4: Haskell-Meeting in Germany, 12th June 2009
by Janis Voigtlaender 02 Jun '09

02 Jun '09

Hi all, If you are anyway near Halle/Saale in June, be sure not to miss out on: http://iba-cg.de/hal4.html We have already close to 50 registered participants, so expect a very lively meeting. See you there? (Late registration still possible.) Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Trouble with types
by Vladimir Reshetnikov 02 Jun '09

02 Jun '09

Hi, I tried this code: ----------------------- f, g :: a -> a (f, g) = (id, id) ----------------------- Hugs: OK GHC: Couldn't match expected type `forall a. a -> a' against inferred type `a -> a' In the expression: id In the expression: (id, id) In a pattern binding: (f, g) = (id, id) What does mean this error message? And what of them (Hugs, GHC) is correct? Thanks Vladimir

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RE: Haskell-Cafe Digest, Vol 70, Issue 2
by R J 02 Jun '09

02 Jun '09

Sorry -- the vocabulary template is attached. > From: haskell-cafe-request(a)haskell.org > Subject: Haskell-Cafe Digest, Vol 70, Issue 2 > To: haskell-cafe(a)haskell.org > Date: Mon, 1 Jun 2009 21:47:16 -0400 > > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Re: Missing a "Deriving"? (Daniel Fischer) > 2. Re: Bool as type class to serve EDSLs. (Claus Reinke) > 3. Re: ANN: new version of uu-parsinglib (S. Doaitse Swierstra) > 4. Re: Re: Error message reform (Claus Reinke) > 5. Re: Re: Error message reform (was: Strange type errorwith > associated type synonyms) (Claus Reinke) > 6. Re: Missing a "Deriving"? (michael rice) > 7. Re: Bool as type class to serve EDSLs. (Sebastian Fischer) > 8. Checking a value against a passed-in constructor? (Dan Cook) > 9. Re: Checking a value against a passed-in constructor? > (Jason Dagit) > 10. Re: Missing a "Deriving"? (michael rice) > 11. Re: Missing a "Deriving"? (Ross Mellgren) > 12. Re: Missing a "Deriving"? (michael rice) > 13. Re: Missing a "Deriving"? (Ross Mellgren) > 14. Re: Missing a "Deriving"? (michael rice) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 1 Jun 2009 19:51:35 +0200 > From: Daniel Fischer <daniel.is.fischer(a)web.de> > Subject: Re: [Haskell-cafe] Missing a "Deriving"? > To: haskell-cafe(a)haskell.org > Message-ID: <200906011951.35775.daniel.is.fischer(a)web.de> > Content-Type: text/plain; charset="iso-8859-15" > > Am Montag 01 Juni 2009 19:02:36 schrieb michael rice: > > > All good so far, but then tried to convert Failable from Computation to > > Monad > > > > > > instance Monad Failable where > > return = Success > > fail = Fail > > >>= (Success x) f = f x > > >>= (Fail s) _ = Fail s > > mplus (Fail _) y = y > > mplus x _ = x > > > > > > and got the following error. > > > > > > Prelude> :l graph5 > > [1 of 1] Compiling Main ( graph5.hs, interpreted ) > > > > graph5.hs:34:4: parse error on input `>>=' > > Failed, modules loaded: none. > > Prelude> > > > > When you use an operator in prefix position, you must enclose it in parentheses, like you > must enclose a function in backticks if you use it infix. > > So the definition of (>>=) should read > > > (>>=) (Success x) f = f x > (>>=) (Fail s) _ = Fail s > > or, defining it in infix position, > (Success x) >>= f = f x > (Fail s) >>= _ = Fail s > > > > > Complete code follows. > > > > Michael > > > > > > ------------------------------ > > Message: 2 > Date: Mon, 1 Jun 2009 19:22:10 +0100 > From: "Claus Reinke" <claus.reinke(a)talk21.com> > Subject: Re: [Haskell-cafe] Bool as type class to serve EDSLs. > To: "haskell Cafe" <haskell-cafe(a)haskell.org> > Message-ID: <76CE077128014A86A2D61E5918F8BFE8@cr3lt> > Content-Type: text/plain; format=flowed; charset="iso-8859-1"; > reply-type=original > > >> Do you argue that overloading logical operations like this in Haskell > >> sacrifices type safety? Could programs "go wrong" [1] that use such > >> abstractions? > > > > If I understand your point correctly, you are suggesting that such programs > > are still type safe. I agree with the claim that such features are > > detrimental in practice though. Instead of lumping it with type safety, > > then what do we call it? I think I've heard of languages that do such > > conversions as "weakly" typed. Really the issue is with implicit > > conversions, right? > > Isn't it merely a matter of balance? In order for typed programs not > to go "wrong", one has to define "right" and "wrong", and devise a type > system that rules out anything that might go "wrong", usually at the > expense of some programs that might go "right". > > Advanced type system features like overloading take that unused space > and devise ways to redirect code that would go "wrong" (in simpler > systems) to go "right" in useful new ways (eg: adding two functions or > matrices or .. does not have to be "wrong", there are interpretations in > which all of these make perfect sense, and Haskell can express many > of them). > > What is happening then is that more and more of the previously "wrong" > space is filled up with meaningful ways of going "right", until nearly every > syntactically valid program goes somewhere. That can make for an > extremely expressive and powerful language, but it renders the naive > notion of going "wrong" or "right" rather meaningless: "wrong" just > means we haven't figured out a meaningful way to interpret it, and > going "right" can easily be a far cry from where you wanted it to go. > > Claus > > PS. this problem can be made worse if the implicit conversions > aren't consistent, if small "twitches" in source code can lead to > grossly different behaviour. There is a fine line between advanced > and uncontrollable, and opinions on what side of the line any given > definition is on can differ. > > > > > ------------------------------ > > Message: 3 > Date: Mon, 1 Jun 2009 20:27:05 +0200 > From: "S. Doaitse Swierstra" <doaitse(a)swierstra.net> > Subject: Re: [Haskell-cafe] ANN: new version of uu-parsinglib > To: Ross Paterson <ross(a)soi.city.ac.uk> > Cc: haskell-cafe(a)haskell.org > Message-ID: <8F2D399B-451A-45F4-9FE1-B7F5145C3B91(a)swierstra.net> > Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes > > And rename "empty" to "fail"? You managed to confuse me since I always > use pSucceed to recognise the empty string. > > Doaitse > > > On 1 jun 2009, at 01:21, Ross Paterson wrote: > > > On Sun, May 31, 2009 at 09:40:38PM +0200, S. Doaitse Swierstra wrote: > >> A new version of the uu-parsinglib has been uploaded to hackage. It > >> is > >> now based on Control.Applicative where possible. > >> > >> Be warned that functions like some and many will be redefined in the > >> future. > > > > Perhaps we should make some and many methods of Alternative, <* and *> > > methods of Applicative and <$ a method of Functor, all with the > > current > > definitions as defaults. (John Meacham was also asking for the first > > of these.) > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > ------------------------------ > > Message: 4 > Date: Mon, 1 Jun 2009 19:49:44 +0100 > From: "Claus Reinke" <claus.reinke(a)talk21.com> > Subject: Re: [Haskell-cafe] Re: Error message reform > To: <haskell-cafe(a)haskell.org> > Message-ID: <E0BDCB9EE5F84BEA9126D498C0F98621@cr3lt> > Content-Type: text/plain; format=flowed; charset="UTF-8"; > reply-type=response > > > It's too wordy, but it's a start. This is also prime ground for wanting > > to have configurable levels of error reports, since some users will find > > it helpful to see both types but others will find it confusing. > > Indeed. For this simple example, I find Hugs' message nearly optimal, > but as one could just as well construct examples where GHC's message > was nearly optimal, I've tried instead to extend the example, until neither > Hugs nor GHC gives an optimal message. Even so, it is worth comparing > the two, and their complementary approaches to the problem: > > t = (let q=0 in(\y->(\x->(\z->let r=1 in x z))) ()) :: (a->b)->(a->a) > > D:\home\Haskell\tmp\desktop\errormessages.hs:2:41: > Couldn't match expected type `a' against inferred type `b' > `a' is a rigid type variable bound by > an expression type signature > at D:\home\Haskell\tmp\desktop\errormessages.hs:2:56 > `b' is a rigid type variable bound by > an expression type signature > at D:\home\Haskell\tmp\desktop\errormessages.hs:2:59 > In the expression: x z > In the expression: let r = 1 in x z > In the expression: (\ z -> let r = 1 in x z) > > ERROR file:.\errormessages.hs:2 - Inferred type is not general enough > *** Expression : let {...} in (\y -> \x -> \z -> let {...} in x z) () > *** Expected type : (a -> b) -> a -> a > *** Inferred type : (a -> a) -> a -> a > > GHC delivers its messages from in-the-middle of its typing process, > which one hopes might give as little information as neccessary to > identify the issue, without irrelevant context. It then tries to explain > that information nicely, followed by some value-level context. > > Hugs delivers its messages from on-the-outside-looking-in, which one > hopes might give the maximum potentially relevant information. It makes > no attempt to cushion the blow. > > In practice, neither approach works all the time, and even for this > tiny contrived example, one would have to engage in some "type > debugging" (inserting type signatures, to trigger type-level printfs; > commenting out large parts of code to see whether they contribute > to the issue; ..) to figure it out. > > Note that both approaches are fairly precise in locating the error > at the type level - it is the cross-reference from type to value level > that runs into trouble, especially if the former is only implicit in the > latter (which is why type debugging by adding signatures helps). > > > For really intricate type hacking, even this isn't enough because the > > programming errors often happen far from where the type errors are > > finally caught. In an ideal world, ghc could dump the entire proof > > forest generated by inference, so an external tool[1] could be used to > > browse through it and track the complete history of where inferred types > > came from. This gives a partial view of the inferred types for a > > compilation unit, something I've often wanted (rather than the manual > > comment/reload/uncomment routine). The proof forest could even be used > > as an interlingua over which people can write filters to generate > > messages they find most helpful. > > Yes, most research papers on this ended up suggesting visual representations > and separation between generating and presenting/querying error information. > > > Ah the joys of ideal worlds... ;) > > "if you don't have a dream, how you gonna have a dream come true?" (Bloody Mary) > > Claus > > > > > ------------------------------ > > Message: 5 > Date: Mon, 1 Jun 2009 20:00:09 +0100 > From: "Claus Reinke" <claus.reinke(a)talk21.com> > Subject: Re: [Haskell-cafe] Re: Error message reform (was: Strange > type errorwith associated type synonyms) > To: <haskell-cafe(a)haskell.org> > Message-ID: <8C39CAA7170D479D88613F7F77920916@cr3lt> > Content-Type: text/plain; format=flowed; charset="iso-8859-1"; > reply-type=response > > > I once thought, that error messages must be configurable by libraries, > > too. This would be perfect for EDSLs that shall be used by non-Haskellers. > > Yes, that is a problem. > > > But I have no idea how to design that. > > There was some work in that direction in the context of the Helium > project. See the publication lists of Bastiaan Heeren and Jurriaan Hage: > > http://people.cs.uu.nl/bastiaan/#Publications > http://www.cs.uu.nl/research/techreps/aut/jur.html > > Claus > > > > > ------------------------------ > > Message: 6 > Date: Mon, 1 Jun 2009 14:30:00 -0700 (PDT) > From: michael rice <nowgate(a)yahoo.com> > Subject: Re: [Haskell-cafe] Missing a "Deriving"? > To: haskell-cafe(a)haskell.org, Daniel Fischer > <daniel.is.fischer(a)web.de> > Message-ID: <789959.35516.qm(a)web31103.mail.mud.yahoo.com> > Content-Type: text/plain; charset="iso-8859-1" > > Got it. > > Thanks! > > Michael > > --- On Mon, 6/1/09, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > > From: Daniel Fischer <daniel.is.fischer(a)web.de> > Subject: Re: [Haskell-cafe] Missing a "Deriving"? > To: haskell-cafe(a)haskell.org > Date: Monday, June 1, 2009, 1:51 PM > > Am Montag 01 Juni 2009 19:02:36 schrieb michael rice: > > > All good so far, but then tried to convert Failable from Computation to > > Monad > > > > > > instance Monad Failable where > > return = Success > > fail = Fail > > >>= (Success x) f = f x > > >>= (Fail s) _ = Fail s > > mplus (Fail _) y = y > > mplus x _ = x > > > > > > and got the following error. > > > > > > Prelude> :l graph5 > > [1 of 1] Compiling Main ( graph5.hs, interpreted ) > > > > graph5.hs:34:4: parse error on input `>>=' > > Failed, modules loaded: none. > > Prelude> > > > > When you use an operator in prefix position, you must enclose it in parentheses, like you > must enclose a function in backticks if you use it infix. > > So the definition of (>>=) should read > > > (>>=) (Success x) f = f x > (>>=) (Fail s) _ = Fail s > > or, defining it in infix position, > (Success x) >>= f = f x > (Fail s) >>= _ = Fail s > > > > > Complete code follows. > > > > Michael > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 02 Jun '09

02 Jun '09

I didn't know I could do that. Works fine. Output below. Thanks! This is some pretty neat stuff, and I've only scratched the surface. Michael =================== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:37:9: Warning: No explicit method nor default method for `mzero' In the instance declaration for `MonadPlus Failable' Ok, modules loaded: Main. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 :: Failable [Int] Success [1,2,3] *Main> searchAll g 3 1 :: Failable [Int] Fail "no path" *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 3 1 :: Maybe [Int] Nothing *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 3 1 :: [[Int]] [] *Main> --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:43 PM Oh I wasn't clear -- you need multiple instance declarations for a given type (Failable, for example), one for each type class you're implementing. That is, instance Monad Failable where return = ... ... instance MonadPlus Failable where mplus = ... ... -Ross On Jun 1, 2009, at 9:40 PM, michael rice wrote: Hi Ross, I thought of that, but return, fail, and >>= became "not visible" when I changed the instance declaration from Monad to MonadPlus.. Can Failable be in two instance declarations, one for Monad (giving it return, fail, and >>=) and one for MonadPlus (giving it mplus)? Michael --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:33 PM mplus is a method of class MonadPlus, so you need to write it in a separate instance from the one for Monad, e.g. instance MonadPlus Failable where mplus = ... -Ross On Jun 1, 2009, at 9:28 PM, michael rice wrote: Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 02 Jun '09

02 Jun '09

Hi Ross, I thought of that, but return, fail, and >>= became "not visible" when I changed the instance declaration from Monad to MonadPlus.. Can Failable be in two instance declarations, one for Monad (giving it return, fail, and >>=) and one for MonadPlus (giving it mplus)? Michael --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:33 PM mplus is a method of class MonadPlus, so you need to write it in a separate instance from the one for Monad, e.g. instance MonadPlus Failable where mplus = ... -Ross On Jun 1, 2009, at 9:28 PM, michael rice wrote: Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 02 Jun '09

02 Jun '09

Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 02 Jun '09

02 Jun '09

Got it. Thanks! Michael --- On Mon, 6/1/09, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: From: Daniel Fischer <daniel.is.fischer(a)web.de> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: haskell-cafe(a)haskell.org Date: Monday, June 1, 2009, 1:51 PM Am Montag 01 Juni 2009 19:02:36 schrieb michael rice: > All good so far, but then tried to convert Failable from Computation to > Monad > > > instance Monad Failable where > return = Success > fail = Fail > >>= (Success x) f = f x > >>= (Fail s) _ = Fail s > mplus (Fail _) y = y > mplus x _ = x > > > and got the following error. > > > Prelude> :l graph5 > [1 of 1] Compiling Main ( graph5.hs, interpreted ) > > graph5.hs:34:4: parse error on input `>>=' > Failed, modules loaded: none. > Prelude> > When you use an operator in prefix position, you must enclose it in parentheses, like you must enclose a function in backticks if you use it infix. So the definition of (>>=) should read (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s or, defining it in infix position, (Success x) >>= f = f x (Fail s) >>= _ = Fail s > > Complete code follows. > > Michael > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Error message reform (was: Strange type error with associated type synonyms)
by Max Rabkin 02 Jun '09

02 Jun '09

On Wed, May 27, 2009 at 11:05 PM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > i mean just changing the words to make obvious what type was got in > what way. and check it on beginners who don't yet read your > explanations, for example teachers may test it on their students > > my English is limited... for example, it may be like this: > > read x > > Error: type of x is Integer > while type of read argument should be String For me, this word order is an improvement, but it doesn't help to leave out the words "expected" and "inferred". To me, those words add explanation. Of course they don't help if you don't know what they mean. I think they add something because they explains where the types "came from". When I get type errors, I often think "why do you say True has type Bool" (of course in most cases it is not so transparent). Although the compiler doesn't say *why* it inferred that type, at least it says it did infer it. I prefer this wording: The inferred type of `True' is `Bool', while the type of the first argument of `f' should be `Int'. In the expression: f True I prefer all three to Hugs's ERROR - Type error in application *** Expression : f True *** Term : True *** Type : Bool *** Does not match : Int --Max

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 02 Jun '09

02 Jun '09

I went back and tried to convert the YAHT example to Monad, importing Monad, commenting out all but the data descriptions and the searchAll function, and finally replacing success, failure, augment, and combine in the searchAll function with return, fail, >>=, and mplus. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 3 1 :: Maybe [Int] Nothing *Main> searchAll g 3 1 :: [[Int]] [] All good so far, but then tried to convert Failable from Computation to Monad instance Monad Failable where return = Success fail = Fail >>= (Success x) f = f x >>= (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x and got the following error. Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:34:4: parse error on input `>>=' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail >>= (Success x) f = f x >>= (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es

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MySQL, CouchDB, and Haskell
by Henry Laxen 02 Jun '09

02 Jun '09

Dear Group, I've spent the last few days trying to convert a bunch of mysql tables into couchdb using haskell, and I've documented my efforts, in case anyone else intends to wander in similar waters. The tutorial is at: http://maztravel.com/haskell/mySqlToCouchDB.html comments welcome here at the list or to me privately at nadine.and.henry -AT- pobox.com Best wishes, Henry Laxen

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How to implement this? A case for scoped record labels?
by ntupel＠googlemail.com 02 Jun '09

02 Jun '09

Hi, I would like to get some advice on how to best implement a protocol. The protocol in question is "Bayeux": http://svn.cometd.org/trunk/bayeux/bayeux.html. The details don't matter here - it defines a couple of requests and responses in JSON format, basically JSON objects with different properties, some of which are shared by all (e.g. "channel") and some which are specific for certain kinds of requests/responses (e.g. "subscription"). To give an example, a connect request would look like this: [ { "channel": "/meta/connect", "clientId": "Un1q31d3nt1f13r", "connectionType": "long-polling" } ] Now I leave the actual JSON parsing to the excellent Text.JSON library. My problem is how to get the types right. At first I started with a big discriminated union, e.g. data BayeuxMessage = HandshakeRequest { channel :: String , ... } | HandshakeResponse { channel :: String, successful :: Bool, ... } | ... This way I could create BayeuxMessage values by copying the Text.JSON parsed values over. However what I don't like is that many selector functions, e.g. successful, are only partial and using them with a BayeuxMessage value constructed with HandshakeRequest for example will result in a runtime error. So I think it would be better to have individual types for the protocol requests/responses, e.g. data HandshakeRequest = HandshakeRequest { channel :: String , ... } data HandshakeResponse = HandshakeResponse { channel :: String, successful :: Bool, ... } ... data BayeuxMessage = HSReq HandshakeRequest | HSRes HandshakeResponse ... This however does not work because record selectors have module scope, so the compiler will complain that channel et. al. are defined multiple times. As a workaround I could put each type into its own module, but at least GHC requires a file per module (which is *very* inconvenient IMO). If we would have scoped labels (e.g. like proposed here: http://legacy.cs.uu.nl/daan/pubs.html#scopedlabels) it seems like it would have been straightforward. So certainly I am missing something and there is a better way to design this. Hence this e-mail. I welcome any advice how this would best be done in Haskell with GHC. Many thanks, nt

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GUI and background processing
by Dmitry V'yal 01 Jun '09

01 Jun '09

Greetings, fellow haskellers. Currently I'm writing some kind of web crawler in haskell with gtk2hs gui. All network operations are run in separate thread, but sometimes input from user is needed. Afaik, gtk2hs is not thread safe, so I came up with following: I create two mvars, A and B, one for question, one for answer. Worker thread puts question in A and blocks trying to take B. Gui thread checks for question by polling with tryTakeMVar. Then question is ready, it asks user and puts his answer into B. Worker awakens and continues processing. Also, for progress report, I use a Chan which is written to by worker and is read by polling by gui thread. This scheme works, but I don't like it. All these mvars and chans create too much clutter, and polling looks rather ugly too. Is there a better way for implementing similar patterns of interaction? Best regards, Dmitry.

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RE: iota
by Paul Keir 01 Jun '09

01 Jun '09

That is quite spectacular. I revised my knowledge of sequence with a little function, akin to "sequence [xs1,xs2]": seq2 xs1 xs2 = do x1 <- xs1 x2 <- xs2 return [x1,x2] > seq2 [0,1] [0,1,2] > [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2]] I like your point-free style too; and that's a nice use of pred. Many thanks, Paul The iota function you're looking for can be a whole lot simpler if you know about monads (list monad in particular) and sequence. For lists, sequence has the following behaviour: sequence [xs1,xs2, ... xsn] = [[x1,x2, ... , xn] | x1 <- xs1, x2 <- xs2, ... , xn <- xsn] Using this, you can reduce your iota function to a powerful one-liner: iota = sequence . map (enumFromTo 0 . pred) Kind regards, Raynor Vliegendhart From: Paul Keir Sent: 01 June 2009 10:01 To: haskell-cafe(a)haskell.org Subject: iota Hi all, I was looking for an APL-style "iota" function for array indices. I noticed "range" from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: > let (a,b) = (2,3) > index ((0,0),(a-1,b-1)) > [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn't long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) => [a] -> [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] >>= genericReplicate r lists = map buildRepList tups in transpose lists > length $ iota [2,3,4] > 24 Thanks, Paul

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iota
by Paul Keir 01 Jun '09

01 Jun '09

Hi all, I was looking for an APL-style "iota" function for array indices. I noticed "range" from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: > let (a,b) = (2,3) > index ((0,0),(a-1,b-1)) > [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn't long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) => [a] -> [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] >>= genericReplicate r lists = map buildRepList tups in transpose lists > length $ iota [2,3,4] > 24 Thanks, Paul

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problem with inf-haskell + ghci?
by Alex Ott 01 Jun '09

01 Jun '09

Hello all I recently found strange problem in use of inf-haskell + ghci on my Mac OS X Tyger. I haven't used inf-haskell for a some time, and several days ago i found, that it stopped to work - when I run C-c C-l (load file) it signal error, and when I perform C-c C-b (start interpreter) it load it, but entering of any text in it, leads to message: Leaving GHCi. Process haskell finished In terminal ghci works fine, but I couldn't find why ghci terminates when it called from Emacs. May be somebody had such problem with ghci? P.S. I use Carbon Emacs 22 + ghc 6.10.1 installed from macports -- With best wishes, Alex Ott, MBA http://alexott.blogspot.com/ http://xtalk.msk.su/~ott/ http://alexott-ru.blogspot.com/

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(Pre-) Announce: Data.GDS 0.1.0
by Uwe Hollerbach 01 Jun '09

01 Jun '09

Hello, all, I'm hereby announcing Data.GDS, a small module to write and (eventually -- that's part of the "pre") read GDS files. For those of you not in the semiconductor biz, GDS-II is one of the classic formats of the industry. It's perhaps ever so slightly obsolete at this point, as the OASIS format is in the process of displacing it, but there are still huge numbers of designs in GDS format, and lots and lots of tools deal with it. Since I'm a sad sick weirdo(*), I spent a perfectly nice & sunny NorCal day hacking up this initial version of this module. It is to the point where it can generate a GDS file of your devising, although your specification of it still has to be at a very low level. It would be, and eventually will be, nicer to specify things at a higher level of abstraction. Also, it will eventually be nice to be able to read GDS files, returning an array of GDSRecord. I know how to do that, and I plan to, but I haven't got there yet. This ought to already be properly cabalized, and there's a small test program included; run it, save the output somewhere, and compare that with the sample GDS file which I also included in the tarball. If you examine the GDS file itself, you will see that, although it is small, it does in fact contain vital bits of design which will no doubt enable the biz to continue Moore's law for at least another century or so. Once I've implemented the reader, I'll upload this to hackage; in the meantime, if any of you are especially interested in what the rest of the interface to this should look like, I'm happy to hear your suggestions! Uwe (*) In point of fact, I am neither sad nor sick; I am in fact mostly happy & healthy. The reason I wasn't out taking a long walk today was because, alas, I dinged one achilles tendon a few days ago, and wanted to let it heal a bit... as to the "weirdo" charge, I beg you, gentle readers, avert your eyes while I plead no contest! :-)

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Re: [Haskell-cafe] Parsec float
by Bartosz Wójcik 01 Jun '09

01 Jun '09

On Friday 29 May 2009 08:34:36 you wrote: > myfloat = try (do{ symbol "-"; n <- float; return (negate n) }) <|> > try float <|> > do { i<-integer; return(fromIntegral i) } Thank you, this is an easy and nice solution. I've made it a bit prettier optically: myFloat = try (symbol "-" >> float >>= return . negate) <|> try float <|> (integer >>= return . fromIntegral) Best regards, Bartek

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Lazy Parsing
by Günther Schmidt 01 Jun '09

01 Jun '09

Hi all, is it possible to do lazy parsing with Parsec? I understand that one can do that with polyparse, don't know about uulib, but I happen to be already somewhat familiar with Parsec, so before I do switch to polyparse I rather make sure I actually have to. The files it has to parse is anywhere from 500 MB to 5 GB. Günther

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Re: Lazy Parsing
by Guenther Schmidt 31 May '09

31 May '09

Dear Doaitse, thank you very much for your help. > > I am curious to know what made you go wrong with the tutorial, and > caused that you could not find the solution below? > Well let's first agree that I'm not very bright. I hate to admit it, but it's a simple fact ;-). Second let's agree that the uu-parsinglib is a *very* sophisticated beast, I have not seen anything else like it out there, my sincere congratulations for it. Thirdly the tutorial is also a very sophisticated beast, and forthly, well just see point 1 :-). And I just figured out why I was unable to write even that simple parser. The code you sent me works just fine, I copied and pasted it, no problems. But, as soon as I comment out the "main" function the type checker complains, because now the ghci's type checker can no longer infer the types of pDate or pDot. And this is exactly what happened. I kept getting error messages from ghci, eventhough I had defined my parsers possible correctly, but, *minus* the type signatures *and* minus any main function that called it. In hindsight I realize that this is a trap I have walked into many times before, I guess I still have not acquired a Haskellers intuition. I promise to do better next time :) Günther

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Type families in export lists
by Maurício 31 May '09

31 May '09

Hi, How do I include type families (used as associated types) in a module export list? E.g.: class MyClass a where type T a :: * coolFunction :: Ta -> a (...) If I just include MyClass and its functions in the list, instances in other modules complain they don't know T, but I wasn't able to find how (where) to include T in the list. Thanks, Maurício

3 3

(no subject)
by Vladimir Reshetnikov 31 May '09

31 May '09

Hi, Seems that Haskell allows to specify "dummy" type variables in a declaration of a type synonym, which do not appear in its right-hand side. This can lead to interesting effects, which appears differently in GHC and Hugs. I would like to know, what behavior is correct according to the haskell 98 report. 1) ------------------------------------------ type F a = Int class A a where foo :: A b => a (F b) ------------------------------------------ GHC - OK Hugs - Illegal type "F b" in constructor application 2) ------------------------------------------ type F a = Int class A a where foo :: F a instance A Bool where foo = 1 instance A Char where foo = 2 xs = [foo :: F Bool, foo :: F Char] ------------------------------------------ GHC: M.hs:14:6: Ambiguous type variable `a' in the constraint: `A a' arising from a use of `foo' at M.hs:14:6-8 Probable fix: add a type signature that fixes these type variable(s) M.hs:14:21: Ambiguous type variable `a1' in the constraint: `A a1' arising from a use of `foo' at M.hs:14:21-23 Probable fix: add a type signature that fixes these type variable(s) Hugs: [1,2] Thanks, Vladimir

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GHCi vs. Hugs (record syntax)
by Vladimir Reshetnikov 31 May '09

31 May '09

Hi, I tried to evaluate this expression: head[[]{}] GHCi: [] Hugs: ERROR - Empty field list in update What is the correct behavior? Thanks, Vladimir

3 2