- Haskell-Cafe - Haskell.org

Uploading files to the wiki
by Wouter Swierstra 16 Mar '09

16 Mar '09

I can't manage to upload files to the Haskell wiki. I've tried different browsers, different internet connections, different machines, different operating systems, and different user accounts - all without success. Is this a new anti-spam measure? This is slightly annoying. I was looking to release the next Monad.Reader on the wiki. Thanks for any advice, Wouter

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Re: [Haskell-cafe] Dynamically typing TH.Exp at runtime
by Martin Hofmann 16 Mar '09

16 Mar '09

Sorry, maybe it I didn't made it clear enough. > Perhaps I'm misunderstanding something, but since one can infer types > in GHCI, that implies one can infer types in the GHC API; since Hint > wraps the GHC API, that implies one can infer types in Hint, doesn't > it? And indeed, there are functions to infer the type of a String; > iirc I've even used them myself in mueval. > That's true, inferring the type of a string is not the problem. However, I am not only interested in the top-level type, but also in the type of any arbitrary subexpression. Consider for example following recursive definition: foo [] = [] foo (x:xs) = (:)(x+1)(foo xs) Typing only the first rule one would says its type is '[a] -> [a]'. However, the second rule specialises the type to 'Num a => [a] -> [a]'. So, if I am interested in the type of the 'x' in the pattern of the second rule, I need to be aware of its binding on the right-hand side. Consequently the type of 'x' is 'Num a'. AFAIK, I need Hendley-Milner type inference for that (please correct me if I am wrong) and I was wondering if I really need to implement it by myself, when it is already somewhere inside GHC. Thanks, Martin

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examples for error handling in takusen?
by Gü?nther Schmidt 16 Mar '09

16 Mar '09

Hi, can someone please point me to error handling examples with takusen? I try to run a piece of code with takusen but just get the very sparse "Database.InternalEnumerator.DBException

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A new haskell tools project
by L.Guo 16 Mar '09

16 Mar '09

Hi all: I have just created an haskell work toolset project in google code. Currently there are only a few tools to process YUV image files I wrote before. Welcome you attend in to improve my code or to add in your tools. And also pleasure to get your advices. http://code.google.com/p/haskellworkingtools/ Regards -------------- L.Guo 2009-03-16

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Re: is there a version of hsffig that builds on a recent versin of haskell?
by Dmitry Golubovsky 16 Mar '09

16 Mar '09

On Mar 15, 4:03 am, Anatoly Yakovenko <aeyakove...@gmail.com> wrote: > is there a version of hsffig that builds on a recent versin of haskell? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe Unfortunately no. There have been too many changes in GCC and GHC since, and too low interest to the project from other developers, so I did not maintain it. Dmitry.

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Most elegant funciton for removing adjacent duplicates from a list using foldl and foldr
by R J 16 Mar '09

16 Mar '09

I need to write an implementation using foldl, and a separate implementation using foldr, of a function, "remdups xs", that removes adjacent duplicate items from the list xs. For example, remdups [1,2,2,3,3,3,1,1]= [1,2,3,1]. My approach is first to write a direct recursion, as follows: remdups :: (Eq a) => [a] -> [a] remdups [] = [] remdups (x : []) = [x] remdups (x : xx : xs) = if x == xx then remdups (x : xs) else x : remdups (xx : xs) This code works, but it has three cases, not usual two, namely [] and (x : xs). What, if any, is the implementation using only two cases? Also, if three cases are required, then how can it be implemented using foldr, and how using foldl? Thanks. _________________________________________________________________ Express your personality in color! Preview and select themes for Hotmail®. http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGT…

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Help with Bird problem 4.5.6: sequence of successive maxima
by R J 16 Mar '09

16 Mar '09

This Bird problem vexes me, in the first instance because it doesn't seem to specify a unique solution: Given a list xs = [x_1, x_2, . . . , x_n], the sequence of successive maxima "ssm xs" is the longest subsequence [x_j1, x_j2, x_j3..x_jk] such that j_1 = 1 and j_m < j_n => x_jm < x_jn. For example, xs = [3, 1, 3, 4, 9, 2, 10, 7] => ssm xs = [3, 4, 9, 10]. Define "ssm" in terms of "foldl". >From this specification, I infer: ssm [] = [] ssm [1] = [1] ssm [1, 2, 3] = [1, 2, 3] ssm [1, 0, 3, 2] = [1, 3] However, what is ssm [1,0,100,2,3,4,5]? Is it [1, 100] or [1, 2, 3, 4, 5]? I think the latter, but am not certain. Whichever it is, what's the solution? Thanks. _________________________________________________________________ Windows Live™ Groups: Create an online spot for your favorite groups to meet. http://windowslive.com/online/groups?ocid=TXT_TAGLM_WL_groups_032009

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State monad is missing Applicative instance
by Peter Verswyvelen 16 Mar '09

16 Mar '09

I think. Or is it defined in some other package?

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big discussion about Haskell on Reddit
by FFT 16 Mar '09

16 Mar '09

I noticed that on Programming Reddit, where I lurk, there is a big discussion about the disconnect between how much Haskell is advocated there and the number of applications written in it. http://www.reddit.com/r/programming/comments/84sqt/dear_reddit_i_am_seeing_… The difficulty of reasoning about memory and CPU-efficiency in nontrivial programs was suggested as an explanation.

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I can't install hs-plugins beceause of Linker.h
by Yuri Kashnikoff 16 Mar '09

16 Mar '09

Hi! I was trying to install hs-plugins both from sources "$./Setup.lhs ..." and with "$cabal fetch&&cabal install" and both failed. It always reports that Linker.h is missing. I have tried to use --extra-include-dirs with path to ghc-6.10.1 dir and ghc-6.10.1/include dir. Nothing changed. Here is a report from Setup.lhs: Setup.lhs:2:2: Warning: In the use of `defaultUserHooks' (imported from Distribution.Simple): Deprecated: "Use simpleUserHooks or autoconfUserHooks, unless you need Cabal-1.2 compatibility in which case you must stick with defaultUserHooks" Warning: defaultUserHooks in Setup script is deprecated. Configuring plugins-1.4.0... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package ghc-6.10.1 requires Cabal-1.6.0.1 package plugins-1.4.0 requires Cabal-1.6.0.2 checking build system type... i386-apple-darwin9.6.0 checking for ghc... ghc checking for value of __GLASGOW_HASKELL__... 610 checking for ghc library directory... /opt/local/lib/ghc-6.10.1/. checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ANSI C... none needed checking for arc4random... yes checking for a BSD-compatible install... /usr/bin/install -c configure: creating ./config.status config.status: creating config.mk config.status: creating testsuite/makewith/io/TestIO.conf config.status: creating testsuite/makewith/unsafeio/Unsafe.conf config.status: creating config.h config.status: config.h is unchanged Setup.lhs: Missing dependency on a foreign library: * Missing header file: Linker.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. -- Yuri S. Kashnikov Novosibirsk State University, Russia 2 Pirogova street 630090, Novosibirsk-90 yuri.kashnikoff(a)gmail.com

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Link errors
by Lingappan, Loganathan 16 Mar '09

16 Mar '09

Hi, If I include import Text.Regex.Posix ((=~)) into a Haskell code, I get the following link error: FindBBUsage.o:fake:(.text+0x44d): undefined reference to `__stginit_regexzmposixzm0zi72zi0zi3_TextziRegexziPosix_' collect2: ld returned 1 exit status Any ideas on how to fix this? I am using GHC version 6.10.1 on Windows XP. Thanks, Logo

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ANNOUNCE: dzen-utils 0.1
by Felipe Lessa 15 Mar '09

15 Mar '09

Hello! I'm please to announce dzen-utils 0.1. I don't feel like it is ready to be released, but let's see how everything goes. :) == WHAT == dzen-utils contains various utilities for creating dzen input strings in a type-safe way using some combinators, including the ability to apply colors locally (instead of applying for everything beyond some point). It can also emulate dbar and gdbar, do automatic padding, and more. == WHY == I don't like to write bash scripts for dzen, and I don't like the way xmobar treats its configuration and its inability to show anything other than text. This package is a short-term solution for my problems. Maybe I'll write xmozen in the future. ;) == WHERE, HOW, etc. == It is in Hackage already, go get it[1]. Please see the documentation of System.Dzen for some small examples. [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dzen-utils Thanks for your time, -- Felipe.

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How to catch error in array index when debugging
by Colin Paul Adams 15 Mar '09

15 Mar '09

I'm getting a runtime failure "Error in array index". This causes ghci to exit. Is there a way to get it to break instead, so I can find out which function is failing? -- Colin Adams Preston Lancashire

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Re: [Haskell-cafe] Default Tuple Orderings
by Martijn van Steenbergen 15 Mar '09

15 Mar '09

Mark Spezzano wrote: > In other words my best guess is that the ordering of tuples is by > comparing the first tuple elements and then the seconds, thirds etc, > until there’s an element less than another. ...until there's an element not equal to the other, yes. It finds the first position at which the two elements in the tuples are unequal and returns the result of comparing those two elements (LT or GT). If there's no such position, then the tuples are equal. See also: http://en.wikipedia.org/wiki/Lexicographical_order HTH, Martijn.

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is there a version of hsffig that builds on a recent versin of haskell?
by Anatoly Yakovenko 15 Mar '09

15 Mar '09

is there a version of hsffig that builds on a recent versin of haskell?

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Default Tuple Orderings
by Mark Spezzano 15 Mar '09

15 Mar '09

Hi, Does anyone know how Haskell’s tuple ordering works? Presumably for some tuple T1 (x1,x2,...xn) and another tuple T2 (y1,y2,...yn) Haskell would compare T1 and T2 as follows: T1 < T2 if: x1 < y1 OR otherwise if x1 == y1 then if x2 < y2 OR otherwise if x2 == y2 then if x3 < y3 OR otherwise etc xn < yn OR otherwise both tuple are equal In other words my best guess is that the ordering of tuples is by comparing the first tuple elements and then the seconds, thirds etc, until there’s an element less than another. Is this reasoning correct? So minimum xs would yield the “smallest” tuple (according to the above rules and if xs is a list of tuples)? Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.13/2001 - Release Date: 14/03/2009 6:54 AM

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A systematic method for deriving a defintion of foldl using foldr?
by R J 15 Mar '09

15 Mar '09

foldl and foldr are defined as follows: foldr :: (a -> b -> b) -> b -> [a] -> b foldr f e [] = e foldr f e (x : xs) = f x (foldr f e xs) foldl :: (b -> a -> b) -> b -> [a] -> b foldl f e [] = e foldl f e (x : xs) = foldl f (f e x) xs 1. I understand how these definitions work, and yet I'm unable to implement foldl in terms of foldr. What's a systematic approach to identifying such an implementation, and what is the implementation? 2. I believe that the reverse implementation--namely, implementing foldr in terms of foldl--is impossible. What's the proof of that? 3. Any advice on how, aside from tons of practice, to develop the intuition for rapidly seeing solutions to questions like these would be much appreciated. The difficulty a newbie faces in answering seemingly simple questions like these is quite discouraging. _________________________________________________________________ Express your personality in color! Preview and select themes for Hotmail®. http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGT…

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Alternative to Data.Binary
by Grzegorz Chrupala 15 Mar '09

15 Mar '09

Hi all, Is there a serialization library other than the Data.Binary from hackage? I am using Data.Binary in a couple of projects, but I have found its stack and memory usage very hard to control. Its very common that decoding a map or list of non-trivial size uses up all available RAM, or causes a stack overflow. I don't care that much about it being extremely fast, I just want to stop worrying that if I try to read a file a few percent larger than the last time, my program will suddenly stop working. Best, -- Grzegorz -- View this message in context: http://www.nabble.com/Alternative-to-Data.Binary-tp22512229p22512229.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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RE: Hashing over equivalence classes
by Galchin, Vasili 15 Mar '09

15 Mar '09

Hi Roman, So are you really talking about an equivalence relation on the function's domain? The reason I ask is that it is well known that "f(a)=f(b)" establishes an equivalence relation on f's co-domain! Vasili

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Does anybody dislike implicit params as much as I do?
by Thomas Hartman 15 Mar '09

15 Mar '09

http://blog.patch-tag.com/2009/03/09/implicitparams-are-evil-thoughts-on-ad… I understand there are arguments for using IPs, but after this experience, the ImplicitParams extension is a "code smell" for me.

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Style Guide for Haskell Code
by Manlio Perillo 15 Mar '09

15 Mar '09

After a quick search with Google, it seems that there is not yet an "official" document for Style Guide for Haskell Code. I was only able to found: http://www.cs.caltech.edu/courses/cs11/material/haskell/misc/haskell_style_… http://urchin.earth.li/~ian/style/haskell.html http://github.com/tibbe/haskell-style-guide/tree/master and, of course, the GHC style guide. Any plans for a common document? Thanks Manlio Perillo

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Proof of induction case of simple foldl identity.
by R J 15 Mar '09

15 Mar '09

Can someone provide the induction-case proof of the following identity: foldl (-) ((-) x y) ys = (foldl (-) x ys) - y If foldl is defined as usual: foldl :: (b -> a -> b) -> b -> [a] -> b foldl f e [] = e foldl f e (x : xs) = myFoldl f (f e x) xs The first two cases, _|_ and [], are trivial. Here's my best attempt at the (y : ys) case (the left and right sides reduce to expressions that are obviously equal, but I don't know how to show it): Case (y : ys). Left-side reduction: foldl (-) ((-) x y) (y : ys) = {second equation of "foldl"} foldl (-) ((-) ((-) x y) y) ys = {notation} foldl (-) ((-) (x - y) y) ys = {notation} foldl (-) ((x - y) - y) ys = {arithmetic} foldl (-) (x - 2 * y) ys Right-side reduction: (foldl (-) x (y : ys)) - y = {second equation of "foldl"} (foldl (-) ((-) x y) ys) - y = {induction hypothesis: foldl (-) ((-) x y) ys = (foldl (-) x ys) - y} ((foldl (-) x ys) - y) - y = {arithmetic} (foldl (-) x ys) - 2 * y Thanks as always. _________________________________________________________________ Express your personality in color! Preview and select themes for Hotmail®. http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGT…

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Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Henning Thielemann 15 Mar '09

15 Mar '09

On Fri, 13 Mar 2009, Mark Spezzano wrote: > 1. Don’t bother. Just use Integer. > > 2. Use the type > > data Natural = Zero | Succ !Natural > > 3. Use the following definition taken from the Gentle Introduction to Haskell 98 > > newtype Natural = MakeNatural Integer This option looks like non-negative package.

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What's new on Hackage this week? March 14, 2009
by Don Stewart 15 Mar '09

15 Mar '09

http://archhaskell.wordpress.com/2009/03/14/arch-haskell-news-mar-14-2009/ A regular update of Haskell in Arch Linux Arch now has 974 Haskell packages in AUR. That’s 12 new packages this week, and lots of updates as well. See the blog for the full list of updates. -- Don

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Microsoft PhD Scholarship at Strathclyde
by Conor McBride 15 Mar '09

15 Mar '09

Apologies for crossposting. Please forward this message to individuals or lists who may be interested. In addition to the recently advertised PhD position at Strathclyde on "Reusability and Dependent Types", I am delighted to advertise the following PhD opportunity. {--------------------------------------------------------- -- Haskell Types with Numeric Constraints ---------------- ---------------------------------------------------------} We are grateful to Microsoft Research for their sponsorship of this project, which includes an internship, and with it the chance to make a real difference to world of principled but practical programming. The project investigates the practical and theoretical impact of extending Haskell's type system with numeric expressions (representing sizes, or ranges, or costs, for example) and constraints capturing richer safety properties than are currently managed by static typing. It has three strands: (1) to investigate type inference with numeric constraints, (2) to investigate new programming structures, patterns, and techniques which exploit numeric indexing, and (3) to study the performance benefits derivable from richer guarantees. A bright student could bring significant benefits to developers using Haskell, a language with increasing industrial traction — not least at Microsoft. Work on the Glasgow Haskell Compiler, at Strathclyde! {---------------------------------------------------------} The position is fully funded, covering stipend, fees (at the home/EU rate), equipment, and travel, starting in October 2009. The closing date for applications is 15th April 2009. For further details, see: http://personal.cis.strath.ac.uk/~conor/phds/ or email me (conor(a)cis.strath.ac.uk) I look forward to hearing from you. Yours &c Conor McBride

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building c-callable shared objects and dlls with Haskell
by Anton Tayanovskyy 15 Mar '09

15 Mar '09

Hello, What's the current best practice to build shared objects / dlls to call Haskell from C? I think I figured out the basics, but I would like to understand the picture completely and document the current state of affairs in the Haskell wiki. In particular, I'd like to make sure that the shared libraries are usable from multiple applications, and ideally that applications can dynamically link to multiple shared libraries built from Haskell. I was trying to do that to call Pandoc from C, and the information was rather scattered. Here are two links that I used: http://www.haskell.org/haskellwiki/Calling_Haskell_from_C http://www.haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html Based on that information, I was able to compile and run small examples. However, I'm not sure I completely understand how it works in production. Specifically, for the UNIX version, I followed a recipe for loading and unloading the Haskell runtime with library constructors/destructors: #include <HsFFI.h> static void library_init(void) __attribute__((constructor)); static void library_init(void) { hs_init(..); } static void library_exit(void) __attribute__((destructor)); static void library_exit(void) { hs_exit(); } Does this work correctly? Another concern is that every library compiled this way will include its own copy of the Haskell runtime. Will there be problems when using multiple libraries? For the Windows counterpart, there is this very suspicious code I adapted from the GHC user guide: #include <windows.h> #include <Rts.h> extern void __stginit_LibPandoc(void); BOOL STDCALL DllMain( HANDLE hModule, DWORD reason, void* reserved) { if (reason == DLL_PROCESS_ATTACH) { static char* args[] = { "ghcDll", NULL }; startupHaskell(1, args, __stginit_LibPandoc); } return TRUE; } It does not seem to unload the Haskell runtime, indeed the Guide warns that it is unsafe to do so from DllMain. What are the implications of this? Thanks! --A

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broken IO support in uvector package, when using non primitive types
by Manlio Perillo 15 Mar '09

15 Mar '09

Hi. I'm sure this is a know bug, but I can't find details with Google. The offending code is here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2362 When I execute the program I get: uio: readChunkBU: can't read What's the problem? I'm using uvector from: http://code.haskell.org/~dons/code/uvector/ Thanks Manlio Perillo

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Re: [Haskell-cafe] Using a monad to decompose a function into functions
by Marcin Kosiba 15 Mar '09

15 Mar '09

On Friday 13 March 2009, you wrote: > 2009/3/13 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > > On Thursday 12 March 2009, you wrote: > >> 2009/3/12 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > >> > Hi, > >> > I'm doing a bit of research into mobility models and I'm > >> > currently exploring implementation language choices for the simulator > >> > (*snip*) > >> > The simulation algorithm requires expressing > >> > the node's mobility so that it is "stateless". The mobility model > >> > algorithm's type should be something like: > >> > mobility_model :: WorldState -> NodeState -> OtherInput -> (Action, > >> > NodeState) > >> > > >> > where Action can alter WorldState and the second NodeState is an > >> > altered input NodeState. I perform a form of speculative execution on > >> > mobility_model so sometimes I need to backtrack to a previous world > >> > and node state. This is all fairly simple stuff, and was just an > >> > introduction. What I do now is store an enum in NodeState and > >> > implement mobility_model as one big case statement. Yes, this is very > >> > imperative of me, I know. What I'd like to do is to express > >> > mobility_model, so that the code would look like: > >> > > >> > mobility_model world node input = do > >> > do_calculus > >> > emit_action > >> > if something > >> > then emit_action > >> > else emit_action > >> > do_calculus > >> > emit_action > >> > mobility_model world node input > >> > >> Hi, > >> > >> It seems you can use > >> http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control- > >>Mon ad-State-Lazy.html Just have a look at the exemple : > >> > >> tick :: State Int Int > >> tick = do n <- get > >> put (n+1) > >> return n > >> > >> your code would become something like > >> mobility_model :: OtherInput -> State (WorldState,NodeState) () > >> mobility_model input = do > >> world <- gets fst > >> node <- gets snd > >> .... > >> let (world',node') = ... > >> put (world',node') > > > > ok, that solves part of the problem. what this doesn't solve is that > > somewhere between these lines (which corespond to emit_action in my > > example) > > > >> let (world',node') = ... > >> put (world',node') > > > > I need to return a value and an Action and NodeState to the simulation > > algorithm. and then, after the simulation algorithm calculates a new > > WorldState it will want the mobility_model to where it left off, but with > > a new WorldState. > > > > I hope I'm clear about what I wish to achieve: each emit_action should > > return a value (Action, NodeState) and maybe a function > > mobility_model_cont which I then could call with the new WorldState to > > continue from where emit_action returned. > > I'm not entirely sure ... but I think it doesn't matter that much :) > Here is why. > > This was just an exemple : > mobility_model :: OtherInput -> State (WorldState,NodeState) () > > You could also have > mobility_model :: OtherInput -> NodeState -> State WorldState > (NodeState,Action) > or whatever. > > In fact, the State monad makes it easy to thread (in this context, it > means 'pass around') an argument to many functions, providing a nice > syntax reminiscent of imperative language. But it lets you completely > free of what is passed around. It depends on what you want to be > explicitely passed by argument, and what you want to pass in the state > of the monad (that is, what you want to appear, inside the monad only, > as some global variable). > > So in your code, if you often need to pass a WorldState to a function > which should return a modified WorldState, it makes sense to put > WorldState inside the state monad. But, maybe, if there is just a few > functions which act on NodeState, it has not to be part of the state > carried by the state monad. > > I'm not entirely sure of what is a problem to you : is it the use of > the State monad, or something else ? > If it can help you to formulate your question you can post some code > (or past it to http://hpaste.org/) Threading the state is not the problem. Maybe this will help: what I have now: fsm world state = case state of first -> do_stuff_one (move_up, succ state) second -> do_stuff_two (move_left, succ state) third -> do_stuff_three (move_right, first) what I'd want to have is to say: fsm world state = do do_stuff_one yield move_up do_stuff_two yield move_left do_stuff_three yield move_right fsm world state and have it "translated" to: fsm world state = do_stuff_one (move_up, \world' state' -> do_stuff_two (move_left, \world'' state'' -> do_stuff_three (move_right, fsm world'' state'') Thanks! Marcin Kosiba

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Haskell-Wiki Account registration
by Henning Thielemann 15 Mar '09

15 Mar '09

How long will the Wiki account registration be disabled? Would it be possible to ask a question, that real Haskellers could easily answer, but a spambot cannot? E.g. "What's Haskell's surname?"

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Problem with cabal-install where package requires* mutually exclusive versions of another package
by Mark Wassell 15 Mar '09

15 Mar '09

I am trying install a package using cabal-install however the package requies an older version of QuickCheck and one of the required packages requires the latest version: $ cabal fetch Reactive Resolving dependencies... cabal.exe: cannot configure Stream-0.3.1. It requires QuickCheck >=2.0 For the dependency on QuickCheck >=2.0 there are these packages: QuickCheck-2.1 and QuickCheck-2.1.0.1. However none of them are available. QuickCheck-2.1 was excluded because checkers-0.1.3 requires QuickCheck <2.0 QuickCheck-2.1 was excluded because reactive-0.10.5 requires QuickCheck <2.0 QuickCheck-2.1.0.1 was excluded because checkers-0.1.3 requires QuickCheck <2.0 QuickCheck-2.1.0.1 was excluded because reactive-0.10.5 requires QuickCheck <2.0 How can I get around this? I could work around this and install the packages individually. Can I have two versions of a package installed (ie QuickCheck) and will everything get resolved correctly? Mark

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bytestring vs. uvector
by Alexander Dunlap 15 Mar '09

15 Mar '09

Hi all, For a while now, we have had Data.ByteString[.Lazy][.Char8] for our fast strings. Now we also have Data.Text, which does the same for Unicode. These seem to be the standard for dealing with lists of bytes and characters. Now we also have the storablevector, uvector, and vector packages. These seem to be also useful for unpacked data, *including* Char and Word8 values. What is the difference between bytestring and these new "fast array" libraries? Are the latter just generalizations of the former? Thanks for any insight anyone can give on this. Alex

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Re: [Haskell-beginners] folds -- help!
by Adrian Neumann 14 Mar '09

14 Mar '09

That's why I said "for appropriate g and f". But I see that my wording was misleading. However you can say: > foldl' oplus alpha bs =(foldr f id bs) alpha where > f a g = \alpha -> g (alpha `oplus` a) > > foldr' oplus alpha bs = (foldl f id bs) alpha where > f g a = \alpha -> g (a `oplus` alpha) And it works as long as oplus is strict in both arguments. Am 10.03.2009 um 21:54 schrieb John Dorsey: > Adrian Neumann wrote: > >> Notice that there is no difference between >> >> foldr g a >> foldl f a >> >> (for appropriate g and f) if g and f are strict in both arguments. > > Be careful... as apfelmus noted elsewhere in this thread, that's > not (in > general) true. > > Prelude> foldr (^) 2 [3,5] > 847288609443 > Prelude> foldl (^) 2 [3,5] > 32768 > > The reason? Integer exponentiation (^) isn't associative and > commutative. So the first is (3 ^ (5^2)) = 3^25, while the second is > ((2 ^ 3) ^ 5) = 2^15. > > Cheers, > John > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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Haskell Weekly News: Issue 109 - March 14, 2009
by Brent Yorgey 14 Mar '09

14 Mar '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090314 Issue 109 - March 14, 2009 --------------------------------------------------------------------------- Welcome to issue 109 of HWN, a newsletter covering developments in the [1]Haskell community. Congratulations to the authors of RWH on their [2]Jolt award! Some cool libraries released this week (as usual), and some really cool PhD [3]opportunities at Strathclyde. Also, it seems that I was censured last week for not including any quotes in the HWN, which is because tunes.org (which hosts the #haskell logs) was down while I was putting it together. So, this time I've included quotes going back two weeks, I HOPE YOU'RE HAPPY Community News Tom DuBuisson (TomMD) has [4]moved to Portland and will be starting a PhD at Portland State soon. Announcements darcs fundraising drive - only $720 left to go!. Eric Kow [5]announced that [6]donations are still being accepted to help pay for travel to the upcoming Haskell hackathon. So far we have raised $280, so we're almost a third of the way there. Think you can help? Vintage BASIC 1.0. Lyle Kopnicky [7]announced the initial release of [8]Vintage BASIC, an interpreter for microcomputer-era BASIC. Fully unit-tested, it faithfully implements the common elements of the language. On the web site, you can find 102 games from the classic book BASIC Computer Games, all of which run flawlessly. Have fun! ThreadScope: Request for features for the performance tuning of parallel and concurrent Haskell programs. Satnam Singh [9]requested feedback on infrastructure for logging run-time events and a graphical viewer program called [10]ThreadScope. The goal is for these features to make it into the next release of GHC. torch-0.1. Yusaku Hashimoto [11]announced a new unit test framework, [12]torch-0.1. sparsebit 0.5 - Sparse Bitmaps for Pattern Match Coverage. Ki Yung Ahn [13]announced the release of the [14]sparsebit library. This library packages the functional peal paper [15]Sparse Bitmaps for Pattern Match Coverage submitted to ICFP 2009 by Ki Yung Ahn and Tim Sheard. happs-tutorial 0.8. Crieghton Hogg [16]announced the release of [17]happs-tutorial 0.8, which is compatible with [18]happstack-0.2. A number of changes have occurred in this release, including general code cleanup, migration to the new Happstack.Server.SimpleHTTP API, and more. Future 1.1.0 concurrency library. ChrisK [19]announced the [20]future package, which ought to do what C++ standard futures/promises do, plus a bit more. The main operation is forkPromise :: IO a -> IO (Promise a), which sets the "IO a" operation running in a fresh thread; the eventual result can be accessed in many ways (non-blocking, blocking, blocking with timeout). Holumbus-MapReduce 0.0.1. Stefan Schmidt [21]announced three new libraries: [22]Holumbus-MapReduce, [23]Holumbus-Distribution, and [24]Holumbus-Storage, which provide tools for building distributed systems. These libraries are used as the backbone of the [25]Holumbus search engine. Turbinado V0.6. Alson Kemp [26]announced the release of [27]Turbinado 0.6, a Rails-ish Model-View-Controller web serving framework for Haskell. New features include support for CGI serving, statically compiled Layouts, Views, and Controllers, lower case paths, support for cookies and encrypted cookie sessions, easier installation, and support for GHC 6.10. iteratee-0.1.0. John Lato [28]announced the hackage release of [29]iteratee-0.1.0. This library implements enumerators and iteratees [30]as proposed by Oleg Kiselyov. Harpy 0.4.4 - Runtime code generation for x86 machine code. Dirk Kleeblatt [31]announced the release of [32]Harpy 0.4.1, a library for runtime code generation for x86 machine code. The new release features additional Eq instances, support for new prefetching instructions, and some bug fixes. Discussion Suggestion for a Haskell mascot. Maur�cio [33]suggested using a sloth as the Haskell mascot. If you would like to know how to say 'sloth' in just about every language ever, read this thread. Jobs Microsoft PhD Scholarship at Strathclyde. Conor McBride [34]announced another PhD opportunity at Strathclyde, sponsored by Microsoft Research, to investigate the practical and theoretical impact of extending Haskell's type system with numeric expressions (representing sizes, or ranges, or costs, for example) and constraints capturing richer safety properties than are currently managed by static typing. Blog noise [35]Haskell news from the [36]blogosphere. * Braden Shepherdson: [37]Pimp Your XMonad #4: Urgency Hooks. * Thomas M. DuBuisson: [38]Explicit Parallelism via Thread Pools. * Ketil Malde: [39]Current developments. Ongoing development of the biohaskell libraries for bioinformatics. * Philip Wadler: [40]Cafe Scientifique. Philip will be giving a talk, "Proofs are Programs: 19th Century Logic and 21st Century Computing", on Monday in Edinburgh. * Yi: [41]Yi 0.6.0 Release Notes. * Alson Kemp: [42]ANNOUNCE: Turbinado V0.6. * GHC / OpenSPARC Project: [43]Thread activity plotting. * Real-World Haskell: [44]We won a Jolt Award!. * Real-World Haskell: [45]Real World Haskell on the Kindle 2. * Christophe Poucet (vincenz): [46]Bootstrapping cabal. * Gtk2HS: [47]Mickinator File Manager. * Osfameron: [48](rough) Grids in Haskell. Some notes on representing 2D grids in Haskell. * Manuel M T Chakravarty: [49]These graphs summarise the performance of Data Parallel Haskell.... * GHC / OpenSPARC Project: [50]Project midpoint. * GHC / OpenSPARC Project: [51]The GNU Debugger and me. * Don Stewart (dons): [52]Evolving faster Haskell programs. Using genetic algorithms to find optimal flag combinations. * Xmonad: [53]Xmonad and the Gimp. * Xmonad: [54]xmonad on eee. * Mark Wassell: [55]Conversations with a type checker. * Holumbus: [56]Holumbus-MapReduce on Hackage. Some neat libraries for building distributed systems! * Darcs: [57]darcs weekly news #20. * Nick Mudge: [58]First Bay Area Haskell Meeting. * Dan Piponi (sigfpe): [59]Dinatural Transformations and Coends. Quotes of the Week * mmorrow: when i first saw haskell i was like "holy shitfork! that's what i've been trying to do in C for forever!" * pastah: the maybe monad is like cheating. everything is so awesomelly easy. * wli: Monads are like constipation. Comonads are like Ex-Lax. * TomMD: Never trust IO. * Axman6: let blah f x = f (blah f x) in blah ("blah "++) "" * MyCatVerbs: Lazy IO is implemented in terms of unsafePerformIO, you, you, you silly bipedal carbon-based organism. * MyCatVerbs: Amdahl's law is mostly to be used for making people feel depressed. * Axman6: -ddump-occur-anal <- another terrible name... * AchimSchneider: Finite automata don't go bottom in any case, at least not if you don't happen to shoot them and their health drops below zero. * ski: the truth (semantics), the whole truth (completeness), and nothing but the truth (soundness) * f4hy: wait you can do a show on an infinite list?! (I am starting to think haskell is not a programming language, it is evil wizardry) * Baughn: concat $ forM [(1,2), (4,5)] $ \(a,b) -> show (b,a+b) About the Haskell Weekly News New editions are posted to [60]the Haskell mailing list as well as to [61]the Haskell Sequence and [62]Planet Haskell. [63]RSS is also available, and headlines appear on [64]haskell.org. To help create new editions of this newsletter, please see the information on [65]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [66]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.realworldhaskell.org/blog/2009/03/12/we-won-a-jolt-award/ 3. http://article.gmane.org/gmane.comp.lang.haskell.general/16944 4. http://tommd.wordpress.com/2009/03/13/explicit-parallelism-via-thread-pools/ 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54758 6. http://www.darcs.net/donations.html 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54652 8. http://www.vintage-basic.net/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54521 10. http://raintown.org/threadscope 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54445 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/torch 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54436 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sparsebit 15. http://kyagrd.dyndns.org/wiki/SparseBitmapsForPatternMatchCoverage 16. http://www.haskell.org//pipermail/haskell-cafe/2009-March/057305.html 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happs%2Dtutorial 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happstack 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54409 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/future 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54325 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus%2DMapRe… 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus-Distrib… 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus-Storage 25. http://www.holumbus.org/redmine/wiki/holumbus 26. http://article.gmane.org/gmane.comp.lang.haskell.general/16953 27. http://www.turbinado.org/ 28. http://article.gmane.org/gmane.comp.lang.haskell.general/16945 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/iteratee 30. http://okmij.org/ftp/Haskell/Iteratee/ 31. http://article.gmane.org/gmane.comp.lang.haskell.general/16942 32. http://uebb.cs.tu-berlin.de/harpy/ 33. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/54418 34. http://article.gmane.org/gmane.comp.lang.haskell.general/16944 35. http://planet.haskell.org/ 36. http://haskell.org/haskellwiki/Blog_articles 37. http://braincrater.wordpress.com/2009/03/14/pimp-your-xmonad-4-urgency-hook… 38. http://tommd.wordpress.com/2009/03/13/explicit-parallelism-via-thread-pools/ 39. http://blog.malde.org/index.php/2009/03/13/current-developments/ 40. http://wadler.blogspot.com/2009/03/cafe-scientifique.html 41. http://yi-editor.blogspot.com/2009/03/yi-060-release-notes.html 42. http://www.alsonkemp.com/haskell/announce-turbinado-v06/ 43. http://ghcsparc.blogspot.com/2009/03/thread-activity-plotting.html 44. http://www.realworldhaskell.org/blog/2009/03/12/we-won-a-jolt-award/ 45. http://www.realworldhaskell.org/blog/2009/03/13/real-world-haskell-on-the-k… 46. http://cpoucet.wordpress.com/2009/03/12/bootstrapping-cabal/ 47. http://haskell.org/gtk2hs/archives/2009/03/10/mickinator-file-manager/ 48. http://greenokapi.net/blog/2009/03/10/rough-grids-in-haskell/ 49. http://justtesting.org/post/85103645 50. http://ghcsparc.blogspot.com/2009/03/project-midpoint.html 51. http://ghcsparc.blogspot.com/2009/03/gnu-debugger-and-me.html 52. http://donsbot.wordpress.com/2009/03/09/evolving-faster-haskell-programs/ 53. http://xmonad.wordpress.com/2009/03/08/xmonad-and-the-gimp/ 54. http://xmonad.wordpress.com/2009/03/08/xmonad-on-eee/ 55. http://www.poundstone.org/blog/?p=23 56. http://holumbus.fh-wedel.de/blog/?p=18 57. http://blog.darcs.net/2009/03/darcs-weekly-news-20.html 58. http://nickmudge.info/?post=115 59. http://blog.sigfpe.com/2009/03/dinatural-transformations-and-coends.html 60. http://www.haskell.org/mailman/listinfo/haskell 61. http://sequence.complete.org/ 62. http://planet.haskell.org/ 63. http://sequence.complete.org/node/feed 64. http://haskell.org/ 65. http://haskell.org/haskellwiki/HWN 66. http://code.haskell.org/~byorgey/code/hwn/

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Vim support for cabal
by Pavel Perikov 14 Mar '09

14 Mar '09

Hi café ! I googled for some kind of vim support for cabal but found nothing. I mean syntax highlighting of .cabal and probably integration with haskellmode. Did anyone hear about such thing? Pavel

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Re: [Haskell-cafe] ANNOUNCE: Vintage BASIC 1.0
by Lyle Kopnicky 14 Mar '09

14 Mar '09

I've posted a new version, 1.0.1, that has a usage message if you run it with no arguments. There is a package for Windows now (not just a binary) as well as Linux. Both packages include documentation and example programs. Thanks for pointing that out! On Fri, Mar 13, 2009 at 1:31 AM, Wolfgang Jeltsch < jeltsch(a)informatik.tu-cottbus.de> wrote: > When running the executable, nothing happens. The executable should show a > usage message instead. Where are the BASIC programs? I think, you have to > grab them from the source. They should be installed, too. > > Best wishes, > Wolfgang >

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Re: [Haskell-cafe] Abuse of the monad [was: monadic logo]
by Andrew Wagner 14 Mar '09

14 Mar '09

Can you expand on this a bit? I'm curious why you think this. On Thu, Mar 12, 2009 at 10:13 AM, Thomas Davie <tom.davie(a)gmail.com> wrote: > > On 12 Mar 2009, at 15:04, Gregg Reynolds wrote: > > At risk of becoming the most hated man in all Haskelldom, I'd like to >> suggest that the Haskell logo not use lambda symbols. Or at least not as >> the central element. Sorry, I know I'm late to the party, but the thing is >> there is nothing distinctive about lambda; it's common to all FPLs. >> Besides, Lisp/Scheme already have that franchise. >> >> What is distinctive about Haskell it's use of the monad. The Pythagorean >> monad symbol is wonderfully simple: >> > No, what's distinctive about Haskell is usually the abuse of the monad. > > Encouraging people to think Haskell is all about monadic programming even > more is a recipe for disaster. > > Just my 2¢ > > Bob_______________________________________________ > > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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reddit.com/r/haskell_proposals: Relevance to Google Summer of Code?
by fallintothis 14 Mar '09

14 Mar '09

NOTE: This message was originally posted to the subreddit in question (and has been slightly modified), where I was told that I should post the question on haskell-cafe. See http://www.reddit.com/r/haskell_proposals/comments/847by/ask_relevance_to_g… I'm currently researching the possibility of applying for the Google Summer of Code[1] and (by way of dons's post[2]) have found myself perusing the Haskell Proposals subreddit for project ideas. As an accepted MS CS student, I've spent my undergraduate life annoying professors by using a slew of different programming languages -- most recently, Haskell. While not altogether unfamiliar with the language, I rather doubt (or at least try to honestly question) how useful a contributor I'd be to some of the larger projects that have been posted (e.g., the Immix garbage collector[3] or loop unrolling[4]), despite some of their acceptances as tickets on the Summer of Code Trac[5] and my deep interest in compiler & language theory. There are projects that match other interests of mine and (in my approximation) my experience level, but they also tend to be smaller. For example, the reddit-alike[6] or wiki-embedded Haddock[7]. So, background sap-story notwithstanding, I suppose my general question is: with respect to the aforementioned Summer of Code post, is this subreddit meant to be a place for appropriate SoC projects? It doesn't feel quite like it is, but I would be excited to work on some of those that I indicated. The telltale sign for a project's worthiness seems to be that a ticket is already opened on the Trac that spells out the appropriateness, but this is limiting. More obviously, to test the fitness of a project, should the idea be opened as a ticket for the cursory review of others? Should someone who wants to do Summer of Code work be at any particular level of experience ("dive into GHC's backend" versus "use Haskell to hack on a project")? Intuitively, 1 summer's worth of work could probably be found in any corner of code that meshes with any student's expertise and interests -- even a reddit-alike can actually be a lot of work. But the catch (for me) is that when familiarity is relatively low, it's hard to come up with such ideas. I realize that applications are competitive against other students, so the range of acceptable submissions would depend more on the other applicants, but I think it's also important to address haskell.org's ethos with regards to who they're looking for. If there's a more appropriate place to post this question (ha! singular) I'm more than happy to transplant it. [1] http://code.google.com/soc/ [2] http://www.reddit.com/r/programming/comments/7zfog/anybody_done_the_google_… [3] http://www.reddit.com/r/haskell_proposals/comments/7wjsc/implement_the_immi… [4] http://www.reddit.com/r/haskell_proposals/comments/7z1uf/implement_loop_unr… [5] http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2009 [6] http://www.reddit.com/r/haskell_proposals/comments/7u48x/a_simple_redditlik… [7] http://www.reddit.com/r/haskell_proposals/comments/7qnwq/haddock_with_embed…

1 0

Pointless functors
by Martijn van Steenbergen 14 Mar '09

14 Mar '09

Bonjour café, Are there any functors f for which no point/pure/return :: a -> f a exists? Thank you, Martijn.

9 14

Applicative combinators from Parsec
by Martijn van Steenbergen 13 Mar '09

13 Mar '09

Hello, Looking at Parsec 3 I see: > chainr1 :: (Stream s m t) => ParsecT s u m a -> > ParsecT s u m (a -> a -> a) -> ParsecT s u m a > chainr1 p op = scan where > scan = do x <- p; rest x > rest x = (do f <- op; y <- scan; return (f x y)) <|> return x > But if I remove the type signature and let GHC infer it for me, I get a much more generic type: > chainr1 :: (Alternative m, Monad m) => m a -> m (a -> a -> a) -> m a But we don't really need m to be a monad since we're only doing applicative operations, so after some rewriting we get: > chainr1 :: Alternative f => f a -> f (a -> a -> a) -> f a > chainr1 p op = scan where > scan = flip id <$> p <*> rest > rest = (flip <$> op <*> scan) <|> pure id Would it be a good idea to: 1) make the Parsec combinators as generic as possible and 2) move the really generic applicative ones to Control.Applicative? Thanks, Martijn.

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Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Brandon S. Allbery KF8NH 13 Mar '09

13 Mar '09

On 2009 Mar 12, at 22:54, Mark Spezzano wrote: > I was wondering what the best way to implement Natural number would > be. Is there a package which already does this? type-level on Hackage. > 2. Use the type > data Natural = Zero | Succ !Natural One of the reasons people use type-level naturals is to get laziness; you've made this strict. Is there a reason? -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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Re: [Haskell-cafe] ANNOUNCE: Vintage BASIC 1.0
by Lyle Kopnicky 13 Mar '09

13 Mar '09

On Fri, Mar 13, 2009 at 1:31 AM, Wolfgang Jeltsch < jeltsch(a)informatik.tu-cottbus.de> wrote: > When running the executable, nothing happens. The executable should show a > usage message instead. Where are the BASIC programs? I think, you have to > grab them from the source. They should be installed, too. > Thanks for your feedback, and for checking out Vintage BASIC! I will add a usage message, as you suggested. Currently the usage is only described in the User's Guide. There are no options; you just give it the name of one or more BASIC source files. (Naturally this extends to doing nothing when supplied with zero source files.) The BASIC games are at http://www.vintage-basic.net/games.html. There is a tarball of all the games (bcg.tar.gz<http://www.vintage-basic.net/downloads/bcg.tar.gz>) as well. These are separated from the Vintage BASIC distribution for legal reasons. Vintage BASIC is copyrighted by me under a BSD license. The games were written by many authors and compiled into a book copyrighted by Creative Computing. I have secured permission from the editor to post the games. I do not wish to imply that they are covered by the same license as the interpreter. If anyone has any suggestions as to how to make the process simpler, I'd be happy to hear it. Thanks, Lyle P.S. I need to get my other e-mail account subscribed to this list....

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Sugestion for a Haskell mascot
by Maurício 13 Mar '09

13 Mar '09

Hi, Here in Brazil we have a forest animal we name 'preguiça' -- literally, lazyness. What better mascot we could have for Haskell? It lives (and sleeps) in trees, and if you see the main picture in wikipedia articles you can easily imagine the tree branch beeing replaced by a lambda: http://en.wikipedia.org/wiki/Sloth http://pt.wikipedia.org/wiki/Bicho-pregui%C3%A7a Best, Maurício

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Against cuteness
by Gregg Reynolds 13 Mar '09

13 Mar '09

Regarding logos/mascots: nothing personal folks, but I would like to cast a loud firm vote against all forms of cuteness, especially small furry animal cuteness. It's been done half to death, and we are not O'Reilly. Of all the billions of images from all the cultures in the world available to us we can surely find something that is witty or charming without being cute. -g

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Re: [Haskell-cafe] DSLs with {in,}equalities
by Alberto G. Corona 13 Mar '09

13 Mar '09

even at the level of expressions you can be sure that inequalities hold. So you can create an instance of Ord. for example: (Sum x z) < y | x== y && z>0 = True | .... here x and y can be expressions and == can have its own rules with this you can compute symbolically, because you don´t need to reduce the expression to a numerical value. It´s a matter of finding rules. 2009/3/13 Alberto G. Corona <agocorona(a)gmail.com> > >(<) :: (Ord a) => a -> a -> Bool > > what´s the problem? > > make your Expr an instance of Ord. > > By the way > > > instance Num Expr where > > fromInterger = Const > > (+) = Plus > > (*) = Times > > does not work. you have not defined (+) and (*) for Const Integer. > > (+) (Const a) (Const b)= Const (a+b) > > With this you have an evaluator. > > In the same way: > > (Const a) < (Const b) = Const (a < b) > > > > > 2009/3/12 Adam Vogt <vogt.adam(a)gmail.com> > > This seems to be in ghc for those reasons: >> http://www.haskell.org/haskellwiki/Quasiquotation >> >> * On Monday, March 02 2009, Andrew Hunter wrote: >> >> >Several times now I've had to define an EDSL for working with >> >(vaguely) numeric expressions. For stuff like 2*X+Y, this is easy, >> >looking pretty much like: >> > >> >> data Expr = Const Integer | Plus Expr Expr | Times Expr Expr >> >> >> >> instance Num Expr where >> >> fromInterger = Const >> >> (+) = Plus >> >> (*) = Times >> > >> >&c. This lets me get a perfectly nice AST, which is what I want. >> >When I want to be able to express and work with inequalities and >> >equalities, this breaks. Suppose I want to write 2*X + Y < 3. I >> >either have to: >> > >> >a) Hide Prelude.(<) and define a simple < that builds the AST term I >> want. >> >b) Come up with a new symbol for it that doesn't look totally awful. >> > >> >Neither of these work decently well. Hiding Eq and Ord operators, >> >which is what I effectively have to do for a), is pretty much a >> >nonstarter--we'll have to use them too much for that to be practical. >> > >> >On the other hand, b) works...but is about as ugly as it gets. We >> >have lots and lots of symbols that are already taken for important >> >purposes that are syntactically "near" <,<=,==, and the like: << and >> >>> and >>= for monads, >>> for arrows, etc. There...are not good >> >choices that I know of for the symbols that don't defeat the purpose >> >of making a nice clean EDSL for expressions; I might as well use 3*X + >> >Y `lessthan` 3, which is just not cool. >> > >> >Does anyone know of a good solution, here? Are there good >> >substitutions for all the six operators that are important >> >(<,>,>=,<=,==,/=), that are close enough to be pretty-looking but not >> >used for other important modules? >> > >> >Better yet, though a little harder, is there a nice type trick I'm not >> >thinking of? This works for Num methods but not for Ord methods >> >because: >> > >> >(+) :: (Num a) => a -> a -> a >> >(<) :: (Ord a) => a -> a -> Bool >> > >> >i.e. the return type of comparisons is totally fixed. I don't suppose >> >there's a good way to...well, I don't know what the *right* answer is, >> >but maybe define a new typeclass with a more flexible type for < that >> >lets both standard types return Bool and my expressions return Expr? >> >Any good solution would be appreciated. >> > >> >Thanks, >> >AHH >> >_______________________________________________ >> >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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Fwd: [Haskell-cafe] DSLs with {in,}equalities
by Alberto G. Corona 13 Mar '09

13 Mar '09

---------- Forwarded message ---------- From: Alberto G. Corona <agocorona(a)gmail.com> Date: 2009/3/13 Subject: Re: [Haskell-cafe] DSLs with {in,}equalities To: Adam Vogt <vogt.adam(a)gmail.com> You need an expression evaluator: with (+) (Const a) (Const b)= Const (a+b) (*) (Const a) (Const b)= Const (a*b) eval :: Exp -> Integer eval (Const i)= i eval ( Plus e1 e2)= eval e1 + eval e2 .. eval( Mul .... and instance Ord Expr where (<) expr1 expr2 = eval expr1 < eval expr2 by the way: simplify expr= Const (eval expr) ...................................... 2009/3/13 Alberto G. Corona <agocorona(a)gmail.com> > Sorry(Const a) < (Const b) = a < b > > also > > (*) (Const a) (Const b)= Const (a*b) > > > 2009/3/13 Alberto G. Corona <agocorona(a)gmail.com> > > >(<) :: (Ord a) => a -> a -> Bool >> >> what´s the problem? >> >> make your Expr an instance of Ord. >> >> By the way >> >> > instance Num Expr where >> > fromInterger = Const >> > (+) = Plus >> > (*) = Times >> >> does not work. you have not defined (+) and (*) for Const Integer. >> >> (+) (Const a) (Const b)= Const (a+b) >> >> With this you have an evaluator. >> >> In the same way: >> >> (Const a) < (Const b) = Const (a < b) >> >> >> >> >> 2009/3/12 Adam Vogt <vogt.adam(a)gmail.com> >> >> This seems to be in ghc for those reasons: >>> http://www.haskell.org/haskellwiki/Quasiquotation >>> >>> * On Monday, March 02 2009, Andrew Hunter wrote: >>> >>> >Several times now I've had to define an EDSL for working with >>> >(vaguely) numeric expressions. For stuff like 2*X+Y, this is easy, >>> >looking pretty much like: >>> > >>> >> data Expr = Const Integer | Plus Expr Expr | Times Expr Expr >>> >> >>> >> instance Num Expr where >>> >> fromInterger = Const >>> >> (+) = Plus >>> >> (*) = Times >>> > >>> >&c. This lets me get a perfectly nice AST, which is what I want. >>> >When I want to be able to express and work with inequalities and >>> >equalities, this breaks. Suppose I want to write 2*X + Y < 3. I >>> >either have to: >>> > >>> >a) Hide Prelude.(<) and define a simple < that builds the AST term I >>> want. >>> >b) Come up with a new symbol for it that doesn't look totally awful. >>> > >>> >Neither of these work decently well. Hiding Eq and Ord operators, >>> >which is what I effectively have to do for a), is pretty much a >>> >nonstarter--we'll have to use them too much for that to be practical. >>> > >>> >On the other hand, b) works...but is about as ugly as it gets. We >>> >have lots and lots of symbols that are already taken for important >>> >purposes that are syntactically "near" <,<=,==, and the like: << and >>> >>> and >>= for monads, >>> for arrows, etc. There...are not good >>> >choices that I know of for the symbols that don't defeat the purpose >>> >of making a nice clean EDSL for expressions; I might as well use 3*X + >>> >Y `lessthan` 3, which is just not cool. >>> > >>> >Does anyone know of a good solution, here? Are there good >>> >substitutions for all the six operators that are important >>> >(<,>,>=,<=,==,/=), that are close enough to be pretty-looking but not >>> >used for other important modules? >>> > >>> >Better yet, though a little harder, is there a nice type trick I'm not >>> >thinking of? This works for Num methods but not for Ord methods >>> >because: >>> > >>> >(+) :: (Num a) => a -> a -> a >>> >(<) :: (Ord a) => a -> a -> Bool >>> > >>> >i.e. the return type of comparisons is totally fixed. I don't suppose >>> >there's a good way to...well, I don't know what the *right* answer is, >>> >but maybe define a new typeclass with a more flexible type for < that >>> >lets both standard types return Bool and my expressions return Expr? >>> >Any good solution would be appreciated. >>> > >>> >Thanks, >>> >AHH >>> >_______________________________________________ >>> >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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Typing Dynamic Typing [Was: Dynamically typing TH.Exp at runtime]
by oleg＠okmij.org 13 Mar '09

13 Mar '09

Martin Hofmann asked: > Is there a Haskell implementation of the paper "Typing Dynamic Typing" > by Baars and Swierstra There is a different implementation but in the same spirit http://okmij.org/ftp/tagless-final/IncopeTypecheck.hs http://okmij.org/ftp/Computation/tagless-typed.html#tc-final The first difference between IncopeTypecheck and Baars and Swierstra's one is in the type representation and safe coercions. IncopeTypecheck uses Typeable, and so it needs to define reflection from a type rep to a value of that type. The projection function of Typeable requires a value of a specific type rather than a typerep. The main difference is treating environments and weakening. Baars and Swierstra treat an Identifier, at compile time, as a projection function \env->t. Here env is the run-time env represented as a nested tuple. `Closing the Stage' paper relies on the same idea. To be able to collect those projection functions into a regular list, they wrap them into a Dynamic. IncopeTypecheck, in contrast, represents compile-time variables as functions \x->x, which are weakened as they are embedded into a reacher environment. The functions are wrapped into Dynamic, as this is the result of typechecking. The type environment Gamma in IncopeTypecheck contains only TypeRep but no Dynamics! Here a few examples to illustrate the difference between Baars and Swierstra and IncopeTypecheck: Source expression: Add (Int 2) (Int 3) Baars and Swierstra: \env -> (\_ -> 2) env + (_ -> 3) env IncopeTypecheck: add (int 1) (int 2) Source expression: (Lam "x" Int (x + (Int 1))) Baars and Swierstra: \env -> \x -> (\env -> (\(x,_) ->x) env + (\_ -> 1) env) (x,env) IncopeTypecheck: \x -> (\x -> x) x `add` (\_ -> int 1) x (of course, in IncopeTypecheck, instead of (\x -> ...) there should be lam (\x -> ...) and instead of meta-language application there should be app. I drop them for clarity). For deeply nested functions, IncopeTypecheck probably has to do more redices as it repeatedly applies coercions. Since the environment is known, I could have built the weakening in one step rather than as a sequence of weakening into a progressively richer environment. Each step into this sequence includes an administrative redex. The sequential weakening was easier to implement though.

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Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Sebastian Fischer 13 Mar '09

13 Mar '09

Hi Mark, On Mar 13, 2009, at 3:54 AM, Mark Spezzano wrote: > I was wondering what the best way to implement Natural number would > be. Is there a package which already does this? there are two packages on Hackage that implement natural numbers using algebraic datatypes: 'numbers' has a module Data.Number.Natural that implements them in "unary" notation (zero and successor), 'nat' has Data.Number.Nat that uses a binary encoding. Cheers, Sebastian

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Combining haddock documentation from multiple packages
by Heikki Aitakangas 13 Mar '09

13 Mar '09

Is it possible to generate a "unified" documentation tree similar to http://www.haskell.org/ghc/docs/latest/html/libraries/index.html from packages installed via cabal from hackage? And if so, how does one go about doing it? -- Heikki Aitakangas

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How to compare PortNumbers or Bug in Network.Socket?
by Stefan Schmidt 13 Mar '09

13 Mar '09

Hello, I want to compare two PortNumber-Values from the Network.Socket module and I think I've experienced some unexpected behavior of the derived ordering methods. I'm using the ghc-6.10.1 and the network library 2.2.0.1 on a x86 32-Bit machine. The following program creates two pairs of PortNumber values and compares them with each other. module Main(main) where import Network.Socket -- Block A p1 :: PortNumber p1 = fromIntegral 9000 p2 :: PortNumber p2 = fromIntegral 10000 -- Block B pp1 :: PortNumber pp1 = PortNum 9000 pp2 :: PortNumber pp2 = PortNum 10000 main :: IO () main = do putStrLn $ "A - " ++ (show $ p1 > p2) putStrLn $ "B - " ++ (show $ pp1 > pp2) On my machine, I get the result: A - True A - False At the first glance, it seemed to me, that there is a bug in the fromIntegral Implementation. But after I looked into the code, I saw that fromIntegral calls the system function "htons" to change the byte order from my machine to the network byte order, which on my machine is different. Because of this the values 9000 and 10000 are transferred, so that their ordering changes. Block B preservers this ordering, because the Constructor PortNum does not call htons, but since the byte order is wrong, I cannot use pp1 and pp2 to address port 9000 and 10000. As a work around, I could convert two PortNumbers back to Int-Values before comparing them, but I think the ordering functions for the PortNumber-Type do not work as expected. Or am I wrong? Stefan Schmidt

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Re: [Haskell-cafe] Using a monad to decompose a function into functions
by Marcin Kosiba 13 Mar '09

13 Mar '09

On Thursday 12 March 2009, you wrote: > 2009/3/12 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > > Hi, > > I'm doing a bit of research into mobility models and I'm currently > > exploring implementation language choices for the simulator > > (*snip*) > > The simulation algorithm requires expressing > > the node's mobility so that it is "stateless". The mobility model > > algorithm's type should be something like: > > mobility_model :: WorldState -> NodeState -> OtherInput -> (Action, > > NodeState) > > > > where Action can alter WorldState and the second NodeState is an altered > > input NodeState. I perform a form of speculative execution on > > mobility_model so sometimes I need to backtrack to a previous world and > > node state. This is all fairly simple stuff, and was just an > > introduction. What I do now is store an enum in NodeState and implement > > mobility_model as one big case statement. Yes, this is very imperative of > > me, I know. What I'd like to do is to express mobility_model, so that the > > code would look like: > > > > mobility_model world node input = do > > do_calculus > > emit_action > > if something > > then emit_action > > else emit_action > > do_calculus > > emit_action > > mobility_model world node input > > Hi, > > It seems you can use > http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mon >ad-State-Lazy.html Just have a look at the exemple : > > tick :: State Int Int > tick = do n <- get > put (n+1) > return n > > your code would become something like > mobility_model :: OtherInput -> State (WorldState,NodeState) () > mobility_model input = do > world <- gets fst > node <- gets snd > .... > let (world',node') = ... > put (world',node') ok, that solves part of the problem. what this doesn't solve is that somewhere between these lines (which corespond to emit_action in my example) > let (world',node') = ... > put (world',node') I need to return a value and an Action and NodeState to the simulation algorithm. and then, after the simulation algorithm calculates a new WorldState it will want the mobility_model to where it left off, but with a new WorldState. I hope I'm clear about what I wish to achieve: each emit_action should return a value (Action, NodeState) and maybe a function mobility_model_cont which I then could call with the new WorldState to continue from where emit_action returned. Thanks, Marcin Kosiba

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