- Haskell-Cafe - Haskell.org

RFI: Link errors with random pkg on 6.8.1
by Jim Stuttard 05 Dec '07

05 Dec '07

Hi, Getting same error in 2 diff. apps * built with ghc-6.8.1. * random-1.0.0.0 also built with 6.8.1 * random-1.0.0.0 is registered # HAppS -6.8.1/lib/random-1.0.0.0/HSrandom-1.0.0.0.o: unknown symbol `oldzmtimezm1zi0zi0zi0_SystemziTime_a99_info' ghc-6.8.1: unable to load package `random-1.0.0.0' # Haskell$ ghci DonsGame-0.1.hs GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Game ( DonsGame-0.1.hs, interpreted ) Ok, modules loaded: Game. *Game> main Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package filepath-1.1.0.0 ... linking ... done. Loading package directory-1.0.0.0 ... linking ... done. Loading package unix-2.2.0.0 ... linking ... done. Loading package process-1.0.0.0 ... linking ... done. Loading package random-1.0.0.0 ... linking ... <interactive>: /usr/local/lib/ghc-6.8.1/lib/random-1.0.0.0/HSrandom-1.0.0.0.o: unknown symbol `oldzmtimezm1zi0zi0zi0_SystemziTime_a99_info' ghc-6.8.1: unable to load package `random-1.0.0.0' # ubuntu gutsy 2.6.22-14-server #1 SMP Sun Oct 14 23:34:23 GMT 2007 i686 GNU/Linux Jim I

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NY Functional Programmers Network: F# Talk by Don Syme, Monday De cember 10th at 7pm
by Mansell, Howard 05 Dec '07

05 Dec '07

Credit Suisse will be hosting a talk by Don Syme from Microsoft Research on Monday December 10th. He will talk about F# and Microsoft's plans for it. This talk is arranged by the New York Functional Programmers Network, a group of individuals in the New York area who are interested in functional programming. Many of us also use functional programming in our work life. Despite the URL below, we are typically interested in statically typed functional languages. If you are interested in coming, please RSVP at the meetup site below: http://lisp.meetup.com/59/ <http://lisp.meetup.com/59/> Regards Howard Mansell Global Modelling & Analytics Group Credit Suisse ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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Nofib modifications
by Neil Mitchell 05 Dec '07

05 Dec '07

Hi, Some of the nofib suite are messed up by Yhc/nhc because of the monomorphism restriction. Take imaginary/bernouilli as an example: powers = [2..] : map (zipWith (*) (head powers)) powers Hugs and GHC both see powers :: [[Integer]] and a CAF. Yhc (and nhc) both see powers :: (Enum a, Num a) => [[a]] and no CAF. This completely destroys the performance in Yhc/nhc. Since this is not so much a performance aspect but a compiler bug, based on a feature whose future in Haskell' is as yet unclear, perhaps it would be wise to patch nofib to include an explicit type signature where this matters. I am happy to send in a patch (or just apply it) - but I have no idea who "maintains" the suite. I've CC'd those people who make substantial use of the nofib suite. Thanks Neil

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isSpace
by Ryan Bloor 05 Dec '07

05 Dec '07

HI I will try and explain it better. I am meaning to write a function that takes a string, " apple" and eliminates the spaces at the start ONLY. called........ removeSpace :: String -> String I decided to use the function 'dropWhile' and another one 'isSpace' in the 'removeSpace' function. removeSpace:: String -> String removeSpace a = case dropWhile isSpace a of "" -> I get stuck here... I am not sure how to go about this function. Any ideas. Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Advice for clean code.
by David McBride 05 Dec '07

05 Dec '07

I am still in the early stages learning haskell, which is my first foray into functional programming. Well there's no better way to learn than to write something, so I started writing a game. Mostly the thing looks good so far, far better than the C version did. However, my problem is that code like the following is showing up more often and it is becoming unwieldy. gameLoop :: World -> IO () gameLoop w = do drawScreen w action <- processInput let (result, w') = processAction action w case result of MoveOutOfBounds -> putStrLn "Sorry you can't move in that direction." MoveBadTerrain a -> case a of Wall -> putStrLn "You walk into a wall." Tree -> putStrLn "There is a tree in the way." otherwise -> putStrLn "You can't move there." otherwise -> return () let w'' = w' { window = updateWindowLocation (window w') (location $ player w')} unless (action == Quit) (gameLoop w'') Where world contains the entire game's state and so I end up with w's with multiple apostrophes at the end. But at the same time I can't really break these functions apart easily. This is error prone and seems pointless. I have been reading about control.monad.state and I have seen that I could run execstate over this and use modify but only if each function took a world and returned a world. That seems really limiting. I'm not even sure if this is what I should be looking at. I am probably just stuck in an imperative mindset, but I have no idea what to try to get rid of the mess and it is only going to get worse over time. Any suggestions on what I can do about it?

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Haskell interface file (.hi) format?
by Rob Hoelz 04 Dec '07

04 Dec '07

Hello fellow Haskellers, Does anyone know if/where I can find a specification for the .hi files generated by GHC? I ask because I want to write an omni-completion plugin for Vim to make Haskell hacking a bit nicer. Thanks, Rob Hoelz

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Re: [Haskell] IVars
by Bertram Felgenhauer 04 Dec '07

04 Dec '07

[redirecting to haskell-cafe] Simon Peyton-Jones wrote: > But since the read may block, it matters *when* you perform it. For > example if you print "Hello" and then read the IVar, you'll block > after printing; but if you read the IVar and then print, the print > won't come out. If the operation was pure (no IO) then you'd have > a lot less control over when it happened. Well, the same can happen with any pure computation that does not terminate. It's the compiler's job to avoid that, and as far as I know ghc is pretty careful not to reorder pure computations and IO operations too much. I'd argue for adding both an IO and a pure version of readIVar to an IVar implementation. Both have their uses; the firts gives you a little extra control, while the second one can easily be evaluated on demand, at the risk of creating a mine field of potential deadlocks. Bertram

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regexen no go with 6.8.1?
by Jason Dusek 04 Dec '07

04 Dec '07

Is it just me, or are all the regex implementations broken with new change in lib layout for 6.8.1? Are fixes available in darcs? -- _jsn

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Leopard: ghc 6.8.1 and the case of the missing _environ
by Joel Reymont 04 Dec '07

04 Dec '07

Symptoms: You build 6.8.1 from source on Leopard (x86 in my case) and then junior:ghc-6.8.1 joelr$ ghci GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help ghc-6.8.1: /usr/local/lib/ghc-6.8.1/lib/base-3.0.0.0/HSbase-3.0.0.0.o: unknown symbol `_environ' Loading package base ... linking ... ghc-6.8.1: unable to load package `base' Problem: ghc binaries are being stripped upon installation which eliminates _environ, e.g. junior:tmp joelr$ nm x|grep environ 00002ff0 T ___hscore_environ 0004d004 D _environ junior:tmp joelr$ strip x junior:tmp joelr$ nm x|grep environ Solution: Need to make sure install-sh does _not_ use the -s option. Haven't found out where this needs to be done yet. A temporary workaround is to ask Manuel for the pre-built binaries. Thanks, Joel -- http://wagerlabs.com

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Re: The Opposite of $
by Alan Hawkins 04 Dec '07

04 Dec '07

|> is not defined in the prelude but can be defined as let (|>) = flip ($) to make it the opposite of $ which means the syntax looks more like the Unix pipe '|' command eg. filter (\x-> x >3 && x < 7) $ [1..10] ++ [5] [1..10] ++ [5] |> filter (\x-> x >3 && x < 7) res: [4,5,6] note that with the |> the function applications are read as naturally left to right as apposed to the $ which reads left to right. a style more in line with the syntax f (x) ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs

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The Opposite of $
by PR Stanley 04 Dec '07

04 Dec '07

Hi According to the Haskell Wikibook the is the opposite of $. The text also oints out that the "|>" can be used as a substitute. I tried using |> but to no avail. Is the list au fait with the |> operator? Thanks, Paul

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Fw: [Haskell-cafe] Re: do
by Thomas Hartman 04 Dec '07

04 Dec '07

It took me forever to get comfortable with monads. I think it helps if you've seen continuations, or done FP before, or a variety of things that build familiarity. But probably the only thing that I think will work for the masses of plodders (there are always a few stars to crash the curve) is a desire to learn and tons of practice. I wouldn't worry so much about the ideal way to introduce the material. Oh, one other thing. You learn monads when you need them. You need IO right away... well, after you tired of playing with pure functions in the ghci sandbox. my 2c. t. "Dan Piponi" <dpiponi(a)gmail.com> Sent by: haskell-cafe-bounces(a)haskell.org 12/03/2007 07:19 PM To haskell-cafe(a)haskell.org cc Subject Re: [Haskell-cafe] Re: do On Dec 3, 2007 3:54 PM, Ben Franksen <ben.franksen(a)online.de> wrote: > I don't buy this. As has been noted by others before, IO is a very special > case, in that it can't be defined in Haskell itself, and there is no > evaluation function runIO :: IO a -> a. I'm not sure what a function of type m a -> a has to do with the concept of a monad. And I don't really see what the problem is with the IO monad not being definable in (pure) Haskell. IO exposes the same interface as every other Monad, and you need to use that interface to get visible results. So people have to learn it whatever. And the whole point of the Monad is that it's an interface, not a specific implementation. > I'd rather use a simple example like Maybe (modeling failure as an effect). And I'd like to see more people getting off the ground doing interesting stuff with Haskell before their attention spans for pointless-seeming new stuff run out. I'm not talking about the smartest people in computer science courses here. I'm talking about the millions of people writing everyday Python and Ruby scripts, say, who might benefit from a more expressive, type-safe, well-thought out, fast and compiled language. -- Dan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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doing builds using cygwin on Windows
by Galchin Vasili 04 Dec '07

04 Dec '07

To Nanonthief ...... VasiliIGalchin wrote: > > > " .. however, I don't see in which file PATH can be set. Any help? I > really > want to get my Haskell build environment set up and cranking away. > Unless I misunderstood what you want, you can add a path to the PATH variable by adding the line: export PATH="$PATH:/path/to/folder" to the ~/.bashrc file (and also possibly the ~/.bash_profile file). I've also tried to build haskell with cygwin, but didn't have any luck. Could you describe how you managed to do it? -- nanothief ... here is what I decided to add to unix.cabal .. and I am getting farther ... build-depends: base, directory extensions: CPP, ForeignFunctionInterface include-dirs: include c:/cygwin/usr/include <<< added cygwin include path includes: HsUnix.h execvpe.h install-includes: HsUnix.h HsUnixConfig.h execvpe.h c-sources: cbits/HsUnix.c cbits/execvpe.c

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Possible Improvements
by PR Stanley 04 Dec '07

04 Dec '07

Hi data Tree = Leaf Int | Node Tree Int Tree occurs :: Int -> Tree -> Bool occurs m (Leaf n) = m == n occurs m (Node l n r) = m == n || occurs m l || occurs m r It works but I'd like to know if it can be improved in any way. Thanks, Paul

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Monads
by PR Stanley 04 Dec '07

04 Dec '07

Hi Does the list consider http://en.wikibooks.org/w/index.php?title=Haskell/Understanding_monads&oldi… a reliable tutorial on monads and, if not, could you recommend an onlien alternative please? Thanks, Paul

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ANN: atom 2007.12
by Tom Hawkins 04 Dec '07

04 Dec '07

Hello, Atom is a language embedded in Haskell for describing reactive software, primarily for realtime control applications. Based on conditional term rewriting, an atom description is composed of a set of state transition rules. The name "atom" comes from the atomic behavior of rules: if a rule is selected to fire, all its transitions occur or none at all. A hallmark of the language, rule atomicity greatly simplifies design reasoning. This release of atom is a major redirection. Atom is no longer a hardware description language (I changed jobs. I'm now in software.). Much of the frontend language and backend generators have changed, though rule scheduling remains nearly the same. On the frontend, atom's Signal datatypes have been replaced with Terms and Vars, which leverage Haskell's GADTs. The 4 supported Term and Var types include Bool, Int, Float, and Double. At the backend, atom generates C and Simulink models. The Verilog and VHDL generators have been dropped, but they may reappear in the future. Enjoy! http://funhdl.org/ darcs get http://funhdl.org/darcs/atom -Tom

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A tale of three shootout entries
by Sterling Clover 04 Dec '07

04 Dec '07

In some spare time over the holidays I cooked up three shootout entries, for Fasta, the Meteor Contest, and Reverse Complement. I should probably have tossed them to haskell-cafe before submission, for review and ideas, but they're up now. In any case, all three were great learning experiences, but could use some other eyes and ideas to be the best that they can. First up is the meteor-contest entry. http://shootout.alioth.debian.org/gp4/benchmark.php? test=meteor&lang=ghc&id=5 This is the clear win of the bunch, with significantly improved time thanks to its translation of the better algorithm from Clean. However, it's still missing something. Nearly all its time is spent in a tight loop in the solveCell function near the end of the code. I tried unboxing this, but failed because it spends the bulk of its time applying a bitwise and between the recursively passed value and a piece of data retrieved from the masksAtCell data structure which is of type :: Array (Row,Col) (Array Color [Mask]). (Note that Mask, Color, Row and Col are all type synonyms for Int that I added for readability). As each Mask is stored in this list, the masks can't easily be unboxed -- some sort of custom data structure built using the FFI seems in order here. If anyone wants to tackle this, I think it could be a big win for performance. Next is reverse-complement. http://shootout.alioth.debian.org/gp4/benchmark.php? test=revcomp&lang=ghc&id=3 This *would* be a big win except I dimly doubled memory usage from the previous entry due to filtering newlines explicitly -- which does, one should note, provide a large performance gain. The solution here seems the most obvious -- roll the newline stripping into the destructive modifications performed in the revcomp function, as the winning C++ entry does. I'll probably get around to this eventually, but if someone else wants to try to implement this or any other performance improvements, please jump right in. Additionally, there might be some other tricks to reducing its memory usage that escape me at the moment (noting, of course, that using a strict bytestring, as we should, its unavoidable that we consume the entire contents of the input at once... I think?) Finally, there's fasta. http://shootout.alioth.debian.org/gp4/benchmark.php? test=fasta&lang=ghc&id=2 This one really depresses me. It outperforms the previous version by roughly 20% on my machine (PPC) but underperforms by roughly the same amount on the shootout box. If you compare it to dons previous version, the "optimizations" I attempted should be pretty obvious. First, I precompute the partial sums for the frequency tables and alter the choose function accordingly. This is a pretty basic measure that all the better entries seem to do. Next, I unboxed the random function, which yielded big speedups However, given that we use an unfoldN, as we should, I couldn't very well pass it a function that returned something of kind #, (especially as Maybe, which unfoldN uses, is of kind *->*). Thus, I hid the unrolled loop in a lazy list of floats that is passed instead of a random seed. I suspect that for some reason I don't understand relating to differences in processors, GHC's internal handling of floating point math, or who knows what, this somehow is the source of the slowdown. If someone with an Intel Pentium 4 machine comparable to that of the shootout box wants to take a look at this code and see why it underperforms, I'd be much obliged. It really seems to me that GHC's fasta performance is far below where it should be (4x slower than Java!) , and I'd like to get its numbers up somehow. Thanks, Sterl p.s. It looks like they've depreciated chameneos in favor of a new version, chameneos-redux. As this was one of the places Haskell really rocked the competition, it would probably be worth updating the Haskell entry for the new benchmark. Also, the n-bodies benchmark seems like another that could be much improved.

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[Announce] FPSIG @ Southampton
by Paulo J. Matos 04 Dec '07

04 Dec '07

Hello all, Just to let you all know I'm leading a Special Interest Group in the University of Southampton, UK which is studying Haskell. I've put up a blog which I'll try to regularly update _at least_ with our meetings summary, exercises and code. We welcome any suggestions/comments/etc either via my email or as comments in the blog. I've added the group to the Haskell Wiki User Groups (I hope there's no problem). Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Re: [Haskell-cafe] Parsing unstructured data
by Olivier Boudry 04 Dec '07

04 Dec '07

On 12/2/07, Steven Fodstad <flarelocke(a)hotpop.com> wrote: > > Sorry for not responding earlier. The haskell-cafe list is hard to keep > up with. > > The process of finding geographic (lat/long) coordinates from a text > address is called geocoding. Obviously extracting the parts of an > address is part of that, so you might find better results looking for > geocoding, rather than the more general and more difficult topic of > extracting structure from unstructured data. Unfortunately, I don't > have any references at hand on that part of geocoding. > Hi Steven, The idea of using the geocoding approach seems appealing. I already thought of using geocoding for address validation (after the parsing) but not of looking at how geocoding tools parse addresses. But I'm not sure geocoding tools would be suitable to handle my addresses. I used a few geocoding tools and usually you have to provide the address in a very specific format if you want it to be recognized. Also most of the time it work quite well for US addresses but not for other countries addresses. I need to recognize very specific parts of an address. More than what a geocoding tools will require. Like dock #, doors, suite #, contact person, etc... I'm currently using the ZipFourCE web service from BCCSoftware for validating my addresses against the USPS address database. This tool is built for parsing and correcting addresses but I just use it for validation as it's not "smart enough" to parse them or maybe they are just too scrambled for the parsing to be automated using an out of the box tool. ;-) Thanks for your input, Olivier.

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Trees
by Adrian Neumann 04 Dec '07

04 Dec '07

Good morning, as an exercise for my Algorithms and Programming course I have to program a couple of simple functions over trees. Until now everything we did in Java could be done in Haskell (usually much nicer too) using the naive > data Tree a = Leaf a | Node a [Tree a] But now the assignments require more than a simple top-down traversal. For example: given a tree t and two nodes u,v, find the first common ancestor. In Java this is really simple, because each node has a "parent" reference. That way I only need to follow those references until I find the first common ancestor. This should take something like O(log n) in the average case. In Haskell however the best way I've come up with so far is doing a BFS and looking for the last common node in the paths to u and v. This is neither fast, nor particularly elegant. So how would you smart guys do it? With a Zipper? It would be nice if there was an elegant solution without monads. --Adrian

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Optimizing cellular automata evaluation (round 2)
by Justin Bailey 03 Dec '07

03 Dec '07

I posted awhile back asking for help improving my cellular automata program. I am competing with a C program which evolves CAs using a fairly simple genetic algorithm. The algorithm involves evaluating 100 rules on 100 CAs, 100 times. In C this takes about 1 second. In my Haskell version, it takes about 20 minutes. That's an improvement from my first attempts, which took 7 hours. All the time is spent in the function that evaluates one iteration of a given CA. It examines each cell and its neighbors and, based on the values found, updates the cell to be white or black. Here is the code, followed by an explanation of what it's doing. I'm looking for any suggestions on making this faster. Prettier helps too but not at the expense of speed. http://hpaste.org/4151#a0 I represent the automata as an array of integers, where each bit represents a cell. Automata 'wrap' around, so marching through the array and determing the value of all neighbors is tricky around the edges. My goal was to minimize conditionals as much as possible and to avoid any "modulo" arithmetic. I wanted to use straightforward array access. My implemention uses three functions: fill, fillS and fillE. Each evalutaion of fill will determine the new values for cells represented by the current array element. fill handles two conditions - when the array element being examined is the last element or when it is not. It calls fillS with appropriate arguments, the results of which become the value for the integer in the current array position. fillS then handles two conditions - when the "neighborhood" of the bit being examined crosses to the "next" array element and when it does not. When the "neighborhood" crosses to the next array element, fillE is called. fillE finishes determining the values for the remaining cells in the integer and returns, ending the current evaluation. There are some other details in the code (for example firstRule) which could be made faster and/or cleaner, but their contribution is far outweighed by the fill function. If there is one thing I'm happy about, its the memory performance of this code. It runs in a constant 7 - 10MB of memory, even when I'm doing 100 generations with 100 rules and 100 CAs. Each CA is evaluated for 250 steps, which means every generation does 2.5M steps. That kind of memory usage is pretty sweet. Thanks in advance to those willing to take the time to look at this code! Justin

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Re: [Haskell-cafe] Possible Improvements
by PR Stanley 03 Dec '07

03 Dec '07

That's a very good point. Yes, let's have some detailed explanations accompanied by some good examples. Cheers, Paul At 08:43 03/12/2007, you wrote: > > I agree that (in this context, beginning learning Haskell) it is a > > somewhat minor issue. But I disagree that this is something you should > > ignore until it becomes a problem and I do think that it should be part > > of learning Haskell. Properly using strictness is an important part of > > using Haskell. It makes the difference between code that stack > > overflows and code that doesn't, code that takes 100 seconds and code > > that takes 10, code that uses 3MB of RAM and code that uses 600. At > > least the first of these is not, in my mind, the difference between > > "optimized" and "unoptimized", but rather the difference between correct > > and incorrect. Writing better code at the beginning is much easier than > > trying to figure out what the problem is later. Furthermore, writing > > better code is not more difficult. In this case it merely means adding > > two characters. Of late, the "rules of thumb" for this sort of thing > > are becoming more widely known. Such things need to be "instinctively" > > part of how you write code, much like writing code tail-recursively or > > not using (++) left associatively. It's not that you should immediately > > know that this is better, but (more strongly) that you should not even > > think of the worse ways to begin with in many cases. > >It would be great if someone could exemplify these "rules of thumb", >e.g. "Primitive types such as Int should be strict unless in the three >canonical examples X, Y and Z." My strictness radar is still quite >poor and I feel I can't make informed decisions on when I need to make >something more strict or lazy. > >-- Johan >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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more info on building the Unix package on cygwin
by Galchin Vasili 03 Dec '07

03 Dec '07

Hello, I put an ugly kludge(on my laptop) in the unix.cabal file For the includes-dir "attribute" I explicited specified the path to the cygwin include directory. The Unix package build gets farther and then gets errors. At this point, it appears that the cygwin sys/types.h has a "bug" .. id_t is not defined but id_t is referenced by sys/resources.h/(getpriority/setpriority). Please see see http://www.opengroup.org/onlinepubs/000095399/functions/setpriority.html for getpriority function signature. In any case, I want compare the Linux sys/types.h include. To summarize: 1) I am just learning about the .cabal file format from the Cabal .pdf. In any case, it appears that for the case of cygwin that unix.cabal is deficient. 2) It also appears that the cygwin sys/types.h failed to defined id_t which is needed by sys/resources.h which in turn is needed by System/Posix/DynamicLinker/Module.hsc (error at line 57 where HsUnix.h is refd .. and in HsUnix.h at line # 38 sys/resource.h is referenced.) I can provide a stderr "log". Kind regards, Vasya

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doing builds using cygwin on Windows ....help
by Galchin Vasili 03 Dec '07

03 Dec '07

Hello, I have suceeded in doing a build on top of cygwin. However, after editing some of the cygwin "bashrc" files, teh darcs path and haddock path still don't show up when I do a "echo $PATH". I am now reading http://cygwin.com/cygwin-ug-net/setup-env.html. "The PATH environment variable is used by Cygwin applications as a list of directories to search for executable files to run. This environment variable is converted from Windows format (e.g. C:\WinNT\system32;C:\WinNT) to UNIX format (e.g., /WinNT/system32:/WinNT) when a Cygwin process first starts. Set it so that it contains at least the x:\cygwin\bin directory where " x:\cygwin is the "root" of your cygwin installation if you wish to use cygwin tools outside of bash. " .. however, I don't see in which file PATH can be set. Any help? I really want to get my Haskell build environment set up and cranking away. Kind regards, Vasya

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who develops and maintains the Unix package?
by Galchin Vasili 03 Dec '07

03 Dec '07

Hello, I am starting to get in the groove related to runhaskell -- config/builds on top of cygwin (I haven't had time to carve up hard drive and install Linux). In any case, I really want to get the Unix package to build on cgywin (as advertised at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-2.2.0.0) When i do "runhaskell Setup.lhs configure" things seem to go fine, .log fine is clean, and pertinent cygwin include files like times.h are found, but when i do a "runhaskell Setup.lhs build" then the include files that are allegedly found during the configure "probe" cannot be found. ???? 1) During the "runhaskell Setup.lhs build" step, what human readable file is used and contains paths to the cygwin Posix include file directories? 2) thatis foloowing is a better approach for me .. => 2) Somebody obviously put " (except under Cygwin)". Can you point at who made this claim and why? Kind regards, Vasya

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Re: Haskell-Cafe Digest, Vol 51, Issue 180
by manu 03 Dec '07

03 Dec '07

>> >> I'm trying to build diverse packages from Hackage with ghc 6.8.1, >> they usually fail to build because of missing language extensions. >> >> Sometimes I am unable to determine the proper name of the extension >> missing in .cabal >> I tend to slap {- #OPTIONS -fglasgow-exts #-} at the top of the >> troublesome file. >> It works, but out of curiosity, what is the downside of such an >> approach ? (bigger executables ?) > > Can you list which packages failed to build out of the box? > > -- Don > Well I'd say none of the packages I've tried, build out of the box, that include : haskelldb haskelldb-hdbc haskelldb-hdbc-sqlite3 HDBC HDBC-sqlite3 Agreed some of these compile right after tweeking the build-depends: and extensions: lines in the .cabal file (apart from the pesky PatternSignatures which require a LANGUAGE pragma in the source file) haskelldb-hdbc-sqlite3 is the one that I couldn't get past... I didn't bother with hsql-mysql-1.7 since Duncan Coutts mentioned it required significant work (and I cannot do much) Manu

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Peg solitaire in Haskell - anyone interested?
by Maurício 03 Dec '07

03 Dec '07

Hi, I've just finished a command line application that solves peg solitaire. It does that by building a database of (possible ^ solvable) boards, and you can walk through them from the beggining of the game to the end. It always shows all movements you can do where you are guaranteed to finish with just one peg. Today, it's messy. So I would like to know if people are interested on that. If at least five people say they would REALLY like to play with that program, I will make it into a nice gtk2hs application and release it on the net. If not, I'll keep it like it is and use for my pleasure only. Thanks, Maurício

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fast Array operations: foldl, drop
by Henning Thielemann 03 Dec '07

03 Dec '07

I thought operations like "foldl'" and "drop" must be very fast on arrays (especially UArray) with appropriate pointer tricks, I mean pointer incrementing instead of indexing for "foldl'" and a pointer into the array for "drop". Is it planned to add such functions? Ok, if "foldl f x . elems" and "listArray (i,sufficientlyBig) . drop n . elems" are fused to high speed code, then these functions do not need to materialize in the API.

8 11

ANN: Teach Yourself Gtk2Hs in 21 Hours
by Hans van Thiel 03 Dec '07

03 Dec '07

Hello All, The Gtk2Hs basics tutorial, based on the Tony Gale and Ian Main GTK+2.0 tutorial, is now available for review and comment. The TOC: 1. Introduction 2. Getting Started 3. Packing 3.1 Packing Widgets 3.2 Packing Demonstration Program 3.3 Packing Using Tables 4. Miscellaneous Widgets 4.1 The Button Widget 4.2 Adjustments, Scale and Range 4.3 Labels 4.4 Arrows and Tooltips 4.5 Dialogs, Stock Items and Progress Bars 4.6 Text Entries and Status Bars 4.7 Spin Buttons 5. Aggregated Widgets 5.1 Calendar 5.2 File Selection 5.3 Font and Color Selection 5.4 Notebook 6 Supporting Widgets 6.1 Scrolled Windows 6.2 Event Boxes and Button Boxes 6.3 The Layout Container 6.4 Paned Windows and Aspect Frames 7. Action Based Widgets 7.1 Menus and Toolbars 7.2 Popup Menus, Radio Actions and Toggle Actions This covers about half of Gtk2Hs (guesstimate) but (hopefully) most of what's needed for the run of the mill GUI. It's available or will be on http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/ http://j-van-thiel.speedlinq.nl/gtk2hs/index.html Thanks to Alex Tarkovsky it's in XHTML and well structured for easy maintenance and adaptation. I hope a Gtk2Hs basics tutorial will help to popularize this large, powerful and sophisticated gui library for Haskell. Needless to say, please don't hesitate to comment and report errors. Regards, Hans van Thiel

4 3

Happy parsing wierdness
by Mike Menzel 02 Dec '07

02 Dec '07

Hi, I'm using the Happy parser, and I've threaded a Success/Failure monad-like thing through it so that if the parse succeeds, Success AST (AST is the datatype I want to turn my tokens into) is returned and if it fails, Fail String is returned (in a similar manner to how such threading is described in the Happy Documentation). That is, data SorF a = Succeed a | Fail String. Now, if the input is valid, I get the correct result (namely Success AST). If invalid input is given (ie the wrong token is encountered), happyError is triggered correctly, and and a Fail String is returned. However, if incomplete info is given (let's say a rule is expecting two tokens, and only one is read before the end of input), the parser generated by happy throws error("Reading EOF"). I have defined happyError on both the empty list of tokens, and the non-empty list as returning a Fail String. I'm stumped as to why I get this error. Any advice? Best, Mike.

1 0

Modelling a mutable variable store
by Kannan Goundan 02 Dec '07

02 Dec '07

I'm implementing an interpreter for the lambda calculus augmented with mutable variables. I'm having problems doing the mutable state stuff in Haskell. Here's what I have so far: type Expr = ... terms in the language ... type Value = ... values in the language ... type Ident = String type VarId = String type Env = Map Ident Target data Target = TValue Value | TVar VarId -- idents might refer to values or vars in the store. type Store = Map VarId (Maybe Value) -- Using "Maybe" because vars start out uninitialized. eval :: (Env, Store, Expr) -> (Store, Value) -- The Store is threaded through the evaluator. One problem is that the link between an identifier and it's entry in the store is via VarIds, which are just strings. For example, if I see the identifier "x", I first look it up in the Env, and if it is a "TVar", I then look it up in the store. I'd like to get something stronger (in Java, I would use a pointer). Another problem is that entries in my Store never get garbage collected. Again, if I were using pointers in Java, this wouldn't be an issue. There's also the issue of finding a more elegant way of threading the Store through my evaluator, but I'm not concerned too much about that at this point. I can probably define a state-carrying monad like Parsec. My real concerns are the first two issues. Thanks. - Kannan

4 5

Illegal type def
by PR Stanley 02 Dec '07

02 Dec '07

Hi > type assoc k v = [(k, v)] works beautifully and everything makes sense. > type Assoc v = (Ord k) => [(k, v)] This doesn't work. Is there any wayof defining k as an element of type Ordinal. I could redefine k by putting Char or Int in its place. Why can't I be more general? Thanks, Paul

3 2

Re: [Haskell-cafe] Sequencing Parsers: a Simple Example
by PR Stanley 02 Dec '07

02 Dec '07

> > PRS: (>>=) :: Parser a -> Parser b -> Parser b > > p >>= f = \inp -> > > case p inp of > > [] -> [] > > [(v, out)] -> parse (f v) out > >You probably want: > >(>>=) :: Parser a -> (a -> Parser b) -> Parser b >p >>= f = \inp -> case parse p inp of > [] -> [] > [(v,out)] -> parse (f v) out > >Assuming that you're following Graham Hutton's book. > >Note that this definition won't actually compile; you probably need a >Monad instance and a newtype to get this to work properly (see >http://www.cs.nott.ac.uk/~gmh/Parsing.lhs for a working version of the >same code). PRS: That explains it then. [(v, out)] -> parse (f v) out caused a type mismatch. Cheers, Paul

1 0

Sequencing Parsers: a Simple Example
by PR Stanley 02 Dec '07

02 Dec '07

Hi (>>=) :: Parser a -> Parser b -> Parser b p >>= f = \inp -> case p inp of [] -> [] [(v, out)] -> parse (f v) out based on a lot of guesswork, after the mess created by the OCR, I managed to get the above example to work syntactically but is it semantically correct? Thanks, Paul

2 1

trouble building "unix-2.2.0.0" on cygwin
by Galchin Vasili 01 Dec '07

01 Dec '07

Hello, 0) All work being done on cygwin. Version 6.8.1 of ghc. 1) I ran "runhaskell Setup.lhs configure" and did a "tail -f config.log" in order to follow the config process. 2) Next I did the build "runhaskell Setup.lhs build" but there were many include files referenced in HsUnix.h that couldn't be found, e.g. sys/times.h, sys/resources.h, sys/wait.h, .... 3) I went back through the file config.log and all of the so-called missing include files had supposedly been found during the config process. 4) Next I went to c:/cygwin/usr/include/sys and found all of the so-called missing include files. I am trying to get my confidence level up with respect to the config/build/install (and along with darcs and haddock) process up high so I can make a significant contribution to the Haskell effort. please .. any help will be appreciated. Kind regards, Vasya

2 2

Categories list in French (off-topic?)
by Maurício 01 Dec '07

01 Dec '07

Hi, I'm learning about categories using Saunders Mac Lane book. I'm also learning French. Do you guys know of a nice French mailing list, or forum, where people discuss about categories (and where beginners are accepted)? Sorry to ask that in a Haskell list, but you guys are the ones I know with interest in category theory. Thanks, Maurício

3 3

Editorial error or something meaningful?
by PR Stanley 01 Dec '07

01 Dec '07

Hi taken from ch.8.3 in the Hutton book: "Whereas return v always succeeds, the dual parser failure always fails regardless of the contents of the input string:" The dual parser failure? Cheers, Paul

3 2

Design of a Physics Interface
by Luke Palmer 01 Dec '07

01 Dec '07

I'm currently working on idioms for game programming using FRP. After going through several representations of physics as arrows[1] I decided that physics objects must not be implemented as arrows, because introducing new arrows in the middle of a computation[2] leads to ugly pain. So far the best approach I have is to represent the physics world as a single object World, with a function > integrate :: TimeStep -> World -> World But I can't figure out a good way to represent bodies in this world. I considered: > newBody :: (Position,Velocity) -> World -> (Body,World) Where Body is an ADT with an internal representation of an Integer or something. The problem with this is that (1) there is no way to guarantee that a Body actually exists in a World (which is a minor but still annoying issue), and (2) that there's a possibility that you could make a Body in one world and use it in another and there would be no way to detect the error. (2) could be solved using an internal representation of Data.Unique, but (1) still remains a problem. And as long as the function > deleteBody :: Body -> World -> World exists, it will always remain a problem. Are there any clever idioms I can use for this interface or implementation? Is there a way to store the data for bodies along with the Body object instead of with the world in a way that the relationships between them respect different generations of the World (through integrate)? Any other ideas? Thanks, Luke [1] Once as SF () (Position,Velocity), and again as SF PhysIn PhysOut where PhysIn = (Impulse,Momentum) and PhysOut = (Position,Velocity). [2] Using the arrow joinSF :: SF (Event (SF [a] a)) [a], and other similar style functions taking streams of SF events...

4 4

cabal-install
by Ben Franksen 01 Dec '07

01 Dec '07

Just thought I install the latest version (0.4.0) from hackage and test it. Build and install went fine, but then it gets strange: ben@sarun: .../haskell/cabal-install-0.4.0 > sudo runhaskell Setup.lhs install Installing: /usr/local/bin ben@sarun: .../haskell/cabal-install-0.4.0 > rehash ben@sarun: .../haskell/cabal-install-0.4.0 > cabal --help Usage: cabal [GLOBAL FLAGS] or: cabal COMMAND [FLAGS] [...etc...] Commands: fetch Downloads packages for later installation or study. install Installs a list of packages. update Updates list of known packages clean Removes downloaded files list List available packages on the server. info Emit some info ben@sarun: .../haskell/cabal-install-0.4.0 > cd .. ben@sarun: .../software/haskell > cabal info Config file /home/ben/.cabal/config not found. Writing default configuration to /home/ben/.cabal/config. cabal: dist/Conftest.c: openFile: does not exist (No such file or directory) When later I tried the 'install' command, it reminds me to 'cabal update' first, which I do (no response, so probably worked), however next 'cabal install' gives the same funny answer about a missing dist/Conftest.c. Schade. Cheers Ben

4 9

Rigid type-var unification failure in existentials used with parametrically polymorphic functions
by Pablo Nogueira 01 Dec '07

01 Dec '07

A question about existential quantification: Given the existential type: data Box = forall a. B a in other words: -- data Box = B (exists a.a) -- B :: (exists a.a) -> Box I cannot type-check the function: mapBox :: forall a b. (a -> b) -> Box -> Box -- :: forall a b. (a -> b) -> (exists a.a) -> (exists a.a) mapBox f (B x) = B (f x) Nor can I type check: mapBox :: forall a. (a -> a) -> Box -> Box -- :: forall a. (a -> a) -> (exists a.a) -> (exists a.a) mapBox f (B x) = B (f x) The compiler tells me that in both functions, when it encounters the expression |B (f x)|, it cannot unify the universally quantified |a| (which generates a rigid type var) with the existentially quatified |a| (which generates a different rigid type var) -- or so I interpret the error message. However, at first sight |f| is polymorphic so it could be applied to any value, included the value hidden in |Box|. Of course, this works: mapBox :: (forall a b. a -> b) -> Box -> Box mapBox f (B x) = B (f x) Because it's a tautology: given a proof of a -> b for any a or b I can prove Box -> Box. However the only value of type forall a b. a -> b is bottom. This also type-checks: mapBox :: (forall a. a -> a) -> Box -> Box mapBox f (B x) = B (f x) When trying to give an explanation about why one works and the other doesn't, I see that, logically, we have: forall a. P(a) => Q implies (forall a. P(a)) => Q when a does not occur in Q. The proof in our scenario is trivial: p :: (forall a. (a -> a) -> (Box -> Box)) -> (forall a. a -> a) -> Box -> Box p mapBox f b = mapBox f b However, the converse is not true. Yet, could somebody tell me the logical reason for the type-checking error? In other words, how does the unification failure translate logically. Should the first mapBox actually type-check? Isn't the code for mapBox :: forall a. (a -> a) -> Box -> Box encoding the proof: Assume forall a. a -> a Assume exists a.a unpack the existential, x :: a = T for some T apply f to x, we get (f x) :: a pack into existential, B (f x) :: exists a.a Discharge first assumption Discharge second assumption The error must be in step 3. Sorry if this is obvious, it's not to me right now.

3 4

Re: [Haskell-cafe] A tale of Project Euler
by Sebastian Sylvan 01 Dec '07

01 Dec '07

On Nov 28, 2007 12:12 PM, Kalman Noel <kalman.noel(a)bluebottle.com> wrote: > Sebastian Sylvan: > > primes :: [Integer] > > primes = 2 : filter (null . primeFactors) [3,5..] > > > > primeFactors :: Integer-> [Integer] > > primeFactors n = factor n primes > > where > > factor m (p:ps) | p*p > m = [] > > | m `mod` p == 0 = p : factor (m `div` p) (p:ps) > > | otherwise = factor m ps > > Your definition gives a strange meaning to primeFactors. I'd want that for all > n, product (primeFactors n) == n. I think this law holds for the code posted > by Olivier. Yes you're right. That is property should clearly hold. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

9 20

Haskell and DB : giving up
by manu 01 Dec '07

01 Dec '07

Hello I've spent a few days trying to install all the packages required to use HaskellDB with either MySQL or SQlite3 (the only 2 DB the host I was thinking about is supporting) Well, I am giving up ! I seriously regret replacing ghc-6.6 with ghc-6.8, I didn't expect that building packages would be so ... unsucessfull and time-wasting. I just thought I'd let you know what's in store for "PHP-style-users- like-me" who want to use a database with ghc-6.8.1 at the moment, the hurdle is high ! I hope these packages will be fixed soon (I can't figure how to fix them myself obviously). And now I'm going to stay away from my computer for a while :) cheerio M

9 16

Progress indications
by Andrew Coppin 01 Dec '07

01 Dec '07

In a "normal" programming language, you might write something like this: for x = 1 to 1000000 print x ...do slow complex stuff... next x In Haskell, you're more likely to write something like result k = filter my_weird_condition $ map strange_conversion $ unfoldr ... That means that when you try to process the result, lots of processing happens, and your program just appears to lock up until a result is produced. So, like, how do you make it so that some kind of progress information is output while it's working? (Aside from dunking everything into the IO monad and ruining all your beautiful abstractions.) There doesn't seem to be a clean solution to this one...

9 13

Hit a wall with the type system
by Chris Smith 01 Dec '07

01 Dec '07

I was talking to a few people about this on #haskell, and it was suggested I ask here. I should say that I'm playing around here; don't mistake this for an urgent request or a serious problem. Suppose I wanted to implement automatic differentiation of simple functions on real numbers; then I'd take the operations from Num, Fractional, and Floating, and define how to perform them on pairs of values and their differentials, and then I'd write a differentiate function... but finding an appropriate type for that function seems to be a challenge. We have: 1. Differentiating a function of the most general type (Num a => a -> a) should produce a result of type (Num a => a -> a). 2. Differentiating a function of the more specific type (Fractional a => a -> a) should produce a result of that type (Fractional a => a -> a). 3. Differentiating a function of the most specific type (Floating a => a -> a) should produce a result of type (Floating a => a -> a). 4. BUT, differentiating a function which is of a more specific type than (Floating a => a -> a) is not, in general, possible. So differentiate should have type A a => (forall b. A b => b -> b) -> a -> a, but ONLY if the type class A is a superclass of Floating. Two partial solutions are: I can just define the differentiate function for Floating; but that means if I differentiate (\x -> x + 1), the result is a function only on floating point numbers, which is less than desirable. Or, I can define several functions: say, diffNum, diffFractional, and diffFloating... all of which have precisely the same implementation, but different types and require copy/paste to make them work. Any thoughts? For reference, here's the code I kludged together. (Again, I'm only playing around... so I wrote this very quickly and may have gotten some things wrong; don't use my code without checking it first! In particular, I know that this code produces derivative functions whose domain is too large.) > data AD a = AD a a deriving Eq > > instance Show a => Show (AD a) where > show (AD x e) = show x ++ " + " ++ show e ++ " eps" > > instance Num a => Num (AD a) where > (AD x e) + (AD y f) = AD (x + y) (e + f) > (AD x e) - (AD y f) = AD (x - y) (e - f) > (AD x e) * (AD y f) = AD (x * y) (e * y + x * f) > negate (AD x e) = AD (negate x) (negate e) > abs (AD 0 _) = error "not differentiable: |0|" > abs (AD x e) = AD (abs x) (e * signum x) > signum (AD 0 e) = error "not differentiable: signum(0)" > signum (AD x e) = AD (signum x) 0 > fromInteger i = AD (fromInteger i) 0 > > instance Fractional a => Fractional (AD a) where > (AD x e) / (AD y f) = AD (x / y) ((e * y - x * f) / (y * y)) > recip (AD x e) = AD (1 / x) ((-e) / (x * x)) > fromRational x = AD (fromRational x) 0 > > instance Floating a => Floating (AD a) where > pi = AD pi 0 > exp (AD x e) = AD (exp x) (e * exp x) > sqrt (AD x e) = AD (sqrt x) (e / (2 * sqrt x)) > log (AD x e) = AD (log x) (e / x) > (AD x e) ** (AD y f) = AD (x ** y) (e * y * (x ** (y-1)) + > f * (x ** y) * log x) > sin (AD x e) = AD (sin x) (e * cos x) > cos (AD x e) = AD (cos x) (-e * sin x) > asin (AD x e) = AD (asin x) (e / sqrt (1 - x ** 2)) > acos (AD x e) = AD (acos x) (-e / sqrt (1 - x ** 2)) > atan (AD x e) = AD (atan x) (e / (1 + x ** 2)) > sinh (AD x e) = AD (sinh x) (e * cosh x) > cosh (AD x e) = AD (cosh x) (e * sinh x) > asinh (AD x e) = AD (asinh x) (e / sqrt (x^2 + 1)) > acosh (AD x e) = AD (acosh x) (e / sqrt (x^2 - 1)) > atanh (AD x e) = AD (atanh x) (e / (1 - x^2)) > > diffNum :: Num b => (forall a. Num a => a -> a) -> b -> b > diffFractional :: Fractional b => (forall a. Fractional a => a -> a) -> b -> b > diffFloating :: Floating b => (forall a. Floating a => a -> a) -> b -> b > > diffNum f x = let AD y dy = f (AD x 1) in dy > diffFractional f x = let AD y dy = f (AD x 1) in dy > diffFloating f x = let AD y dy = f (AD x 1) in dy -- Chris Smith

8 16

Strings and utf-8
by Maurício 30 Nov '07

30 Nov '07

Hi, Are 'String's in GHC 6.6.1 UTF-8? Thanks, Maurício

11 17

Re: [Haskell-cafe] Re: New slogan for haskell.org
by Ivan Miljenovic 30 Nov '07

30 Nov '07

Speaking of Stackless Python, its homepage (http://www.stackless.com/) has a rather nice layout... maybe slightly less emphasis on the About section, but there you've got the links, the info and the news all on the one page.

3 3

Functional dependencies / monotonic boolean functions in Haskell
by Henning Thielemann 30 Nov '07

30 Nov '07

I know there are Haskell people who are busy with hardware verification and relational algebra. (as indicated by http://www.haskell.org/haskellwiki/Relational_algebra http://www.haskell.org/haskellwiki/Applications_and_libraries/Hardware_veri… ) Is there a Haskell library for working with functional dependencies, namely for computing minimal keys given a set of functional dependencies? Since this problem can also be posed in terms of monotonic boolean functions a library for processing monotonic boolean functions might also help.

1 0

Sillyness in the standard libs.
by Arthur van Leeuwen 30 Nov '07

30 Nov '07

LS, here is a puzzle for you: try converting a System.Posix.Types.EpochTime into either a System.Time.CalendarTime or a Data.Time.Clock.UTCTime without going through read . show or a similar detour through strings. The problem comes up when trying to easily nicely display the access, modification or status change times of a full directory of files, using System.Posix.Files.Filestatus to get at the times. A closely related issue: fromIntegral is in Integral which also requires quotRem. However, the two are semantically quite disjoint. I can *easily* see the semantics of fromIntegral on EpochTime, but not the semantics of quotRem on EpochTime. Having fromIntegral would solve the above puzzle... :) With kind regards, Arthur. -- /\ / | arthurvl(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

7 12

Haskell packaging on Windows "cygwin" (<< POSIX on Windows ; ^)
by Galchin Vasili 30 Nov '07

30 Nov '07

Hello, I trying to get a library to build ... I am following the instructions in http://en.wikibooks.org/wiki/Haskell/Packaging. Under the "Build your project" section in this web page, when I try to do an actual build (runhaskell Setup.lhs build), the imported module System.Environment cannot be found. I suspect that there is a "cygwin" ("Linux/Posix") environment variable that needs to be set to point to the Haskell libraries/packages. ?? Kind regards, Vasya

2 3

Question about takeMVar
by Maurício 29 Nov '07

29 Nov '07

Hi, 'takeMVar' documentation says "if there are multiple threads blocked in takeMVar, and the MVar becomes full, only one thread will be woken up." Since 'takeMVar' is a reading function, i.e., it doesn't change the value of the "variable", why waking up only one thread? If we wake multiple threads, there's no risk of one changing the value while the other is reading. Does that mean that all threads waiting for that MVar will get the same value, even if one of those threads change it's value just after reading? Thanks, Maurício

4 3