- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Very simple parser
by Gregory Propf 03 Jul '07

03 Jul '07

OK, I get the reference to currying now. A function of 2 variables can be seen as a function of one that returns another function of one. So a function of one variable can be seen as a function of no variables that returns a function of one. Very nice. ----- Original Message ---- From: Tim Chevalier <catamorphism(a)gmail.com> To: Gregory Propf <gregorypropf(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Sent: Monday, July 2, 2007 5:37:41 PM Subject: Re: [Haskell-cafe] Very simple parser On 7/2/07, Gregory Propf <gregorypropf(a)yahoo.com> wrote: > > > > This was a bit baffling too. It appears that there's an implied argument to runTick. This also works and makes it more explicit. I suppose the compiler just works out that the only place to put the 'n' is after tick. > > runTick :: Int -> (String,Int) > runTick n = runState tick n > Not exactly. Look up "currying". (Writing out the same definition with the argument "n" specified explicitly like you did is called "eta-expansion", by the way.) Cheers, Tim -- Tim Chevalier* catamorphism.org *Often in error, never in doubt "My writing is all completely autobiographical. That's why I write lesbian vampire stories." -- Jewelle Gomez ____________________________________________________________________________________ Got a little couch potato? Check out fun summer activities for kids. http://search.yahoo.com/search?fr=oni_on_mail&p=summer+activities+for+kids&…

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Re: [Haskell-cafe] Callback functions?
by Dan Piponi 03 Jul '07

03 Jul '07

On 7/2/07, Hugh Perkins <hughperkins(a)gmail.com> wrote: > Anyway, so the question is: how do we write callback functions in > FP/Haskell? Can someone provide a simple, but functional example? What makes a callback different from any other kind of function? In Haskell, as in other functional programming languages, you're completely free to pass functions into other functions and store them in datastructures. Take a look at the 'callbacks' used in the HOpenGL examples. They're just ordinary functions: http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/GLUT/examples/R… -- Dan

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Re: [Haskell-cafe] Very simple parser
by Gregory Propf 03 Jul '07

03 Jul '07

This was a bit baffling too. It appears that there's an implied argument to runTick. This also works and makes it more explicit. I suppose the compiler just works out that the only place to put the 'n' is after tick. runTick :: Int -> (String,Int) runTick n = runState tick n ----- Original Message ---- From: Arie Peterson <ariep(a)xs4all.nl> To: haskell-cafe(a)haskell.org Sent: Monday, July 2, 2007 4:51:59 PM Subject: Re: [Haskell-cafe] Very simple parser To get you going, start with the example from the documentation, modified slightly: > tick :: State Int String > tick = do > n <- get > put (n+1) > return (show n) If you want to actually run 'tick', use the 'runState' function: > runTick :: Int -> (String,Int) > runTick = runState tick ____________________________________________________________________________________ Sucker-punch spam with award-winning protection. Try the free Yahoo! Mail Beta. http://advision.webevents.yahoo.com/mailbeta/features_spam.html

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Very simple parser
by Gregory Propf 03 Jul '07

03 Jul '07

As a programming exercise I'm trying to use the State monad to create a simple parser. It's for a very simple assembly language for a simple virtual machine. The state is a string of instructions. I want to be able to call something like getNextInstruction to pull out the next instruction and then update the state (string). I know I can do this non-monadically by just passing the string explicitly each time but I'd like to learn more about the State monad. I also know about Parsec and Happy and so forth but this is just an exercise so I want to do it this way. Any ideas? I can't seem to get anything to work. I've tried different things but I suspect I'm just missing something basic. Can someone post a simple prototype for this? Just assume the instructions are integers. ____________________________________________________________________________________ Get the Yahoo! toolbar and be alerted to new email wherever you're surfing. http://new.toolbar.yahoo.com/toolbar/features/mail/index.php

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Callback functions?
by Hugh Perkins 03 Jul '07

03 Jul '07

This sounds something like using a continuation function. In imperative languages, we might have something like: class MyDisplayForm { void ButtonGo_Press() { Processor.Go( new CallbackDelegate( this.SetStatus ) ); } public void SetStatus( string message ) { StatusLbl.Text = message; Application.DoEvents(); } } So: we call Processor.Go, passing in a pointer to the function SetStatus (a "delegate" in C# terminology, an "interface" in Java, a pointer in C, ...). The Processor.Go function is going to update the status bar in MyDisplayForm as it goes along with useful messages "Processing blah.txt...", etc. It does that by calling the passed in "callback" function, with the appropriate status message as a parameter. This is also implicit to the Observer pattern, which may or may not be useful in FP (?) Anyway, so the question is: how do we write callback functions in FP/Haskell? Can someone provide a simple, but functional example?

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XmlSerializer.deserialize?
by Hugh Perkins 03 Jul '07

03 Jul '07

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Re: [Haskell-cafe] N00b question
by Jim Burton 02 Jul '07

02 Jul '07

poop wrote: > > So I'm working my way thorough haskell, doing some programming problems, > and I have this one so far: > Hi, I haven't spotted the problem in your code but there's an alternative solution to Euler Problem 14 on the wiki: http://www.haskell.org/haskellwiki/Euler_problems/11_to_20#Problem_14 -- it may be helpful as a comparison? Regards, > p14next n = if (mod n 2 == 0) then (div n 2) else (3*n + 1) > > p14seqlen n = f' 1 n where > f' accum n > | n == 1 = accum > | otherwise = f' (accum + 1) (p14next n) > > sndmax a b = if snd a > snd b then a else b > > p14 n = fst (f' (0,0) n) where > f' maxTuple n > | n == 0 = maxTuple > | otherwise = f' (sndmax maxTuple (n, p14seqlen n)) (n-1) > > the goal is to be able to run: > p14 999999 > > and not get a stack overflow message. I read what was available on tail > recursion and I think I have it working with tail recursion now, at least > it gives me a stack overflow message really quickly compared to before :) > > I have not been able to find any documentation on seq and ($!) which was > also mentioned as a way to get rid of stack overflow messages so if those > are what is required an explanation of those would be most helpful. > > -- View this message in context: http://www.nabble.com/N00b-question-tf4010616.html#a11395620 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Tree Guidance
by Hans van Thiel 02 Jul '07

02 Jul '07

Hello All, Both CFP and SOE have chapters on trees and there is a standard library Data.Tree. I expected to find all kinds of functions there, as in Data.List, but instead the functions are defined as instances of more general structures. It uses: import Control.Applicative (Applicative(..), (<$>)) import Control.Monad import Data.Monoid (Monoid(..)) import Data.Sequence (Seq, empty, singleton, (<|), (|>), fromList, ViewL(..), ViewR(..), viewl, viewr) import Data.Foldable (Foldable(foldMap), toList) import Data.Traversable (Traversable(traverse)) import Data.Typeable When looking at those it is not clear (to me) what those all mean and I have not been able to find much on the WiKi either. Could someone with more experience provide some guidance on how to learn to use these libraries (and ultimately on the tree type in Data.Tree, which is what I want). There is a Zipper Monad with defined functions, which look promising, but I have not heard of this before, and it appears not to be standard. http://www.haskell.org/haskellwiki/Zipper_monad/TravelBTree http://www.haskell.org/haskellwiki/Zipper_monad Does anybody have any experience with these libraries they could share? Ideally I would want a N-ary tree, where the branches are what's actually the represented data, and so I would like to access it both from the root and the leaves. In an imperative language I would just add an up-pointer in each node, but I have no idea how expensive this would be in Haskell, or if it's necessary at all. Many thanks for your help, Hans van Thiel

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Re: [Haskell-cafe] Re: Parsers are monadic?
by Gregory Propf 02 Jul '07

02 Jul '07

Thanks, that was helpful. I didn't realize that there were pure functional monads. ---------------------------------------------------------- "Monadic" just means a calculation using a mathematical structure called a monad. All impure calculations in Haskell are monadic, but not all monadic calculations are impure. Does this answer your question? --Eric _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ____________________________________________________________________________________ Be a better Globetrotter. Get better travel answers from someone who knows. Yahoo! Answers - Check it out. http://answers.yahoo.com/dir/?link=list&sid=396545469

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TT check
by Andrew Coppin 02 Jul '07

02 Jul '07

Can somebody check that I've implemented this correctly? *****X*X*X*XX***X*X*XXX*X*X*XX***X*X*X*XX**X*X*XX**X*X*X*XX ***X*X*XXX*X*X*XX**XX*X*XX***X*X*XXX*X*XX****X*X*X*XX****X* X*X*XX***X*X*XXX*X*X*XXX*X*XXXX****X*X*XXX****X*X*X*XX***X* X*XXX*X*X*XXX*X*XXXX A parser for this is given by decode (c:cs) = case c of 'X' -> X '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs0 in (e0 `apply` e1, cs1) The letter X stands for the following combinator: X = \x -> xSK K = \xy -> x S = \fgx -> fx(gx) Assuming my Haskell code is correct, the above *should* be a program to compute the sum of the Church numberal 2 and the Church numeral 2. In other words, this is the world's most verbose way to define the act of putting two and two together. ;-) But... I don't know... that's A LOT of expression there! Have a slipped up somewhere? Does anybody have a way to check?

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Name Decoration and Undefined References
by SevenThunders 01 Jul '07

01 Jul '07

I have a very complex project that has to play nice with a lot of C code written by other people. My own code is half Haskell and half C. My build process is rather complex since I generate about 5 different libraries, some of them export Haskell routines. A supreme effort was made to try to make Haskell somewhat invisible, although ultimately it requires a lot of gcc command line work with a bunch of undefines and a links to Haskell static libraries. In the end I'm not sure if it's such a good idea and probably would be better off completing the final link stage just using ghc. In the process of doing all this I attempted to link one of my libraries to some test code written in Haskell and I proceeded to get a few unresolved links whilst running ghc. What happened is that I have a Haskell binary, lets say called foo.o generated from foo.hs that appears in two libraries say called libmyh.a and libmyc.a and that contains a Haskell function called bar. Library libmyh.a ultimately gets built using cabal and libmyc.a gets built using gcc directly. The binary is compiled twice (for complicated reasons) prior to linking to each library. However in the library libmyh.a, the bar symbol is prepended with an ascii string that contains the library name (say libmyh). However the other library shows that the symbol does not have this name decoration since it was linked directly with gcc. It appears however when trying to link against libmyc.a that ghc expects to see the original name decoration for bar. I presume this is due to some side information, perhaps what's contained in foo.hi that informs ghc what names to actually link to. I now ask the question, how can I control this process? Build tools such as CMake and Cabal seem to have their own ideas about what directories to place binaries. It seems like I need to somehow make sure that all files related to the compilation process of a given library are isolated in the same directory or something. Thus it's not enough to specify binaries on the command line of ghc, it's got to have access to some other files; I presume the .hi files. This makes things a bit confusing I'm afraid. -- View this message in context: http://www.nabble.com/Name-Decoration-and-Undefined-References-tf4003458.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Read-only" functions in State Monad
by Ken Takusagawa 01 Jul '07

01 Jul '07

I'd like to have a state monad with the feature that I can somehow annotate using the type system that some functions are only going to read the state and not modify it. Such read-only functions are only permitted to call other read-only functions, whereas state-modifying functions can call both read-only and other state-modifying functions. How can I do this? It does not seem to be straightforwardly doable with Control.Monad.State. --ken

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Fwd: Re: [Haskell-cafe] avoiding command window with wxHaskell on Windows?
by Dean Herington 01 Jul '07

01 Jul '07

>Date: Mon, 25 Jun 2007 20:19:50 -0400 >To: Jens Fisseler <Jens.Fisseler(a)FernUni-Hagen.de> >From: Dean Herington <heringtonlacey(a)mindspring.com> >Subject: Re: [Haskell-cafe] avoiding command window with wxHaskell on Windows? >Cc: haskell-cafe(a)haskell.org, wxhaskell-users(a)lists.sourceforge.net > >At 10:10 PM +0200 6/23/07, Jens Fisseler wrote: >>On Sat, 23 Jun 2007, Dean Herington wrote: >> >>> But if I double-click the .exe file, I get a command window that >>>hangs around >>> (but doesn't appear to do anything, fortunately) until the >>>program terminates. >>> How can I avoid this command window? >> >>With gtk2hs, using "-optl-mwindows" as a command line option for GHC lets >>me get rid of this window. Perhaps it will do the same for wxHaskell? > >Yes, that did the trick! Thanks a lot! But now I've discovered that using "-optl-mwindows" creates a program that doesn't work when invoked from a command line. Is there any way to create a program that can work when invoked either from a command line or through double-clicking? Dean

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advice: instantiating/duplicating modules
by Nicolas Frisby 30 Jun '07

30 Jun '07

I wrote a combination reader/writer monad (a la the RWS monad in the mtl) and I find myself wanting to use multiple instances of it in the same stack of transformers. The functional dependencies prevent this from working out. The class is called MonadRW and the transformer is called RWT. I find myself wishing I could import the same module twice, but instead of having two names for the same entity, I want the definitions proper of the module to be duplicated: I want two separate MonadRW classes and two separate RWT transformers. Since the module integrates the MonadRW and RWT transformer with the mtl, I would then only need to lift the instances of the instantiated MonadRW classes through the other RWTs. I'm rather unfamiliar with Template Haskell, but it sounds like it might fit the bill. Although, if I recall correctly, instances and type declarations splicing are yet to be implemented in GHC? Does anyone have any advice how to do this? I'm envisioning some preprocessing. I'd like to know if someone has developed a tool for this already or if this path has been attempted. Thanks for your time, Nick PS - If this doesn't work out, I'm aware that I could use type-indexed products and coproducts to pull it off, but I think that would drastically reduce the number of people who could understand/maintain the code without having to learn such cool stuff.

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ANNOUNCE: hiccup, a toy tcl impersonator in haskell
by Kyle Consalus 30 Jun '07

30 Jun '07

A while back I saw a toy tcl interpreter in 550 lines of C called 'picol'. I was looking for a simple language to implement in haskell, so I made my own toy tcl interpreter. It was surprisingly easy to make, thanks to the magic of Haskell and Bytestrings. :) It handles a few things incorrectly, and it is not feature complete, but I thought maybe somebody else might find it interesting. For the record (though it means nothing), my interpreter is about half the size of picol and runs about 30% faster in the few tests I've run, despite having a few more features. (it was certainly less than half the size of picol, but I keep tacking on functions when I get bored). Anyway, you can check it out at: http://code.google.com/p/hiccup/ I plan on making it more complete and faithful to tcl, but I'm branching into a different project for this one. However, any suggestions to make hiccup more efficient, elegant, or correct are certainly appreciated. Cheers, Kyle

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Organising teams for ICFP 2007
by dons＠cse.unsw.edu.au 30 Jun '07

30 Jun '07

Interested in competing in the ICFP 2007 programming contest -- the hackers contest of choice! http://www.icfpcontest.org There's 2000 people on this mailing list, and 350 people in #haskell, we must be able to put together a few decent teams out of that talent pool.... To help people get organised, I've created a page on the haskell.org wiki to help Haskell (and FP) people find teams: http://haskell.org/haskellwiki/ICFP_Programming_Contest/Teams_2007 If you've got a team, but need more people, or need a team, add your details, and hopefully people will be able to find you. The more teams the better. Also, drop by the #haskell-icfp07 IRC channel to talk to people and organise. -- Don

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let vs do?
by Greg Meredith 30 Jun '07

30 Jun '07

Haskellians, Once you have a polymorphic let, why do you need 'let' in the base language, at all? Is it possible to formulate Haskell entirely with do-notation where there is a standard monad for let environments? Probably this was all discussed before in the design deliberations for the language standard. Pointers would be very much appreciated. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Updating haskell.org robots.txt
by Justin Bailey 30 Jun '07

30 Jun '07

All, Referencing this thread ( http://www.haskell.org/pipermail/haskell-cafe/2007-May/025769.html) I have prepared a small update to robots.txt. It doesn't do much, but should exclude all dynamic pages from being spidered. Please vet it and let me know of any concerns. Note I'm not really interested in reconfiguring the wiki to have nicer URLs - what is there is what I'm working with right now. Here's the updated robots.txt (availabe as http://www.haskell.org/robots2.txt right now): User-agent: * Disallow: /haskellwiki/Special: Disallow: /haskellwiki/Special%3A Disallow: /haskellwiki/? Disallow: /haskellwiki/%3f Any feedback, let me know. Thanks! Justin

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HGL on Windows
by peterv 30 Jun '07

30 Jun '07

I vaguely remember to have read that HGL is (currently) not supported on Windows, but I can't find this information any more. Is this correct? It seems not to be included in GHC 6.6

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Best idiom for avoiding Defaulting warnings with ghc -Wall -Werror ??
by Dave Bayer 30 Jun '07

30 Jun '07

Hi all, I've been going over my code trying to get it all to compile with "ghc -Wall -Werror", without introducing constructs that would make my code the laughing stock of the dynamic typing community. They already think we're nuts; my daydreams are of a more computer literate society where Jessie Helms stands up in the U.S. Senate to read aloud my type declarations to the derisive laughter of the Ruby and Lisp parties. There's a fine line between my opinion as to how GHC should issue warnings, and a legitimate bug report. I've already submitted a bug report for the need to declare the type of the wildcard pattern, because I believe that the case is clear. Here, I'm seeking guidance. Perhaps I just don't know the most elegant construct to use? My sample code is this: > {-# OPTIONS_GHC -Wall -Werror #-} > > module Main where > > import Prelude hiding ((^)) > import qualified Prelude ((^)) > > default (Int) > > infixr 8 ^ > (^) :: Num a => a -> Int -> a > x ^ n = x Prelude.^ n > > main :: IO () > main = > let r = pi :: Double > x = r ^ (3 :: Int) > y = r ^ 3 > z = r Prelude.^ 3 > in putStrLn $ show (x,y,z) > GHC issues a "Warning: Defaulting the following constraint(s) to type `Int'" for the definition of z. The definition of y glides through, so a qualified import and redefinition of each ambiguous operator does provide a work-around, but the code is lame. (I could always encapsulate it in a module Qualude.) If I import a module that I don't use, then "ghc -Wall -Werror" rightly complains. By analogy, if I use "default (Int)" to ask GHC to default to Int but the situation never arises, then GHC should rightly complain. Instead, if I use "default (Int)", GHC complains about defaulting anyways. In my opinion, this is a bug, but I'd like guidance before reporting it. Is there a more elegant way to handle the numeric type classes with "ghc -Wall -Werror" ? No one is forced to use "ghc -Wall -Werror", but it should be a practical choice. I've enjoyed the recent typing discussions here. On one hand, there's little difference between using dynamic typing, and writing incomplete patterns in a strongly typed language. On the other hand, how is an incomplete pattern any different from code that potentially divides by zero? One quickly gets into decidability issues, dependent types, Turing-complete type systems. My personal compromise is to use "ghc -Wall -Werror", live with the consequences, and get back to work. Perhaps I'll get over it, but that's a slippery slope back to Lisp.

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formatTime %Q not working
by Bit Connor 30 Jun '07

30 Jun '07

Hello, I am using ghc 6.6. I am using the formatTime function from Data.Time.Format. The docs say that the %Q format string gives: "decimal point and up to 12 second decimals, without trailing zeros." But I always just get an empty string. Here is what happens when I try using ghci: > now <- getCurrentTime 2007-06-21 13:48:44.298699 UTC > formatTime defaultTimeLocale "%s" now "1182433724" > formatTime defaultTimeLocale "%Q" now "" "%q" also always gives me an empty string. Is this a bug? Thanks, Bit

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ANN: newports.hs utility for freebsd
by brad clawsie 29 Jun '07

29 Jun '07

i have written a small haskell program to solve a problem many users of freebsd may have - knowing what ports have been updated after a daily/weekly etc cvsup. this is a trivial bit of coding hardly worth attention, but if it might be of use to you, you can find it here: http://www.b7j0c.org/content/haskell-newports.html standard disclaimers - coded & tested mildly by me, a hobbyist haskeller

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Newbie question: alternative for toInt/fromInt
by peterv 29 Jun '07

29 Jun '07

I'm trying to get the SOE graphics library to compile for Win32 using the latest libraries. I fixed a couple of imports, but in the file GraphicsTypes.hs, the functions toInt/fromInt are used, which are now obsolete: type Dimension = Int toDimension :: Win32.INT -> Dimension fromDimension :: Dimension -> Win32.INT toDimension = toInt fromDimension = fromInt I don't have a clue how to fix this, as I can't find alternatives for toInt/fromInt (except toInteger/fromInteger, but that would convert to heavyweight ints), but I guess it must be really easy? Thanks, Peter

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New New newbie question/help
by Balu Raman 29 Jun '07

29 Jun '07

Hi, Hope someone can help me, just starting out with SOE.My code : module Main where import Graphics.SOE.Gtk spaceClose :: WIndow -> IO() spaceClose w = do k <- getKey w if k == ' ' then closeWindow w else spaceClose w equilateralTri :: Window -> Int -> Int -> Int -> IO() equilateralTri w x y side = drawInWindow w (withColor Red (polygon [(x,y),(a,b),(x,y)])) where b = y + side * sin(pi/3) a = x + side * cos(pi/3) main = runGraphics( do w <- openWindow "Equilateral Triangle" (400,400) equilateralTri w 50 300 200 spaceClose w ) all of the above in file triangle.hs when I do a :l triangle.h in ghci, I get the following error triangle.hs:17:36: No instance for (Floating Int) arising from use of 'pi' at triangle.hs:17:36-37 Probable fix: add an instance declaration for (Floating Int) In the first argument of '(/)', namely 'pi' In the first argument of 'cos', namely '(pi / 3)' In the second argument of '(*)', namely 'cos (pi/3)' Failed, modules loaded: none Can someone help me what's going on to a brand new newbie. All I can figure out is that some type mismatch between float and int . I tried various combinations of lets and wheres and I still get the same complaints. I am just linearly studying SOE Thanks, - br

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Status of MIME Strike Force
by Arie Peterson 28 Jun '07

28 Jun '07

Hi, What is the status of the MIME Strike Force? The goals proposed at <http://www.haskell.org/haskellwiki/Libraries_and_tools/MIMEStrikeForce> promise a very useful library. Has the design been initiated? Kind regards, Arie

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Tools for Haskell and COM
by Lewis-Sandy, Darrell 28 Jun '07

28 Jun '07

Are there any currently maintained tools for interfacing Haskell with COM objects? It appears that both Haskell script and Haskell direct haven't been updated since the turn of the century, and have fallen out of step with recent library changes.

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Re: [Haskell-cafe] HDBC and Sqlite3: SqlValue is restricted to SqlString
by Malte Milatz 28 Jun '07

28 Jun '07

Michael T. Richter: > http://www.sqlite.org/datatype3.html > > Short summary: any data you put into SQLite3 may or may not come out > as text. :D Thanks, Michael. (I'm forwarding this to the list again because there is at least one other person interested.) Well, I've read that page before and I think you're wrong. Sqlite3 is “spontaneously typed”, so to say, but it is still typed. Furthermore the column type gives a hint at how to interpret a value. And what's more, Sqlite3 does not store integers as plain strings anymore, so it seems very unnatural to me for a binding to represent them as strings. Although I do not use Python, I made use of the fact that there are a couple of Python libraries installed on my system, and voilà: >>> print cur.fetchall() [(1, 0, u'a'), (2, 1, u'b')] Here I get my integers as integers and my strings as strings. And, well, Boolean values as numbers, but that's Python's truth philosophy. Anyway, this has convinced me that there is room for improvement in HDBC-Sqlite, unless I have entirely missed something. Regards, Malte

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Yet another implementation of cond
by Jon Cast 28 Jun '07

28 Jun '07

I discovered this trick the other day, and didn't remember seeing it anywhere as a cond implementation: head $ [ e1 | cond1 ] ++ [ e2 | cond2 ] ++ [ e3 | cond3 ] etc. You can even use full pattern guards (more powerful than GHC's) in your conditions! Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs

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Cabal Configurations - Beta-Testers Wanted
by Thomas Schilling 28 Jun '07

28 Jun '07

Hello Cafe! Cabal configurations are an often-requested new feature of Haskell's packaging system Cabal. As part of my Google Summer of Code project I implemented it with the feature set of the latest proposal[1]. I would now like people to try it out and give some feedback if it solves their problems, if there are bugs with the current implementation, or if it breaks any install scripts. So, if you have written a package and think your package could profit from Cabal configurations, please adjust your package description and try to build it on as many systems as possible and report any problems (or successes). Following are some instructions to get started. Thanks, / Thomas INSTALLING [Note: The following build instructions are for a recent GHC. If you have another Haskell compiler/interpreter you'll probably have to adjust these steps a bit. Please report any problems you encounter.] You should be able to get Cabal with configurations via: $ darcs get --partial http://code.haskell.org/~nominolo/src/cabal +configs/ Before installing you might want to hide the current cabal package using (tt shouldn't be necessary, though) via $ ghc-pkg hide Cabal or $ sudo ghc-pkg hide Cabal depending on where Cabal is installed. Installing the new Cabal should be as simple as: $ cd cabal+configs $ make install user=y PREF=~/sw This will build Cabal with version 1.1.7, register it in the current user's package database, and install it to the "sw" directory of the current user home directory. Adjust the parameters to your needs. To remove use: $ make remove user=y PREF=~/sw NEW FEATURES The new constructs didn't integrate well with the conventional syntax, so I changed it to a C-style block-style syntax. (This is consistent with Haskell's use of { .. } in do-blocks; adding or replacing this with an indentation-based approach is thinkable and might actually integrate better with the rest of Cabal file syntax. So yell if you want it--but no bike sheds please! ;) The new syntax structures cabal files like this: global properties optional flags descriptions Library { library build properties and conditionals } Executable name { executable build properties and conditionals } For an example, see below. Inside the Library and executable sections you can now use conditionals with the syntax: body: cabal_stanza "\n" body | "if" conditional "{" body "}" [ "else" "{" body "}" ] body | <nothing> conditional: "os(" string ")" | "arch(" string ")" | "flag(" flagname ")" | "True" | "False" | "!" conditional | conditional "||" conditional | conditional "&&" conditional | "(" conditional ")" Note that you cannot test for dependencies directly. The user either specifies certain features she wants by switching flags on or off during the configure step or Cabal picks a suitable flag assignment for you, depending on which dependencies are available. Unless specified otherwise, flag assignments default to True. ATM, the only field specifying dependencies is "build-depends". Options specified inside a conditional are (mostly) added to the values specified outside. E.g. GHC-Options: -Wall if flag(debug) { GHC-Options: -DDEBUG } when built with flag "debug" set to True will be built with GHC-Options: -Wall -DDEBUG For a more detailed description see[1]. The following example .cabal- file should demonstrate the features (it is also available and updated at[2]): Name: Test1 Version: 0.0.1 Cabal-Version: >= 1.1.7 License: BSD3 -- License-File: LICENSE Author: Thomas Schilling <notme(a)nospam.com> Maintainer: Thomas Schilling <notme(a)nospam.com> --Homepage: http://www.example.com/ Synopsis: Test package to test configurations Description: See synopsis. . Really. Category: Useless Flag Debug { Description: Enable debug support Default: False } Flag NoBuild { Description: Inhibit building this package. -- defaults to True } Library { Build-Depends: base Exposed-Modules: Testing.Test1 Extensions: CPP -- flag names are case insensitive if flag(debuG) { CC-Options: "-DDEBUG" GHC-Options: -DDEBUG } if flag(NoBuild) { Build-Depends: nonexistentpackage } } Executable test1e { Main-is: T1.hs Other-modules: Testing.Test1 if flag(debug) { CC-Options: "-DDEBUG" GHC-Options: -DDEBUG } } When configuring it with the usual command line, you now get an additional line, showing which flags were chosen: $ ./Setup.lhs configure configure: Reading installed packages... Configuring Test1-0.0.1... configure: Flags chosen: nobuild=False, debug=False Setup.lhs: Warning: No license-file field. configure: Dependency base-any: using base-2.1.1 [...] Note that, even though the default for the "nobuild" flag was True, the required dependencies weren't present, so it was forced to False. If you want to force flags to certain values you can do so by giving the --flags or -f flag to configure. Listing the (case-insensitive) name forces it to True, putting a "-" in front of the name forces it to False. For example: $ ./Setup.lhs configure -fdebug configure: Reading installed packages... Configuring Test1-0.0.1... configure: Flags chosen: nobuild=False, debug=True Setup.lhs: Warning: No license-file field. [...] or $ ./Setup.lhs configure --flags="-debug nobuild" configure: Reading installed packages... Configuring Test1-0.0.1... Setup.lhs: At least the following dependencies are missing: nonexistentpackage -any $ Note that if you want to change the configuration of a package, you have to ./Setup.lhs clean first, to make sure everything gets recompiled. Good Luck! [1] http://www.mail-archive.com/cabal-devel@haskell.org/msg00282.html [2] http://hackage.haskell.org/trac/hackage/wiki/CabalConfigurations

1 0

Running GranSim on GHC 6.6.1
by Felipe Almeida Lessa 28 Jun '07

28 Jun '07

Hello, everybody! I'd like to know how to compile with GranSim. When compiling anything I get an error about package "concurrent". In fact, it complains even something simple like $ ghc -gransim ghc-6.6.1: unknown package: concurrent This happens with GHC 6.6 from Ubuntu packages and from GHC 6.6.1 compiled from the site tarballs with "./configure --enable-gransim". Is GranSim broken? Or deprecated? Maybe on GHC from Darcs? Or do I have to do something else? I couldn't find anything recent about GranSim (and I've googled quite a lot =). Any pointers are appreciated! Thanks in advance, -- Felipe.

1 0

Reinvention
by Andrew Coppin 28 Jun '07

28 Jun '07

I seem to be forever writing code that looks like this: decode :: String -> (SKI,String) decode (c:cs) = case c of 'S' -> (S,cs) 'K' -> (K,cs) 'I' -> (I,cs) '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs1 in (e0 :@: e1, cs1) In other words, lots and lots of functions with signatures like foo :: [Foo] -> (Bar,[Foo]) that likes to call itself recursively, or that gets called from somewhere else. Is there a Better Way(tm) to do this? It looks like such a common idiom that you'd expect to see "something" in the libraries somewhere. (I remember being puzzled that there was no library function for creating a Cartesian product of two lists. Puzzled until I realised that the monadic nature of lists make it utterly trivial to do this by hand anyway! So it's not always obvious to know what you're looking for...)

4 4

A Query Language for Haskell Terms
by Pasqualino 'Titto' Assini 28 Jun '07

28 Jun '07

Hi, I am writing a Web application using HAppS. As all HAppS apps, it represents its internal state as a Haskell term (HAppS automagically provides persistence and transactions). It is a neat and efficient solution, you can write your data model entirely in Haskell and, at least for read-only transactions (queries) it will be operate as fast as possible as all data is in memory (if your transactions modify the application state, the transactions has to be recorded on disk to make it persistent, but this is pretty fast too). One major component, however seem to be missing, if we are effectively using Haskell as an in-memory database where is the "SQL for Haskell": a generic query language for Haskell terms? There are three basic functions that every web app has to provide, and all of them could be provided by a generic "Haskell SQL": -- query the application state -- transform (possibly monadically) the application state : the result of the query is the new state -- access control: what an user can see is what is returned by an internal access control query The availabilty of such a language would be a major boost for Haskell-based web applications as every application could be accessed via the same API, the only difference being the underlying application-specific data model. So my question is: what ready-made solutions are there in this space, if any? And if there are none, how would you proceed to design/implement such a language? The basic requirements, in decreasing order of importance, are: -- Safe, it must be possible to guarantee that a query: --- cannot cause a system crash --- completes by a fixed time of time --- uses a 'reasonable' amount of space --- cannot perform any unsafe operation (IO, or any unallowed read/write of the application state) -- Expressive (simple queries should be simple, complex queries should be possible) -- Simple to implement -- Efficient: --- Repeated queries should be executed efficiently time-wise (it is acceptable for queries to be executed inefficiently the first time) and all should be space-efficient, so it should not do unnecessary copying. -- User friendly: --- Simple to use for non-haskeller --- Short queries Ah, I almost forgot, it should also be able to make a good espresso. The problem can be broken in two parts: 1) How to implement generic queries on nested terms in Haskell? 2) How to map the queries, written as a string, to the internal Haskell query Regarding the first point, I am aware of with the following options: - SYB (Data.Generics..) - Oleg's Zipper - (Nested) list comprehensions (that are being extended with SQL-like order by and group by operators) Being rather new to Haskell all these options are rather unfamiliar so I would appreciate any advice on what should be preferred and why. Regarding the second point: The simplest solution would be to avoid the problem entirely by using Haskell directly as the query language. This is the LambdaBot way: queries are Haskell expression, compiled in a limited environment (a module with a fixed set of imports, no TH). Lambdabot avoids problems by executing the expression on a separate process in a OS-enforced sandbox that can be as restrictive as required (especially using something like SELinux). However, to get the query to execute efficiently it would probably have to be executed in a GHC thread and I am not sure how safe that would be. Looking at the discussion at http://haskell.org/haskellwiki/Safely_running_untrusted_Haskell_code it seems clear that there are many open issues. For example, how would I enforce limits on the space used by the query? So, it would probably be better to define a separate query language that is less expressive but more controllable than full Haskell, but what form should that take? Any suggestion/tip/reference is very welcome, titto

3 5

HDBC and Sqlite3: SqlValue is restricted to SqlString
by Malte Milatz 28 Jun '07

28 Jun '07

It's always good to know there's a general Haskell mailing list to ask questions. I'm experimenting with the HDBC package in combination with Sqlite3. Here's my example table: sqlite> .schema foos CREATE TABLE foos (id integer primary key, flag bool, comment text); sqlite> select * from foos; id flag comment ---------- ---------- ---------- 1 0 a 2 1 b Here's my query in ghci: Database.HDBC> quickQuery conn "select * from foos;" [] [[SqlString "1",SqlString "0",SqlString "a"], [SqlString "2",SqlString "1",SqlString "b"]] What strikes me is that all values are represented as SqlStrings, although the type SqlValue would allow something more succint, e.g. SqlBool for the flag column. Is this a general limitation of the Sqlite backend, or is this by design (Sqlite seems to do typing differently than I had expected), or is it just that nobody has implemented a better behavior yet? Curiously, Malte

1 0

Preferred way to get data from a socket
by Chad Scherrer 27 Jun '07

27 Jun '07

I've never used sockets before, but I need to now, and I need to be able to get a lot of data quickly. I was thinking about doing something like this (below), but I'm wondering if there's a way that would be faster. Is the obvious way of doing this "the right way"? I'm happy to install outside libraries if that would help, as long as they work on Linux and MS. Thanks! -Chad ----------------------8<------------------------------- import Network import qualified Data.ByteString.Lazy as B hostName = "myComputer" portID = PortNumber 54321 theData :: IO B.ByteString theData = connectTo hostName portID >>= B.hGetContents

3 7

directory tree?
by Chad Scherrer 27 Jun '07

27 Jun '07

Haskell is great at manipulating tree structures, but I can't seem to find anything representing a directory tree. A simple representation would be something like this: data Dir = Dir {dirName :: String, subDirectories :: [Dir], files :: [File]} data File = File {fileName :: String, fileSize :: Int} Maybe these would need to be parametrized to allow a function splitting files by extension or that kind of thing. Anyway, the whole idea would be to abstract as much of the file stuff out of the IO monad as possible. I haven't used the "Scrap Your Boilerplate" stuff yet, but it seems like that could fit in here naturally to traverse a Dir and make changes at specified points. The only problem I can see (so far) with the approach is that it might make "big changes" to the directory tree too easy to make. I'm not sure immediately how to deal with that, or if the answer is just to post a "be careful" disclaimer. So, what do you think? Do you know of any work in this direction? Is there a way to make "dangerous 1-liners" safe? Is there a fundamental flaw with the approach I'm missing? Thanks much, Chad

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ANN: Scripting.Lua 0.1
by Gracjan Polak 27 Jun '07

27 Jun '07

Hi all, I'm pleased to announce the first public release of Scripting.Lua. The package hslua-0.1 contains Haskell FFI bindings for a Lua interpreter along with some Haskell utility functions simplifying Haskell to Lua and Lua to Haskell calls. Full Lua interpreter is included in the package. Example import qualified Scripting.Lua as Lua main = do l <- Lua.newstate Lua.openlibs l Lua.callproc l "print" "Hello from Lua" Lua.close l More information http://home.agh.edu.pl/~gpolak/hslua The Lua language http://www.lua.org -- Gracjan

4 5

Re: [Haskell-cafe] Collections
by Andrew Coppin 27 Jun '07

27 Jun '07

Dan Piponi wrote: > Andrew said: > >> True enough - but that's a rather specific task. I'm still not seeing >> vast numbers of other uses for this... > > Graphs are one of the most ubiquitous structures in the whole of > computer science. Whether you're representing dataflows, or decoding > error-correcting codes, or decomposing an almost block matrix into > independent parts for multiprocessing, or figuring out which registers > to spill in a compiler, or programming neural networks, or finding the > shortest path between two cities, or trying to find dependencies in a > sequence of tasks, or constructing experimental designs, or using an > expert system to diagnose disease symptoms, or trying to find optimal > arrangements of marriage partners, or a million other tasks, graphs > appear everywhere! I see *trees* around the place a lot, but not general graphs. Maybe it's just the type of problems I attempt to solve?

6 9

Parsec
by Michael T. Richter 27 Jun '07

27 Jun '07

I just wanted to say "thank you" to Daan Leijen for Parsec. It's nice to finally find a tool that does parsing that's: a) readable; b) understandable; c) tightly integrated with the target language. I've been dodging Parsec for a long time because my experiences with parser generators and parser libraries in the past have been unpleasant chores. Parsec is a ray of sunshine. Thanks, Daan. -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) The most exciting phrase to hear in science - the one that heralds new discoveries - is not "Eureka!" but "That's funny..." (Isaac Asimov)

1 0

Re: [Haskell-cafe] Haskell version of ray tracer code is much slower than the original ML
by Philip Armstrong 26 Jun '07

26 Jun '07

On Thu, Jun 21, 2007 at 12:25:44PM +0100, Sebastian Sylvan wrote: >Try using floats for the vector, and strict fields (add a ! to the >fields in the data declaration). Because the optimisation page on the haskell wiki is very explicit about never using Float when you can use Double, that's why. An older revision used Float and it was slower than the current one. Making the datatypes strict also makes no difference. I have tried the obvious things :) >That's the simplest possible thing I can think of after about two >seconds of looking anyway. It appears that the ML version uses float >so I don't understand why you would use Double for the Haskell version >at all, and then think you could do any sort of valid comparisons >between them... OCaML floats are Doubles, at least on x86. cheers, Phil -- http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt

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Re: Propositional logic question
by Dave Tapley 26 Jun '07

26 Jun '07

Whoops, okay after two lines (thanks to oerjan) on #haskell I realise that yes, it is as easy as it should have been. For completeness: [A /\ B]1 ------------ (/\ E1) [A => (B => C)]2 A ------------------------------------------------- (=> E) B => C [A /\ B]1 ------------ (/\ E2) B B => C ------------------------------------------------- (=> E) C ------------------ (=> I)1 (A /\ B) => C ------------------------------------------------ (=> I)2 (A => (B => C)) => ((A /\ B) => C) Learning is fun :) Dave,

4 3

Wikipedia archiving bot - code review
by Gwern Branwen 26 Jun '07

26 Jun '07

Hey everyone. So I've been learning Haskell for a while now, and I've found the best way to move from theory to practice is to just write something useful for yourself. Now, I'm keen on editing Wikipedia and I've long wanted some way to stop links to external websites from breaking on me. So I wrote this little program using the TagSoup library which will download Wikipedia articles, parse out external links, and then ask WebCite to archive them. But there's a problem: no matter how I look at it, it's just way too slow. Running on a measly 100 articles at a time, it'll eat up to half my processor time and RAM (according to top). I converted it over to ByteStrings since that's supposed to be a lot better than regular Strings, but that didn't seem to help much. So I'm curious: in what way could this code be better? How could it be more idiomatic or shorter? Particularly, how could it be more efficient either in space or time? Any comments are appreciate. {- Module : Main.hs License : public domain Maintainer : Gwern Branwen <gwern0(a)gmail.com> Stability : unstable Portability : portable Functionality: retrieve specified articles from Wikipedia and request WebCite to archive all URLs found. TODO: send an equivalent request to the Internet Archive. Not in any way rate-limited. BUGS: Issues redundant archive requests. Currently uses Data.ByteString.Lazy.Char8. If I'm understanding the documentation right, this barfs on the full UTF-8 character set, but Wikipedia definitely exercises the full UTF-8 set. USE: Print to stdin a succession of Wikipedia article names (whitespace in names should be escaped as '_'). A valid invocation might be, say: '$echo Fujiwara_no_Teika Fujiwara_no_Shunzei | archive-bot' All URLs in [[Fujiwara no Teika]] and [[Fujiwara no Shunzei]] would then be backed up. If you wanted to run this on all of Wikipedia, you could take the current 'all-titles-in-ns0' gzipped file from [[WP:DUMP]], gunzip it, and then pipe it into archive-bot. -} module Main where import Text.HTML.TagSoup (parseTags, Tag(TagOpen)) import Text.HTML.Download (openURL) import Data.List (isPrefixOf) import Monad (liftM) import Data.Set (toList, fromList) import qualified Data.ByteString.Lazy.Char8 as B (ByteString(), getContents, lines, unlines, pack, unpack, words) main :: IO () main = do mapM_ archiveURL =<< (liftM sortNub $ mapM fetchArticleText =<< (liftM B.words $ B.getContents)) where sortNub :: [[B.ByteString]] -> [B.ByteString] sortNub = toList . fromList . concat fetchArticleText :: B.ByteString -> IO [B.ByteString] fetchArticleText article = liftM (B.lines . extractURLs) (openURL(wikipedia ++ B.unpack article)) where wikipedia = "http://en.wikipedia.org/wiki/" extractURLs :: String -> B.ByteString extractURLs arg = B.unlines $ map B.pack ([x | TagOpen "a" atts <- (parseTags arg), (_,x) <- atts, "http://" `isPrefixOf` x]) archiveURL :: B.ByteString -> IO String archiveURL url = openURL("www.webcitation.org/archive?url=" ++ (B.unpack url) ++ emailAddress) where emailAddress = "%26email%3Dgwern0(a)gmail.com" -- gwern MAC10 M3 L34A1 Walther MPL AKS-74 HK-GR6 subsonic rounds ballistic media special

4 3

avoiding command window with wxHaskell on Windows?
by Dean Herington 26 Jun '07

26 Jun '07

I'm experimenting with wxHaskell. I created a small program with it on Windows. (Well, small in source form, not so small in binary form :-) The program runs fine when invoked at a Windows command line. But if I double-click the .exe file, I get a command window that hangs around (but doesn't appear to do anything, fortunately) until the program terminates. How can I avoid this command window? I'm using GHC 6.6 and wxHaskell 0.9.4 on Windows XP. Dean

2 2

Propositional logic question
by Dave Tapley 26 Jun '07

26 Jun '07

Hopefully this is just about on topic enough.. (Oh and it's not home work, I just can't bring myself to let it go!) Taken from "Simon Thompson: Type Theory and Functional Programming" Section 1.1 Exercise 1.3 Question: Give a proof of (A => (B => C)) => ((A /\ B) => C). Now I can easily perform (and verify, it's given earlier in the section) the proof of implication with the terms flipped around: ((A /\ B) => C) => (A => (B => C)) Thus: [A]2 [B]1 ----------- (/\ I) [(A /\ B) => C]3 A /\ B ---------------------------------------- (=> E) C --------- (=> I) 1 B => C ------------------ (=> I) 2 A => (B => C) --------------------------- (=> I) 3 ((A /\ B) => C) => (A => (B => C)) My problem arrives finding a solution to the exercise question, my approach is to basically run the above proof backwards. Thus: A => (B => C) A --------------------- (=> E) B B => C --------------------- (=> E) C Now at this point I thought "aha, I can use (=> I) to introduce (A /\ B)" and get: --------------------- (=> I) 1 (A /\ B) => C But here I am only entitled to discharge (A /\ B) in the preceding proof and not A and B on their own. What proof which would allow me to discharge my assumptions A and B? I can see in my head how it makes perfect sense, but can't jiggle a way to do it using only the given derivations. Many thanks, Dave

3 2

Re: [Haskell-cafe] Parallel + exceptions
by Andrew Coppin 26 Jun '07

26 Jun '07

Sebastian Sylvan wrote: > On 25/06/07, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: >> Does anybody know where I can find a version of this paper that actually >> has the diagrams in it? >> > > http://citeseer.ist.psu.edu/cache/papers/cs/8487/http:zSzzSzwww.cee.hw.ac.u… > Thanks! :-)

1 0

runInteractiveProcess leaks memory, runInteractiveCommand does not
by Andrea Rossato 26 Jun '07

26 Jun '07

Hi, after many test I found out that System.Process.runInteractiveProcess leaks memory while runInteractiveCommand does. The issue of memory leaks related to running external program was raised here: http://www.haskell.org/pipermail/haskell-cafe/2007-June/027234.html and Bryan noted that after a while the process hits a steady state: http://www.haskell.org/pipermail/haskell-cafe/2007-June/027278.html This is true for runInteractiveCommand, but not for runInceractiveProcess, even though, both relay on the same foreign C imported function. I think this is a ghc bug, but before reporting it I'd like to have someone to confirm it. I wrote this code that demonstrate my point. Could you please have a look? Thanks for you kind attention. Andrea here's the code.: module Main where import System.Process import System.Posix.IO import System.IO import Control.Concurrent readOutput rh = do str <- hGetLine rh return str runWith c f = do (i,o,e,p) <- f c exit <- waitForProcess p str <- readOutput o putStrLn str cHandles i o e threadDelay $ 100000 * 1 runWith c f runRunIntProcess c = do (inp,out,err,p) <- runInteractiveProcess c [] Nothing Nothing return (inp,out,err,p) runRunIntCommand c = do (inp,out,err,p) <- runInteractiveCommand c return (inp,out,err,p) cHandles i o e = do hClose i hClose o hClose e -- this does not leaks -- this reaches a steady state after a while: --virt --5528 3404 668 S 6.6 0.7 13:07.48 procRunInComm main = runWith "date" runRunIntProcess -- this keep on growing --10548 8436 676 S 8.6 1.6 13:15.24 procRunInProc main' = runWith "date" runRunIntCommand

1 0

Parallel + exceptions
by Andrew Coppin 26 Jun '07

26 Jun '07

The two things mentioned in the subject line are both things I've never tried with Haskell. I've seen a lot of papers about these things, but I don't really understand what the current state of play with this is. Are any of these things "production-ready" yet? And how do you use them in the real world?

5 8

Plugin Problem
by Daniel Fischer 26 Jun '07

26 Jun '07

Hi again, I thought, I'd try out the plugins package, cause lambdabot segfaulted while loading plugins (or immediately after). So I wrote a very simpleminded test for eval(_) and unsafeEval(_) from System.Eval.Haskell. Compiled, but: dafis@linux:~/Documents> ghc --make PlugTest [1 of 1] Compiling Main ( PlugTest.hs, PlugTest.o ) Linking PlugTest ... dafis@linux:~/Documents> PlugTest PlugTest: /tmp/MectNX2619.o: unknown symbol `pluginszm1zi0_AltDataziDynamic_toDyn_closure' PlugTest: user error (resolvedObjs failed.) What does that tell more knowledgeable persons? Thanks, Daniel

5 7

Fw: [Haskell-cafe] Re: Best idiom for avoiding Defaulting warnings with ghc -Wall -Werror ??
by Claus Reinke 26 Jun '07

26 Jun '07

i just noticed that i forgot to copy this message to the list.. ----- Original Message ----- From: "Claus Reinke" <claus.reinke(a)talk21.com> To: "apfelmus" <apfelmus(a)quantentunnel.de> Sent: Monday, June 25, 2007 12:20 PM Subject: Re: [Haskell-cafe] Re: Best idiom for avoiding Defaulting warnings with ghc -Wall -Werror ?? >>> there are at least two problems with embedding strongly and dynamically >>> typed languages into strongly and statically typed languages: >>> >>> - it assumes that there is a globally unique static stage, entirely >>> separate from a globally unique dynamic stage >>> - it assumes that there is a unique, unchanging (closed) world of types >>> >>> static typing alone is too static to deal with dynamically evolving >>> types or flexible ideas such as runtime compilation or dynamic linking.. >>> >>> the solution is long-known as type Dynamic >> >> Yes, that's what I meant. The "embedding" is a rather degenerate one, >> making everything to be of type Dynamic. > > one problem is that haskell doesn't have type Dynamic. only hacked > up approximations of it for monomorphic types. i don't recall whether > the following is the best reference (there were many papers with similar > titles), it is just the first one i find right now: > > M. Abadi, L. Cardelli, B. Pierce, G. Plotkin, D. R{\`e}my, > Dynamic Typing in Polymorphic Languages > http://citeseer.ist.psu.edu/abadi94dynamic.html > > the other problem is that there's no type registry outside of individual programs, so only > programs with identical type definitions can communicate. some of the issues are discussed in > 2003: 6. Martijn Vervoort and Rinus Plasmeijer. Lazy Dynamic Input/Output in the lazy > functional language Clean > http://www.st.cs.ru.nl/Onderzoek/Publicaties/publicaties.html > > some more discussion in earlier publications on orthogonal persistent systems. the only losely > related one i can find right now is > > Connor, RCH, Cutts, QI, Kirby, GNC, Morrison, R, > Using Persistence Technology to Control Schema Evolution > http://www-old.cs.st-andrews.ac.uk/research/publications/CCK+94a.php > > where "schema evolution" is the database community term for what > is the evolution of type definitions in an orthogonally persistent language > (another thing haskell doesn't address: try compiling old -previously type-correct!- haskell code > with new ghc/libraries where the library types and apis have changed..; the static typing of > haskell only ever addresses fragments of the lifetime of programs and data - between these > fragments, we're back to untyped strings/object code; so when i say i want Dynamics, i don't want > to weaken the type system, i want to extend its range - more safety, not less;-). > > [that paper also happens to feature some screenshots of an ide that > permits program parts expressed as source text to refer to program > parts that have already been run and stored, a kind of hyperlinked > program representation which might be of independent interest;-] > > imagine a language that can implement something like an advanced > hs-plugins: at runtime of program A, a string representing program B, including new type > definitions and code, is type checked, compiled, and linked into the running program A, so that > entities in A and B > can interact as if they'd been written as parts of a single program. > > the "static" type-checking of B is a "dynamic" type-check if viewed > from A. and if the contents of B are not known when A is compiled, > there's no way of writing down a universal sum type in A that usefully > encompasses all the possible types in B as well. at least not in current > haskell. > >>> if you have a strongly and dynamically typed language, you can embed >>> strongly and statically typed languages into it. by default, that means >>> you get more type-checks than necessary and type-errors later than you'd >>> wish, but you still get them. eliminating runtime type information and >>> runtime type-checks in a strongly and dynamically typed language is a >>> question of optimisation, similar to deforestation. >> >> Well, you can't embed the static checks into dynamic checks without >> loosing the fact that they're static. Which I think is the crucial >> point: "program testing can be used very effectively to show the >> presence of [type] bugs but never to show their absence." To me, dynamic >> typing is next to useless, i want a (partial) proof that the program >> works since a proof is really the only way to know whether it works. > > yes, and no. no, because there's nothing keeping the dynamically > typed system from doing as many type-checks as early as possible; > yes, because there's nothing in the dynamically typed language that > says "i want all my type-checks to be done before this point, and > a guarantee that no type-errors can occur after this point", so the > dynamically typed system can't really report the errors until just > before their execution. > > so, you're right. Dynamic and typecase are necessary extensions > not just to strongly and statically typed languages but also to strongly and dynamically typed > languages. in the former, they > extend the set of expressible programs, in the latter, they extend > the set of expressible restrictions/safety checks. > > i don't want dynamically typed or statically typed languages, i > want the safety of static types whereever possible, combined with the expressiveness of dynamic > types only where needed. and one known way of expressing the phase shift between dynamic and > static typing precisely and safely are Dynamic/ > typecase: > > - in an otherwise statically typed language, they permit me to write programs that couldn't be > expressed otherwise > > - in an otherwise dynamically typed language, they permit > me to extend type-safety over whole regions of code > at once in ways that couldn't be expressed otherwise > > claus >

1 0

Question about HList possibilities
by Scott West 26 Jun '07

26 Jun '07

Hello all, Given an HList (http://homepages.cwi.nl/~ralf/HList/) would it be possible to do the following: Create a class/function/magicks that would essentially do what hOccursMany does, except it would not return a list of elements, but a new HList. For example, would this allow us to be able to write more lax typing constraints and say extract only things that are in lists. ie) HCons "hi" (HCons [2.2,3.3] (HCons 'a' hNil)) -> HCons "hi" (HCons [2.2,3.3] hNil) (removing the Char element). I tried to write something like this but I did not get very far, is it even possible? I'm new to this type-level programming :) Scott

2 1

Haskell version of ray tracer code is much slower than the original ML
by Philip Armstrong 26 Jun '07

26 Jun '07

In odd spare moments, I took John Harrops simple ray tracer[1] & made a Haskell version: http://www.kantaka.co.uk/cgi-bin/darcsweb.cgi?r=ray darcs get http://www.kantaka.co.uk/darcs/ray It's pretty much a straight translation into idiomatic Haskell (as far as my Haskell is idiomatic anyway). Unfortunately, it's a lot slower than the ML version, despite turning all the optimisation options up as far as they'll go. Profiling suggests that much of the time is spent in the intersection' function, and that the code is creating (and garbage collecting) an awful lot of (-|) vector subtraction thunks. Trying to make intersection' or ray_sphere stricter (with seq) appears to have no effect whatsoever: the output of -ddump-simpl is unchanged (with the arguments all staying lazy). Am I missing anything obvious? I don't want to carry out herculean code rewriting efforts: that wouldn't really be in the spirit of the thing. cheers, Phil [1] http://www.ffconsultancy.com/languages/ray_tracer/comparison.html -- http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt

2 1