- Haskell-Cafe - Haskell.org

Processing of large files
by Alexander N. Kogan 04 Nov '04

04 Nov '04

Hi! I'm newbie and I don't understand how to process large files in haskell with constant memory requirements. For example, to count numbers of different words in text file I wrote following program: -- start import System.Environment merge [] x = [(x,1)] merge (e@(a,b):xs) x | x == a = (a,b+1):xs | otherwise = e : merge xs x procFile = putStrLn . show . foldl merge [] . words main = do args <- getArgs readFile (head args) >>= procFile -- end How should I modify it to make it useful on large file? It eats too much memory... -- Alexander Kogan Auto Wave Processes Group Institute of Applied Physics RAS

11 28

Re: [Haskell-cafe] different arities
by Henning Sato Rosen 03 Nov '04

03 Nov '04

> You have met up with one of the rules for function bindings in standard > Haskell (http://www.haskell.org/onlinereport/decls.html) The use of "arity" > in the error message is a poor choice of words. The rule is that each clause > in the function binding must contain the same number of argument patterns. Oh, I see, thanks Scott and Marco for answering! I am beginning to realise that everything worth knowing about Haskell really is in the report:-) Regards/Henning -------------------------------------------------------------------------------------------------- BTW, what might the reason for that be, is it for the sake of ease of translation/implementation, or for the sake of some principle? I mean, as soon as a person realises that there are multiple syntaxes for declaring a function, he/she will unify them to one abstract function in the head (where the resulting type matters, not the syntax). So, even if one probably should avoid mixing as in the example, for readability reasons, one might still argue that the restriction goes against PoLS. Just a thought.

2 1

Fwd: [Haskell-cafe] A simple question
by Marc Charpentier 03 Nov '04

03 Nov '04

Thank you all for the friendly and helpful explanations - and for your patience. The solution to my problem is finally f :: Double -> Double f i = (-1)**i/(2**(10*i)) * (-2^5/(4*i+1)-1/(4*i+3)+2^8/(10*i+1) -2^6/(10*i+3)-2^2/(10*i+5)-2^2/(10*i+7)+1/(10*i+9)) I had confused (^) and (**) due the fact that 1.0/2^16 or (-1.0)^2/2^16 are accepted. The important sentence in the Gentle Tutorial is " It is easy to forget at times that numerals are overloaded, and not implicitly coerced to the various numeric types," (Finding my errors was made somewhat difficult due to the following messages I got while testing my conjectures: *Main> let i=1 in 1/2^i 0.5 *Main> let i=1 in i/2^i Ambiguous type variable `a' in these top-level constraints: `Fractional a' arising from use of `/' at <interactive>:1 `Integral a' arising from use of `^' at <interactive>:1 :-) ) Thanks to all. Marc Charpentier

2 1

A simple question
by Marc Charpentier 03 Nov '04

03 Nov '04

Good evening, Doing some calculations in ghci, I encountered a difficulty which I cannot resolve. Here's a distilled version, prepared for the command line: let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*i+1)) gives the following error messages ghci: Ambiguous type variable `b' in these top-level constraints: `Fractional b' arising from use of `/' at <interactive>:1 `Integral b' arising from use of `^' at <interactive>:1 hugs: ERROR - Unresolved overloading *** Type : (Integral a, Fractional a) => a *** Expression : let {...} in (-1) ^ i / 2 ^ (10 * i) * negate (2 ^ 5 / (4 * i + 1)) However, if I substitute i = 2 in the last fraction, then the transformed expression let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*2+1)) is accepted. Note that both let i=2 in (-1)^i/(2^(10*i)) and let i=2 in (-2^5/(4*i+1)) are accepted. Why? Thank you in advance. Marc Charpentier

3 2

Examples using STRef
by John Velman 02 Nov '04

02 Nov '04

I want to label items on a canvas (using HTk for gui) with a sequence of labels. It looks like STRef would provide the right capability. (Needless to say, at this point, I'm new at using Haskell and am trying to swallow everything at once.) Are there some examples of programs using STRef? I'd like examples that are not too complicated, but not too simple, either. :-) Thanks, John Velman

1 0

different arities
by Henning Sato Rosen 02 Nov '04

02 Nov '04

These two functions have the same type. Is this behaviour intended? (Can functions of the same type have different arities?) ------------------------------------------- data Fun = Increment | Square fun:: Fun -> Int -> Int fun Increment n = n+1 fun Square = (\n -> n*n) ------------------------------------------- Error message in HUGS: ERROR "arity.hs":17 - Equations give different arities for "fun GHC: arity.hs:3: Varying number of arguments for function `fun' /Regards Henning

2 1

installing HaXml
by Tom Spencer 02 Nov '04

02 Nov '04

Hi, I thought i had successfully installed HaXml, ghci loads up the package successfully etc. but then it complains that it can't find XmlLib. Using -v i get the following: Prelude> :load iChing.hs *** Chasing dependencies: iChing.hs: Can't find module `XmlLib' locations searched: XmlLib.hs XmlLib.lhs /usr/lib/ghc-6.2.1/imports/XmlLib.hi /usr/lib/ghc-6.2.1/imports/HaXml/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/data/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/util/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/posix/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/concurrent/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/lang/XmlLib.hi i ran the install script again to see what was happening and noticed this line at the end of the install script: warning: can't find GHCi lib `HSHaXml.o' Saving old package config file... done. Writing new package config file... done. make[1]: Leaving directory `/usr/local/bin/HaXml-1.12/obj/ghc' it doesn't seem to work in ghc or hugs either any ideas? Thanks in advance Tom Spencer

2 2

RE: [Haskell-cafe] Re: www.haskell.org -> applications -> HWS-WP
by Bayley, Alistair 02 Nov '04

02 Nov '04

> From: Benjamin Franksen [mailto:benjamin.franksen@bessy.de] > > The tar ball seems to have vanished already. Do you guys have > any idea where > to get the sources for the HWS (with or without plugins)? http://cvs.sf.net/viewcvs.py/haskell-libs/libs/hws-wp/ I think Shae is mirroring this at: http://www.scannedinavian.org/repos/hlibs/hws-wp/ ... but I don't know if one is more up-to-date than the other. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

Re: www.haskell.org -> applications -> HWS-WP
by Benjamin Franksen 02 Nov '04

02 Nov '04

On Tuesday 02 November 2004 11:53, you wrote: > Benjamin Franksen wrote: > >The Haskell Web Server with Plugins is not easily found if one only has > > the link to the sourceforge announcement. Maybe a link to the home page > > http://www.mdstud.chalmers.se/~md9ms/hws-wp/ would be nice? > > > >Cheers, > >Ben > > Cheers, > > I added the link (although I wonder for how long it will survive, > belonging to a student login). The tar ball seems to have vanished already. Do you guys have any idea where to get the sources for the HWS (with or without plugins)? Ben

2 1

RE: [Haskell-cafe] Newbie Question on type constructors
by Brian Beckman 02 Nov '04

02 Nov '04

Most interesting discussion -- in reading it, I realized that I had a 'hidden agenda' in asking my question (hidden even from myself), and that is: can I put interesting functionality, like precondition checks & data validation, in data constructors? I suspect not, and that's why I tend to write something like the following: data Shape = Circle Float | Square Float deriving (Eq, Show) circle :: Float -> Shape circle x = if (x <= 0) then error "Bad radius!" else Circle x That's fine, but I don't know how to prevent users from calling "Circle" directly (in some other languages, I could declare the raw constructor to be "private", and, in Haskell, there may be some way for me to hack module exports to hide the raw constructor, but I haven't seen a way to do that). In any event, it might be useful if I could have some pattern to prevent circumvention of initialization code. Advice? -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Ben Rudiak-Gould Sent: Monday, November 01, 2004 11:33 AM To: finnw(a)iname.com Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Newbie Question on type constructors Finn Wilcox wrote: >On Mon, 1 Nov 2004, Ben Rudiak-Gould wrote: > >>In particular, one cannot write an invert :: (a->b) -> Maybe >>(b->a) which never returns a wrong answer, except for invert = const >>Nothing > >How about: > >invert = undefined > >This never returns an answer at all, so it can't return a wrong one! Sorry, I should have been clearer: my Nothing return was intended to mean "I don't know an answer", not "there is no answer". So my const Nothing is like your undefined, a function which never returns an answer (and thus never returns a wrong one). -- Ben _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

7 11

RE: [Haskell-cafe] Re: Processing of large files
by Bayley, Alistair 02 Nov '04

02 Nov '04

> From: Alexander Kogan [mailto:alexander@kogan.nnov.ru] > > But I wonder why the very useful function foldl' as I define > it is not > included into Prelude? I think, many people work with large lists or > streams... It's in Data.List: <http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data.List.html#v %3Afoldl'> ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

2 1

Re: [Haskell-cafe] Newbie Question on type constructors
by Antonio Regidor García 02 Nov '04

02 Nov '04

Interesting discussion. I'm still learning Haskell and I also thought about the difference between constructors and functions time ago. I don't think the question about what a constructor really IS is meaningful. It can be can be thought as a label, a special function, ... wathever is more useful in each context or to each person. For me, the main diference between constructors and Haskell functions is that constructors are never evaluated. A program will never simplify "Circle 5", but does simplify "factorial 5" to "5 * factorial 4" and so on, until there are only a numeric literal (a constructor, indeed). So constructors can be tought as some kind of "primitive functions" that can't be evaluated, or blocks to construct "primitive expressions" (literals) that can't be further simplified. Antonio Regidor García ______________________________________________ Renovamos el Correo Yahoo!: ¡100 MB GRATIS! Nuevos servicios, más seguridad http://correo.yahoo.es

1 0

Calling Haskell from .Net Environment
by David Lo 01 Nov '04

01 Nov '04

Dear all, Vincenzo, thank you for your reply and advice. I'll take a look on xmlrpc. I've heard of hugs98 for .Net. Anyone has ever tried to call haskell from .Net using it ? Please kindly advise, since one example provided in the sample is Haskell -> .Net -> Haskell, while I require .Net -> Haskell. When I tried to modify the sample (maybe I modify wrongly), I find that the I'm unable to load new haskell file (when referencing hugsscript.dll from .Net) or BadImageFileFormat exception (when referencing hugs.exe from .Net). Thank you in advance for your help and attention. Best Regards, David Lo On Friday 29 October 2004 03:48, David Lo wrote: > Dear all, > > I'm new in Haskell. I need to port a haskell application written by > someone else to be called by a .Net C# application. Please kindly > advise on which option to pursue. Any helps will really be > appreciated. The easiest path I know would be to use xmlrpc, (or perhaps soap if you know it - but I am unsure of availability of the latter for haskell, you might check the www.haskell.org website). Bye Vincenzo

3 2

Problems with HaXml
by Tom Spencer 31 Oct '04

31 Oct '04

Hi, I thought i had successfully installed HaXml, ghci loads up the package successfully etc. but then it complains that it can't find XmlLib. Using -v i get the following: Prelude> :load iChing.hs *** Chasing dependencies: iChing.hs: Can't find module `XmlLib' locations searched: XmlLib.hs XmlLib.lhs /usr/lib/ghc-6.2.1/imports/XmlLib.hi /usr/lib/ghc-6.2.1/imports/HaXml/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/data/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/util/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/posix/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/concurrent/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/lang/XmlLib.hi i ran the install script again to see what was happening and noticed this line at the end of the install script: warning: can't find GHCi lib `HSHaXml.o' Saving old package config file... done. Writing new package config file... done. make[1]: Leaving directory `/usr/local/bin/HaXml-1.12/obj/ghc' it doesn't seem to work in ghc or hugs either any ideas? Thanks in advance Tom Spencer P.S. apologies if i've accidentally posted this twice

1 0

Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 30 Oct '04

30 Oct '04

>Double already has +Inf and -Inf; it's just that Haskell doesn't have >(AFAIK) syntax to write them as constants. In the source for the GHC libraries it uses 1/0 for +Infinity and -1/0 for -Infinity, so I assume these are the "official" way to do it. Personally I would define nicer names: positiveInfinity :: Double positiveInfinity = 1/0 negativeInfinity :: Double negativeInfinity = -1/0 Keean.

3 2

Set of reals...?
by Stijn De Saeger 30 Oct '04

30 Oct '04

Hi all, I'm new to this list, as well as to haskell, so this question probably has "newbie" written all over it. I'm thinking of a way to represent a set of reals, say the reals between 0.0 and 1.0. Right now I am just using a pair of Float to represent the lower and upper bounds of the set, but i have this dark throbbing feeling that there should be a more haskellish way to do this, using laziness. List comprehensions are out it seems, because they increment with integer steps... (obviously). In other words, 0.5 `inSet` (Set [0.0..1.0]) returns False. I'm sure someone must have hit this problem before me and found a way around it. any suggestions greatly appreciated, regards, stijn.

11 24

RE: [Haskell-cafe] Set of reals...?
by Brian Beckman 29 Oct '04

29 Oct '04

Very pretty, Keean, though to get it to work in Hugs Nov 2002 I had to type the following uglier but equivalent syntax > myInterval = Interval { > isin = (\r -> > if r == 0.6 then True else > if r > 0.7 && r < 1.0 then True else > False ) > } -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Keean Schupke Sent: Wednesday, October 27, 2004 3:53 AM To: Stijn De Saeger Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Set of reals...? I think someone else mentioned using functions earlier, rather than a datatype why not define: data Interval = Interval { isin :: Float -> Bool } Then each range becomes a function definition, for example: myInterval = Interval { isin r | r == 0.6 = True | r > 0.7 && r < 1.0 = True | otherwise = False } Then you can test with: (isin myInterval 0.6) Keean _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

hugs segmentation fault
by Andrej Bauer 29 Oct '04

29 Oct '04

Hi, I am new to haskell, but otherwise experienced in programming languages. My first attempt at Haskell was this (on a Linux Debian) session with hugs: Type :? for help Prelude> :version -- Hugs Version November 2003 Prelude> let p = 1 : [2 * x | x <- p, x < 1] in p [1Segmentation fault Is it the norm for hugs to dump core like this? Or was I just very lucky to discover a bug? Best regards, Andrej

6 8

Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 29 Oct '04

29 Oct '04

>Keith Wansbrough wrote: >>Which brings me to a question: is there a better way to write -inf and >>+inf in Haskell than "-1/0" and "1/0"? Why not do it with types: data InfDbl = Dbl Double | PositiveInfinity | NegativeInfinity Keean.

2 1

Re: [Haskell-cafe] Set of reals...?
by MR K P SCHUPKE 29 Oct '04

29 Oct '04

> | otherwise = contractSet (contract x0 y0:xs) ys I think you'll find the original is correct. The first two cases deal with non-overlapping ranges. The only remaining case is overlapping ranges, (partial and full overlap) both these cases are dealt with by contract, and as a result use up both the ranges at the head of both lists, sdo the merged range is prepended to the output list and the tail is calculated by passing the unused tails of both lists to contactSet... Keean.

2 1

Re: [Haskell-cafe] Set of reals...?
by Keith Wansbrough 29 Oct '04

29 Oct '04

I forgot to mention: With regard to the dangerous practice of using "==" (as in your `elem` example) on Double, every computer scientist or programmer should read the following paper at some point in their training: David Goldberg, "What every computer scientist should know about floating-point arithmetic". ACM Computing Survey Volume 23 , Issue 1 (March 1991) Pages: 5 - 48. http://portal.acm.org/citation.cfm?id=103163 --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

1 0

Calling Haskell from .Net Environment
by David Lo 29 Oct '04

29 Oct '04

Dear all, I'm new in Haskell. I need to port a haskell application written by someone else to be called by a .Net C# application. Please kindly advise on which option to pursue. Any helps will really be appreciated. Best Regards, David

2 1

Are handles garbage-collected?
by Peter Simons 29 Oct '04

29 Oct '04

What happens when a System.IO.Handle falls out of scope without being explicitly hClosed? Is that a resource leak? Or will the RTS close the handle for me? Peter

10 20

intToWord32
by John Velman 28 Oct '04

28 Oct '04

I'm trying to test TclHaskell under Hugs98-Nov2003, compiled from source. My current problem with getting it to work seems to be a failure to load the function intToWord32. Well, here is a partial transcript of a session run from within the TclHaskell src directory (after compiling TclPrim.so): ------------- bash-2.05b$ hugs __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2003 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs(a)haskell.org || || Version: November 2003 _________________________________________ Haskell 98 mode: Restart with command line option -98 to enable extensions Type :? for help Prelude> :load "../demo/Demo.hs" ERROR "./TclPrim.hs":49 - Undefined variable "intToWord32" Word> :names Hugs.Array.! Hugs.Prelude.!! Hugs.Prelude.$ Hugs.Prelude.$! Hugs.Prelude.% Hugs.Prelude.&& Hugs.Prelude.() Hugs.Prelude.* Hugs.Prelude.** Hugs.Prelude.+ ....[snipped many lines] Hugs.Int.intToInt16 Hugs.Int.intToInt32 Hugs.Int.intToInt8 Hugs.Prelude.intToRatio Hugs.Word.intToWord16 Hugs.Word.intToWord32 ... [snipped the rest] ---------------------- So ':names' thinks there is such a thing, but the loader doesn't? I see that intToWord32 is declared with the keyword "primitive" -- i.e., specifically, from Word.hs in the library: primitive intToWord32 "intToWord32" :: Int -> Word32 I'm having trouble finding this syntax in the documentation, but, as I recall -- perhaps incorrectly -- I ran across some gopher documentation that indicated that the "intToWord32" in quotation marks was supposed to be the file name where the foreign primitive could be found. Using "locate" I can't find any files by the name of "intToWord32" in my system. Perhaps this is the Undefined variable in question. Finally, I'm running on a Linux system, Slackware 9, 400Mhz pentium II, 128 mb memory. Any advice or references to documentation will be appreciated. (By the way -- I decided to try TclHaskell because it seemed to be the simplest way to get a graphical interface to an application going! Any advice here will be appreciated as well.) Thanks, John Velman

2 1

exitFailure under forkProcess
by John Goerzen 28 Oct '04

28 Oct '04

Hello, I'm having a little weird situation here. Whenever I call exitFailure under forkProcess, I get: (progname): forkProcess: uncaught exception Test program: import System.Exit import System.Posix.Process main = forkProcess exitFailure If you remove the forkProcess call, the program properly silently exits with exit code 1. I have no idea why it is displaying this message to stderr when called under forkProcess. I argue that it shouldn't. It is perfectly valid to return with an error code from a child process, and absolutely necessary in some cases when your later waitpid() call needs to know what happened. (Or, in Haskell, getProcessStatus()). Thoughts?

6 13

Text.XML.HaXML.HaskellToXml DRIFT example
by S. Alexander Jacobson 27 Oct '04

27 Oct '04

Does someone have an example of how to use DRIFT to convert a Haskell type into XML? The documentation appears rather sparse... -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

3 3

ArrowLoop examples?
by Einar Karttunen 27 Oct '04

27 Oct '04

Hello Are there any examples of using ArrowLoop outside the signal functions? Instances are declared for ordinary functions and Kleisli arrows, but how should they be actually used? - Einar Karttunen

2 1

RE: [Haskell-cafe] exitFailure under forkProcess
by Simon Marlow 27 Oct '04

27 Oct '04

On 26 October 2004 22:29, John Goerzen wrote: > Hello, > > I'm having a little weird situation here. > > Whenever I call exitFailure under forkProcess, I get: > > (progname): forkProcess: uncaught exception > > Test program: > > import System.Exit > import System.Posix.Process > main = forkProcess exitFailure That's a bug, and I've just fixed it. The problem was that we weren't wrapping the child process in the usual default exception handler which catches ExitException and actually performs the exit. Cheers, Simon

1 0

REMINDER: Contributions to the HC&A Report (November 2004 edition)
by Andres Loeh 27 Oct '04

27 Oct '04

Dear Haskellers, the deadline for the November 2004 edition of the Haskell Communities and Activities is approaching quickly -- but there are still a few days left to make sure that the report contains a section on *your* project, on the interesting stuff that you've been doing, using or affecting Haskell in some way. In particular, any information regarding upcoming, Haskell-related, events is highly appreciated (conferences, workshops, summer schools ...). Many FP research groups have not yet updated their reports. Furthermore, please have a look at http://haskell.org/topics.html * Is your project listed, but not yet updated? Please write a short update! * Is a project listed that you are no longer working on? Write up what you are working on instead, and tell me if someone else has picked up the project. * Is something you have heard about not in the list? Please let me know ... There is still time to write a completely new entry, on a new compiler, tool, library, company, user group, idea, ... -- as long as there is a connection to the Haskell language, there is a place for it in the report! Submissions are due by the end of this week, that is Friday, 29 October 2004. Please mail your entries to hcar at haskell dot org, in plain text or pseudo-(La)TeX format. I look forward to receiving your contributions. Thanks a lot, Andres (current editor) -- Haskell Communities and Activities Report (http://haskell.org/communities) 11/2004 edition submission deadline: 29 October 2004 <hcar(a)haskell.org>

2 2

RE: [Haskell-cafe] Are handles garbage-collected?
by Simon Marlow 26 Oct '04

26 Oct '04

On 24 October 2004 20:51, Sven Panne wrote: > IMHO it would be best to use explicit bracketing where possible, and > hope for the RTS/GC to try its best when one runs out of a given > resource. Admittedly the current Haskell implementations could be > improved a little bit in the last respect. Indeed, GHC could/should try to free up file descriptors when they run out. It's a bit tricky though. At the moment performGC doesn't actually run any finalizers. It schedules a thread to run the finalizers, and you hope the thread runs soon. So if you're running performGC for the purposes of finalization, then almost certainly (performGC >> yield) is better. I've been wondering whether having a more synchronous kind of finalizer would be a good thing. Nevertheless, I agree with the general sentiment on this thread that file descriptors shouldn't be treated as a resource in the same way as memory. Cheers, Simon

15 22

RE: [Haskell-cafe] Re: Are handles garbage-collected?
by Simon Marlow 26 Oct '04

26 Oct '04

On 26 October 2004 10:42, Malcolm Wallace wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> I didn't mean fully synchronous, just "more synchronous". For >> example, the finalization routine could be run directly after >> garbage collection. Hugs & nhc98 already do this, because they don't >> support Haskell finalizers. > > Actually, nhc98 supports both C and Haskell finalisers, with the same > signatures as in ghc: > > newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a) > newConcForeignPtr :: IO () -> Ptr a -> IO (ForeignPtr a) > > C finalisers are run during garbage collection, Haskell finalisers > are run immediately afterwards. In fact, you cannot run a Haskell > finaliser during GC, because you don't have a valid heap to run it in. But don't you run into problems in the implementation of newConcForeignPtr, when the Haskell finalizer needs the value of a thunk that was already under evaluation when the GC happened? ie. the need for concurrency that stopped this from becoming part of the official FFI. Cheers, Simon

2 1

Re: Are handles garbage-collected?
by oleg＠pobox.com 26 Oct '04

26 Oct '04

Simon Marlow wrote: > I've been wondering whether having a more synchronous kind of > finalizer would be a good thing. Hans Boehm in his POPL2003 paper "Destructors, Finalizers, and Synchronization" persuasively argued that finalizers _must_ be asynchronous. That assertion is the title of Section 3.5 of the paper. Peter Simons wrote: > input -> iodriver --> connection handler --+ > | /|\ | > output <--+ +-----------------------------+ > (Event, State) > ... > The I/O driver is a StateT monad which calls the (various) > connection handlers every time new input is available from > the network. The handlers are StateT monads, too, and their > respective state is stored in the I/O driver's state when > they return. > ... > The only solution I would know is to put the entire state of > the connection handler into an MVar, so that any changes to > it are immediate for the I/O driver. But that approach feels > like overkill to me. It seems you don't need to store the whole state in MVar: it's enough to store a `clean-up' action. Suppose there exists an MVar known to both a connection handler and the iodriver. The type of MVar's value is Maybe (IO ()). When the connection handler opens a Handle, it places a Just action that will close the handle. If the connection handler makes it to the end without getting an exception, it, right before returning to the iodriver, replaces that cleanup action with Nothing. The iodriver, upon getting control back, checks the MVar and executes whatever action is outstanding. The action can refer to any context it needs for the clean-up. Come to think of it, perhaps MVar is overkill. From your description, it seems that each connection handle has its own private piece of state inside the iodriver state. Perhaps it is not so much to ask to add an IORef (Maybe (IO ())) to that state. The iodriver will check that IORef upon getting control back from the control handler; Or the driver can scan all these IORefs upon receiving an event, an exception, etc. Perhaps a more general approach is a special Handle leasing component. Can we assume that a connection handler has a specific ID? Can we assume that all alive connection handlers can be enumerated (it seems so from the above description: iodriver is aware of all alive connection handlers). If these assumptions hold, we can introduce one level of indirection. Whenever a connection handler needs a Handle, it applies to the Lord of Handles (LoH), passing its own id and the file name. The LoH gives an id of a handle (or the handle itself). The LoH also stores the allocated Handle and the component ID in its tables. From time to time (or on certain events) the LoH checks all the open handles to see if the corresponding connection handlers are still alive.

2 1

RE: [Haskell-cafe] Re: Are handles garbage-collected?
by Simon Marlow 26 Oct '04

26 Oct '04

On 26 October 2004 03:51, oleg(a)pobox.com wrote: > Simon Marlow wrote: >> I've been wondering whether having a more synchronous kind of >> finalizer would be a good thing. > > Hans Boehm in his POPL2003 paper "Destructors, Finalizers, and > Synchronization" persuasively argued that finalizers _must_ be > asynchronous. That assertion is the title of Section 3.5 of the paper. I didn't mean fully synchronous, just "more synchronous". For example, the finalization routine could be run directly after garbage collection. Hugs & nhc98 already do this, because they don't support Haskell finalizers. Just a thought, anyway (and I've read that paper, it's great). Cheers, Simon

2 1

Re: [Haskell-cafe] Relational Algebra post
by Graham Klyne 25 Oct '04

25 Oct '04

Cool! That goes on my list for further study. (I have a work-in-progress of something similar for description logics.) #g -- At 15:01 22/10/04 -0700, Brian Beckman wrote: >Content-class: urn:content-classes:message >Content-Type: multipart/alternative; > boundary="----_=_NextPart_001_01C4B882.A1CF3A2A" > >on my blog >(<http://weblogs.asp.net/brianbec>http://weblogs.asp.net/brianbec) , i've >posted a couple of Haskell gadgets. One is a simulator for the "relational >algebra" of database theory. The other is just an expansion of one of Paul >Hudak's derivations of State monads. > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

1 0

fst and snd as record selectors
by Lemming 24 Oct '04

24 Oct '04

The pair data type is quite a regular data type, where (x,y) is syntactic sugar for (,) x y, right? So, why aren't 'fst' and 'snd' record selectors of the pair data type? The definition of pair could be like data (,) a b = (,) {fst :: a, snd :: b} Then the following would work: Prelude> let a = (1,2) Prelude> a {fst = 3} (3,2)

1 0

executable scripts?
by John Velman 24 Oct '04

24 Oct '04

One of the nice things about perl (for example) is that you can put together a script with #!/usr/local/perl (in bash for example) as the first line of a file and run it immediately. I've used perl a lot this way with simple 'throw away' scripts to do special filtering on a file, or some other processing that I want to do one or a few times. occasionally, a script like this will have a more permanant value to me, and I keep it around. Is there some way to do something similar in with Haskell? I've tried the most obvious (to me) test with Hugs and it doesn't work. Surely there is a way to do this! Thanks, John Velman

4 4

existential type problem
by Andrew Pimlott 23 Oct '04

23 Oct '04

I'm having trouble using an existential type, becasue of what seem to be limitations in how I can express the type. I want values in my existential type to denote, for some monad, a monadic operation and a way to run the monad. Except, I want it mix the operation with operations in another monad, so it use a monad transformer. Specifically: data Foo = forall t. (MonadTrans t) => Foo ((Monad m, Monad (t m)) => t m a -> m a) -- run ((Monad m, Monad (t m)) => t m Int) -- op prog :: Foo -> IO Int prog (Foo run op) = run $ do lift $ putStrLn "Running prog" op ghci gives the error Could not deduce (Monad (t IO)) from the context (MonadTrans t) arising from use of `op' at try.hs:22 Hmm.... I see that my constraints did not mean what I expected. As the "Probable fix" suggests, I can add Monad (t IO) to the existential context to make this code type check, but I want it to work for all monads, not just IO. Something like data Foo = forall t. (MonadTrans t, (forall m. Monad m => Monad (t m))) => Foo ... But I haven't found anything like that. I would also welcome other approaches to this problem. Andrew

3 6

Relational Algebra post
by Brian Beckman 22 Oct '04

22 Oct '04

on my blog (http://weblogs.asp.net/brianbec <http://weblogs.asp.net/brianbec> ) , i've posted a couple of Haskell gadgets. One is a simulator for the "relational algebra" of database theory. The other is just an expansion of one of Paul Hudak's derivations of State monads.

1 0

Stream processors
by Peter Simons 21 Oct '04

21 Oct '04

Hi, I have run across the following idiom several times in the last few weeks: Some part of my program reads input from a 'Handle' and repeatedly calls a "stream processor" to incrementally compute some result, say an SHA1 hash, for example. All my stream processors can be reduced to the following interface: import Foreign ( Ptr, Word8 ) type Buffer = (Ptr Word8, Int) data StreamProc ctx a = SP { start :: IO ctx , feed :: ctx -> Buffer -> IO ctx , commit :: ctx -> IO a } I have to initialize some SP-specific context, then I can feed the data into a stateful computation as it comes in, and after receiving EOF I finalize the SP to obtain the result I wanted. The StreamProc interface works nicely for all kind of things, be it message digests, encryption, encoding, or parsers. And obviously, it's a rather generic concept. One that doesn't have to be re-invented every time ... Which leads to my question: Is there any generic module available that implements such a SP? Or something similar? I know the stream processors as described in Hughes' paper about Arrows, but those are pure stream processors -- they don't allow for I/O, which I need to handle the Ptr. Does anyone have a recommendation for me? Peter

4 11

Re: [Haskell-cafe] Re: Stream processors
by MR K P SCHUPKE 21 Oct '04

21 Oct '04

>No, my stream processors currently have the API I described My point was... using the IO library I posted which abstracts the buffer management, you can treat the IO as a simple list... so you don't need to manage the buffers as such and you can use a simple stream processor like: > process :: [a] -> [a] But of course you have to use the List class operators so that would become: process:: List l a => l a -> l a Keean.

2 1

Re: [Haskell-cafe] Re: Stream processors
by MR K P SCHUPKE 21 Oct '04

21 Oct '04

Okay, maybe I misunderstood, I thought by stream processors you meant functions of the type: process :: [a] -> [a] where both the input and the output are lazy lists (streams) I would assume if you want to have a stateful filter you need to maintain the state by using either: process :: [a] -> IO [a] process :: <state> -> [a] -> [a] process :: [a] -> ST s [a] Keean.

2 1

Re: [Haskell-cafe] Stream processors
by MR K P SCHUPKE 21 Oct '04

21 Oct '04

This is the interface I came up with (and its fairly efficient): data IList a i e = ICons i i (a i e) (IList a i e) | INil class List l e where nil :: l e null :: l e -> Bool head :: l e -> e tail :: l e -> l e (+:) :: e -> l e -> l e class List (l a i) e => ListPlus l a i e where (++:) :: a i e -> l a i e -> l a i e part :: a i e -> i -> l a i e -> l a i e Here's the instance for a normal list: instance List [] e where nil = [] null (_:_) = False null _ = True head (a:_) = a head _ = error "head: empty list" tail (_:l) = l tail _ = error "tail: empty list" a +: l = a:l Here's the instance for a list made of UArray buffers: instance (IArray a e,Ix i,Num i) => List (IList a i) e where nil = INil null INil = True null _ = False head (ICons i _ a _) = a!i head _ = error "head: empty list" tail (ICons i j a l) | i < j = ICons (i+1) j a l | otherwise = l tail _ = error "tail: empty list" a +: l = ICons 0 0 (array (0,0) [(0,a)]) l instance (IArray a e,Ix i,Num i) => ListPlus IList a i e where a ++: l | e >= s = ICons s e a l | otherwise = l where ~(s,e) = bounds a part a i l | e >= i = ICons s i a l | otherwise = l where ~(s,e) = bounds a Here's a feeder reading from a file: hGetIList :: ListPlus l UArray Int Word8 => Int -> Handle -> IO (l UArray Int Word8) hGetIList bufSize h = do mt <- newArray_ (0,bufSize-1) ioLoop mt where ioLoop mt = unsafeInterleaveIO $ do sz <- hGetArray h mt bufSize hd <- freeze mt case sz of 0 -> return nil n | n < bufSize -> do return (part hd (n-1) nil) | otherwise -> do tl <- ioLoop mt return (hd ++: tl) And here's an example consumer: wc :: List l Word8 => l Word8 -> Char -> Int -> Int -> Int -> (Int,Int,Int) wc l p i j k | p `seq` i `seq` j `seq` k `seq` False = undefined | not $ Main.null l, h <- (toEnum . fromEnum . Main.head) l, t <- Main.tail l = case isSpace h of False -> wc t h (i + 1) (j + if isSpace p then 1 else 0) k _ -> wc t h (i + 1) j (k + if h == '\n' then 1 else 0) | otherwise = (i,j,k) Keean.

2 1

Re: Top level mutable data structures problem
by MR K P SCHUPKE 20 Oct '04

20 Oct '04

A slight tangent, (and not GHC specific) so I have put this reply in the cafe. a way to get mutable data outside the IO monad occurs to me. If a file is a mutable data type outside of Haskell, we have hGetContents... which can be implemented with unsafeInterleaveIO, rather than unsafePerformIO. hGetContents lets you read a file from outside the IO Monad. The same applies to hGetChannel. What if there was a way to write in the same way? Say you write a function that reads lazily from a list ... you could then have: main :: IO () ... a <- hGetChannel channel hPutChannel (f a) -- this lazily puts the list to the channel. At the other end of the channel we have a state storage thread, which obeys commands sent over the output channel (read x, write y etc...) You can then write a function, which prepends the next instruction to the output list, and reads data from the input list hence manipulates the state. (the sequence of IO operations is preserved by the lists) Keean.

1 0

unneeded strictness
by Serge D. Mechveliani 20 Oct '04

20 Oct '04

Dear Haskellers, I am still trying to find out how to organize naturally a `lazy' printing of an accumulated data field. People advised me no-buffering and also the tilde usage. But there is some more obstackle. The `trace' is accumulated in fl1 by repeating consD, where data D = D {fl1 :: String, fl2 :: String} consD :: Char -> D -> D consD a d = d {fl1 = a:(fl1 d)} (tilde is not needed here?) The whole process computes res :: D in a certain complex recursive manner. One of the ways to use res is to print out the `trace' field: let res = (complexProcess ...) :: D in show $ fl1 res I need fl1 res to be formed `lazily'. For example, take 2 $ fl1 res may be ready (and shown) immediately, and last $ fl1 res may be ready long after. If it was [Char] istead of D, then this would be easy. Because head $ (a:) xs = a For example, head $ ('a':) $ (error "") = 'a' But is this true for consD and headD, where headD :: D -> Char headD = head . fl1 ? That is headD $ consD a res =?= a If res is not a bottom, then, yes. Right? And if res occurs, say, (error ""), then consD cannot intrude into fl1 res, and the whole expression has a value _|_. Right? So, generally, headD $ consD a res is not equivalent to a; hence the head members of fl1 res have to wait at least until it becomes clear that res is not a bottom. Right? So far, I see the two way-outs. 1. To insert the usage of the `trace' tool -- which does not look nice. 2. To have a result res :: [D] instead of D. And as soon as a new step appears, form res ++[d {fl1 = [step]}] where d = last res, fl2 also can be modified. Now, map fl1 res is the needed trace, which, I believe, would be formed `lazily'. I wonder a) whether there is a nicer solution, b) what happens if the Language treats the constructors as the homomorphisms. For example, head $ (a:) xs = a ==> head $ fl1 $ D {fl1 = (a:) fl1} = a Here (D, fl1) is a constructor, and its insertion between the functions head and (a:) preserves the above relation between them (is this called `bottom chasing' ?). Probably, this will eliminate unneeded strictness -- ? Thank you in advance for the explanations. Copy, please, the answer to mechvel(a)botik.ru ----------------- Serge Mechveliani mechvel(a)botik.ru

1 0

Re: [Haskell-cafe] Re: subsequence
by MR K P SCHUPKE 20 Oct '04

20 Oct '04

Well I don't just want to give you the answer, but this code finds a subsequence in a list... This is from the HList library, and it looks for an ordered subtype ... but it also does what you want (it uses a four valued modal logic, which returns AllFalse, if list a is not a subtype (in-order) of b, (if the types are different there is no point in comparing values) or it returns SomeTrue, which has a value of either True or False depending on whether the list is an ordered sublist of values. -------------------------------------------------------------------- -- test for list a containing list b (elements of b occur in-order, but may -- be interleaved with other elements. class (HList r,HList s) => HContains' r s where hContains' :: r -> s -> Bool instance HContains' HNil HNil where hContains' _ _ = True instance HList r => HContains' (HCons a r) HNil where hContains' _ _ = True instance HList r => HContains' HNil (HCons a r) where hContains' _ _ = False instance (HContains' r (HCons a s),HContains' r s,Eq a) => HContains' (HCons a r) (HCons a s) where hContains' (HCons a r) s@(HCons a' s') = if a == a' then hContains' r s' else hContains' r s instance HContains' r (HCons b s) => HContains' (HCons a r) (HCons b s) where hContains' (HCons _ r) s = hContains' r s class (HList r,HList s,Logic.MBool t) => HContains'' r s t | r s -> t where hContains'' :: r -> s -> t instance HContains'' HNil HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' (HCons a r) HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' HNil (HCons a r) Logic.AllFalse where hContains'' _ _ = Logic.AllFalse instance HContains'' r s t => HContains'' (HCons a r) (HCons a s) t where hContains'' (_ :: HCons a r) (_ :: HCons b s) = hContains'' (undefined :: r) (undefined :: s) instance HContains'' r s t => HContains'' (HCons a r) s t where hContains'' (_ :: HCons a r) s = hContains'' (undefined :: r) s class (HContains' r s,Logic.Modal t) => HContains r s t | r s -> t where hContains :: r -> s -> t instance HContains' r HNil => HContains r HNil Logic.AllTrue where hContains _ _ = Logic.AllTrue instance (HContains'' r (HCons b s) t',Logic.And t' Logic.SomeTrue t,HContains' r (HCons b s)) => HContains r (HCons b s) t where hContains r s = hContains'' r s `Logic.and` Logic.toSomeTrue (hContains' r s) ---------------------------------------------------------------------------- Keean.

1 0

STG questions
by Andrew Harris 19 Oct '04

19 Oct '04

Hi - I am reading Simon Peyton Jones's "Implementing lazy functional languages on stock hardware: the Spineless Tagless G-Machine" (I have version 2.5) paper. I would like very much to understand the contents of Figure 5 which illustrate the mapping of the "map" function from STG code to C. However I have some questions. For example, * It looks like map_closure is the "closure" in that it has an array of StgWords and the first word is a pointer to an info table. This info table contains map_entry as the entry code. I am curious about the relationship between map_entry and map_direct. Why isn't the stuff in map_direct done in map_entry? * The ret_vec1 is a continuation that's placed on the B stack. I am assuming this is a "non-vectored" return that has contents similar to Figure 7 (which is for vectored returns), but is this true? Also, the "upd_nil" and "upd_cons" functions aren't defined in the example. Maybe they just don't do anything? * In the return_cons1() function, how did the head and tail get put into the RegData1 and RegData2 registers? * I may have missed it, but it looks like the fy and mfy use three contiguous words of heap each. Can I assume that the triple {Hp[-5], Hp[-4], Hp[-3]} are of a structure similar to figure 4, the "Layout of a closure", which in this case would have some pointer/non-pointer words? thanks for any help, -andrew

1 0

DSL for business logic
by Bhinderwala, Shoeb 19 Oct '04

19 Oct '04

I understand that there will be issues integrating with Java, but I want to consider a Haskell solution nevertheless. Are monadic programming concepts essential to create a DSL. I am just learning Haskell and haven't grasped fully the concepts of monadic programming yet. Referring to Paul's paper "Modular Domain Specific Languages", it seems like a DSEL in Haskell is just a plain Haskell program that uses functions to model the business domain. I am referring to the DSEL for gemoetric region analysis and Fran both of which look like code in familiar Haskell syntax. Our business problem is in the financial space where we have rules that check compliance on portfolios. Some example rules are: - The portfolio may not invest more than 40% in equity securities. - The portfolio may not invest more than 5% in any sector relative to the S&P500 index. - The portfolio cannot buy any securities whose issuers are domiciled in Japan. Currently we have high level rule expressions that get mapped to low-level code that embodies the business logic of checking the rules. Essentially most of our rules do the following: 1. Group, sort, and filter the portfolio's holdings. 2. Examine individual groups (also called 'buckets') and compute aggregated values. Eg. percent market value of each bucket. 3. Compare the value for each group with threshold values to check compliance I am wondering if is is a good idea to express the above logic in a Haskell DSL by creating data types for Holding, Bucket, Portfolio, etc. Would I be going on the right track by modelling a concentration rule as: -- A concentration rule takes a list of Holdings and a list -- of filter rules and returns an aggregate number. conc :: [Holding] -> [holdingFilter] -> Float holdingFilter :: Holding -> Boolean -- Shoeb -----Original Message----- From: Paul Hudak [mailto:paul.hudak@yale.edu] Subject: Re: [Haskell-cafe] Integrating Haskell into a J2EE environment I wouldn't write off Haskell so quickly. All of what Shoeb describes concerning DSL issues might be much more easily solved in Haskell, and will certainly be more flexible than a hard-wired approach. The J2EE interface might be ugly, but if the functionality needed is not too great it might not be too bad. Generally speaking, these kinds of apps -- in this case "a DSL for high-level business rules" -- sounds like just the sort of thing that Haskell is good for. -Paul Doug Kirk wrote: > You're going to spend alot of time marshalling between Java and > Haskell values, and you'll either have to do it via JNI or by using > pipes [as in System.exec("haskellprogram param param param")], both of > ... > > Take care, > --doug > > > On Oct 5, 2004, at 4:33 PM, Bhinderwala, Shoeb wrote: > > Hi All, > > I am new to Haskell and this mailing list. > > We have a system that uses a custom high-level language to express > high-level business rules. Expressions in the high-level language get > compiled to Java bytecode. We express the grammar using BNF notation > as required by the javacc parser tool. This is then converted to an > AST using jjtree and from there we build the final Java code. Our > language could be considered a domain-specific language (DSL) and is > used by our business users to express very high-level business logic. > The language currently is very limited - we support boolean logic, > function invocations and if-then statements. We want to convert it > into a more powerful scripting language so that even lower level > business logic can be expressed in it. > > I came across a few papers that talk about writing a DSL with Haskell > as the underlying support language. How is this done. Is it possible > to create a sort of domain specific business scripting language > easily. How does that then compile to Haskell code. And how can the > Haskell code be invoked from Java. > > Essentially, I am thinking if I could use a Haskell like DSL language > to express our business rule logic and then be able to integrate into > and invoke the logic from a J2EE app server environment. Has anybody > done anything like this with Haskell. > > -- Shoeb

3 2

One-way and two-way monads.
by Kwanghoon Choi 18 Oct '04

18 Oct '04

Dear All, List and IO are both monads when appropriate operations are defined. The IO monad, which is a one-way monad, does not have a function of type IO a -> a, in general, except an unsafe function "unsafePerformIO :: IO a -> a". A two-way monad, such as List, has such a function "head :: [a] -> a". My rough question is: Is there any intersting theory on the relationship between one-way monads and two-way monads? For example, a restriction on the IO monad would offer a *safe* "unsafePerformIO", and so the IO monad would become a two-way monad. Does this question make sense at all? Thanks in advance. Kwanghoon Choi

3 2

Subsequence near solved hopefully
by Peter Stranney 17 Oct '04

17 Oct '04

Sorry for annoying you all, i'm just very eager to move on with this and solve it. so far i have; equalChar :: Char -> Char -> Bool equalChar x n | x == n = True | otherwise = False equalString :: String -> String -> Bool equalString [] [] = True equalString [] (c':s') = False equalString(c:s) [] = False equalString(c:s)(c':s') = equalChar c c'^ equalString s s' this function is to see if one string is equal to another. but when i compile this i get the error; - Instance of Integral Bool required for definition of equalString does anyone know why i am getting this error and what i will need to do to transform the function above so that it checks is a list a subsequence of another list Peter Stranney --------------------------------- Do you Yahoo!? Yahoo! Mail - You care about security. So do we.

5 12

Re: subsequence
by Peter Stranney 17 Oct '04

17 Oct '04

>> I am stuck on an exercise in a haskell book (the craft of functional >> programming) chapter 7 exercise 7.18 I have to write a function to check >> does one list occur as a subsequence of another, i am having real >> difficulty trying to do this. I have a method which will tell me if one >> character occurs in the list but it won't work for a list of >>characters. >> the function should look like isSubsequence :: [chars] -> [chars] -> >> boolean >> >Try writing a list or string equality function first? That may help. Thanks for getting back to me, have written this so far find :: [Char] -> Char -> Bool find [] n = False find (x:xs) n | x == n = True | otherwise = find xs x this will work to find if one character appears in a list but i dont understand how to call this recursively to find if a list occurs within a list. I have tried writing another function to call this but it won't work. Does anyone have a solution? thanks again Peter Stranney __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

1 0