- Haskell-Cafe - Haskell.org

monad question
by Andrew Harris 20 Sep '04

20 Sep '04

Hi everyone, Well, I'm trying to build a soccer server client and am using Haskell. I thought I was muddling through well enough... but I have some questions. I am using the Robot monad that I copied from Hudak's book HSOE from page 272 (assuming I would understand it later). Here's the monad code I copied: newtype Robot a = Robot (RobotState -> IO (RobotState, a)) instance Monad Robot where return a = Robot (\s -> return (s, a)) Robot sf0 >>= f = Robot (\s0 -> do (s1, a1) <- sf0 s0 let Robot sf1 = f a1 (s2, a2) <- sf1 s1 return (s2, a2)) updateState :: (RobotState -> RobotState) -> Robot () updateState u = Robot (\s -> return (u s, ())) queryState :: (RobotState -> a) -> Robot a queryState q = Robot (\s -> return (s, q s)) I defined myself a comfortable "chunk of state" (comfortable for today at least): data RobotState = RobotState { player_type :: RobotType, team_name :: String, my_current_position :: Position, visible_flags :: [ObjInfo], visible_ball :: [ObjInfo], visible_friends :: [ObjInfo], visible_foes :: [ObjInfo], audible_player_messages :: [String], my_current_mode :: RobotMode, debug_level :: [RobotDebugLevel], my_uniform_number :: Integer, game_mode :: GameMode, game_clock :: Integer } deriving Show In addition, I added some support functions similar to the ones he wrote in Section 19.3.1: assign_flags :: [ObjInfo] -> Robot () assign_flags flaglist = updateState (\s -> s {visible_flags = flaglist}) assign_friends :: [ObjInfo] -> Robot () assign_friends friendlist = updateState (\s -> s {visible_friends = friendlist}) assign_foes :: [ObjInfo] -> Robot () assign_foes foelist = updateState (\s -> s {visible_foes = foelist}) assign_ball :: [ObjInfo] -> Robot () assign_ball balllist = updateState (\s -> s {visible_ball= balllist}) Now, I read several online monad tutorials and kind of understand the notion of a thread of function calls. And I know that the do notation is syntactic sugar for the bind operation. But the code I've written is really nasty. For example, here is part of my code that handles a see record and returns a new RobotState: handleSeeRecord :: [SeeObjInfo_type] -> RobotState -> IO (RobotState, ()) handleSeeRecord seeobjlist p = do flaglist <- return (morphToList flagFinder seeobjlist) balllist <- return (morphToList ballFinder seeobjlist) friendlist <- return (morphToList friendFinder seeobjlist) foelist <- return (morphToList foeFinder seeobjlist) Robot e <- return (assign_flags (flagSpread flaglist)) Robot f <- return (assign_ball balllist) Robot g <- return (assign_friends friendlist) Robot h <- return (assign_foes foelist) (r', ()) <- e p (r'', ()) <- f r' (r''', ()) <- g r'' h r''' This actually works, but I think it can be improved with the bind operation >>= somehow. I tried a few alternatives but couldn't find one that worked. I ended up with this because it seems I had to "strip" the type off of "return (assign_flags (flagSpread flaglist))" and get the "e" that was a function. Might someone have a suggestion on how to improve this? thanks -andrew

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Writing binary files?
by Ron de Bruijn 18 Sep '04

18 Sep '04

Hi, I would like to write and read binary files in Haskell. I saw the System.IO module, but you need a (Ptr a) value for using that, and I don't need positions. I only want to read a complete binary file and write another binary file. In 2001 somebody else came up with the same subject, but then there wasn't a real solution. Now, 3 years later, I can imagine there's *something*. What's that *something*? Regards, Ron __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - Send 10MB messages! http://promotions.yahoo.com/new_mail

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Interoperability with other languages and haskell in industry
by Vincenzo aka Nick Name 17 Sep '04

17 Sep '04

Again, I will try to take benefit of the thread on the "senior" list to ask a question to everybody who uses haskell in industry (so you people at Galois Connection can't avoid to answer, I know you are there :D ): are your solutions entierely written in haskell, or are there parts written in other languages? If so, how do you let all parts interoperate? Do you use some form of RPC, or CORBA, do you just use a database to store common data, do you use custom protocols (e.g. command line arguments) or what? Do you have experience with wrong ways to achieve this goal? I ask this because "it might be" that in the next years our sleeping company will produce some software, and I can easily convince other people to use "new" languages, if I can ensure them that in case it proves difficult for any reason, we can finish with a certain module and implement the rest of the system using "more conventional" technologies. Thanks Vincenzo

2 2

Proofs for program testing
by Henning Thielemann 17 Sep '04

17 Sep '04

I saw that many introductions to Haskell contain proofs of properties of implemented functions. This is probably due to the fact that pure functions can be handled more easily than imperative programs with hidden states. I wondered whether one can use proofs of Haskell functions for testing. I found QuickCheck http://www.cs.chalmers.se/~rjmh/QuickCheck/ which, as far as I understand, relies on random inputs and I found http://homepages.inf.ed.ac.uk/wadler/realworld/era.html which sounds like some GUI driven program. Is there something that can be used for automatical testing, e.g. for darcs' tests to check patch integrity?

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RE: [Haskell-cafe] Re: Writing binary files?
by MR K P SCHUPKE 17 Sep '04

17 Sep '04

>You wouldn't want to have to accumulate the >entire body as a single byte string Ever heard of lazyness? Haskell does it quite well... Accumulating the entire body doesn't really do this because haskell is lazy. You don't need a more complex interface in Haskell! Keean.

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RE: [Haskell-cafe] Re: Writing binary files?
by MR K P SCHUPKE 16 Sep '04

16 Sep '04

>In the general case, it needs to be a bit more complex than that, Thats why the functions handled lists not individual characters, I was assuming that each [Word8] -> [Char] represented a valid and complete encoding block... IE at the start of each call it assumes no escapes. All this means is than when reading in chunks you paste those chunks together before conversion, and you can only break outside of escapes. This in my opinion is better behaviour anyway... I don't want some hidden escape state mangling output, just because some earler code generated invalid output. Keean.

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ghc-6-2-1_1.msi MD5 sum
by Sergey Zaharchenko 16 Sep '04

16 Sep '04

Could anyone give me the MD5 sum of http://www.haskell.org/ghc/dist/6.2.1/ghc-6-2-1_1.msi I've been writing in Haskell for FreeBSD, and now I wanted to compile one under windows. The .msi file seems to be broken, and I just wanted to check... -- DoubleF "The more data I punch in this card, the lighter it becomes, and the lower the mailing cost." -- Stan Kelly-Bootle, "The Devil's DP Dictionary"

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Lifting makes lazy
by hjgtuyl＠chello.nl 16 Sep '04

16 Sep '04

L.S., In my enthusiasm to reduce imperative style coding to a minimum, I changed a program to something too lazy to do anything. The following is an extremely simplified version of the program: > import Monad > displayFile1 :: IO (IO ())displayFile1 = liftM putStr contents -- > Displays nothing > where > contents :: IO [Char] > contents = readFile "DisplayFile.lhs" This should display the contents of a file, but nothing appears. The following function should be exactly the same, but this one does display the file: > displayFile2 :: IO () > displayFile2 = do > contents <- readFile "DisplayFile.lhs" > putStr contents My conclusion is, that "putStr" is evaluated strictly, while "liftM putStr" is not. I have the following questions: - Why is this difference? - Is there some method to predict whether my program is sufficiently strict to really do what it is supposed to do? - Did someone design a method to develop programs not too strict and not too lazy? - The manual "A gentle introduction to Haskell" states in section 6.3: "adding strictness flags may lead to hard to find infinite loops or have other unexpected consequences"; I would like to know when these problems arise; are these cases described somewhere? -- Met vriendelijke groet, Herzliche Grüße, Best regards, Henk-Jan van Tuyl -------------------------------------------------------------------- Festina Lente Hasten Slowly Haast U langzaam Eile langsam Skynd dig langsomt Affrettati lentamente Spěchej pomalu Skynda långsamt Desiderius Erasmus --------------------------------------------------------------------

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Re: [Haskell-cafe] Re: Writing binary files?
by Udo Stenzel 16 Sep '04

16 Sep '04

Glynn Clements <glynn.clements(a)virgin.net> schrieb am 16.09.04 10:46:58: > Gabriel Ebner wrote: > > One more reason to fix the I/O functions to handle encodings and have > > a seperate/underlying binary I/O API. > > The problem is that we also need to fix them to handle *no encoding*. What are you proposing here? Making the breakage even worse by specifying a text based api that uses "no encoding"? Having a seperate byte based api is far better. If you don't know the encoding, all you have is bytes, no text. > Also, binary data and text aren't disjoint. Everything is binary; some > of it is *also* text. No, it isn't. Everything is binary (read: we need a byte based io library), after decoding and only after decoding it becomes text (read: we need explicit support for decoding and probably a convenience layer that looks like the old io library). > > String's are a list of unicode characters, [Word8] is a > > list of bytes. > > And what comes out of (and goes into) most core library functions is > the latter. So System.Directory needs to be specified in terms of bytes, too. Looks like a clean solution to me. Regards, Udo. ________________________________________________________________ Verschicken Sie romantische, coole und witzige Bilder per SMS! Jetzt neu bei WEB.DE FreeMail: http://freemail.web.de/?mc=021193

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Re: [Haskell-cafe] Re: Writing binary files?
by MR K P SCHUPKE 16 Sep '04

16 Sep '04

>CommandLineToArgvW provides a way to obtain a Unicode Don't forget there are multiple encodings for unicode: UTF-8, UTF-16, UTF-32... Keean.

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RE: [Haskell-cafe] Re: Writing binary files?
by Simon Marlow 16 Sep '04

16 Sep '04

On 16 September 2004 10:35, Glynn Clements wrote: > Simon Marlow wrote: > >>> Which is why I'm suggesting changing Char to be a byte, so that we >>> can have the basic, robust API now and wait for the more advanced >>> API, rather than having to wait for a usable API while people sort >>> out all of the issues. >> >> An easier way is just to declare that the existing API assumes a >> Latin-1 encoding consistently. Later we might add a way to let the >> application pick another encoding, or request that the I/O library >> uses the locale encoding. > > But how do you do that without breaking stuff? If the application > changes the encoding to UTF-8 (either explicitly, or by using the > locale's encoding when it happens to be UTF-8), then code such as: > > [filename] <- getArgs > openFile filename ReadMode > > will fail if filename isn't a valid UTF-8 sequence. Similarly for the > other cases where the OS accepts/returns byte strings but the Haskell > interface uses String. And that's the correct behaviour, isn't it? Actually I hadn't really considered filenames, I was just talking about data read & written via the IO library. > I'm less concerned about the handling of streams, as you can > reasonably add a way to change the encoding before any data has been > read or written. I'm more concerned about FilePaths, argv, the > environment etc. Yes, these are interesting issues. Filenames are stored as character strings on some OSs (eg. Windows) and byte strings on others. So the Haskell portable API should probably use String, and do decoding based on the locale (if the programmer asks for it). Argv and the environment - I don't know. Windows CreateProcess() allows these to be UTF-16 strings, but I don't know what encoding/decoding happens between CreateProcess() and what the target process sees in its argv[] (can't be bothered to dig through MSDN right now). I suspect these should be Strings in Haskell too, with appropriate decoding/encoding happening under the hood. Cheers, Simon

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RE: [Haskell-cafe] Re: Writing binary files?
by MR K P SCHUPKE 16 Sep '04

16 Sep '04

>E.g. what happens if you call getDirectoryContents for a directory >which contains filenames which aren't valid in the current encoding? Surely this shows the problem with the idea of a 'current encoding' ... You could be reading files from two remote servers each using different encodings... So you could have read and write raw [Word8] and read and write char, somehting like: readWithEncoder :: ([Word8] -> [Char]) -> IO [Char] writeWithEncoder :: ([Char] -> [Word8]) -> [Char] -> IO () After all, how can you 'normalise' a filename to a standard encoding if you don't have a function to do so. Infact if you encounter a server with an encoding you have no converter for, you have no choice but to treat it as raw bytes? Keean.

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RE: [Haskell-cafe] Re: Writing binary files?
by Simon Marlow 16 Sep '04

16 Sep '04

On 16 September 2004 00:02, Glynn Clements wrote: > Which is why I'm suggesting changing Char to be a byte, so that we can > have the basic, robust API now and wait for the more advanced API, > rather than having to wait for a usable API while people sort out all > of the issues. An easier way is just to declare that the existing API assumes a Latin-1 encoding consistently. Later we might add a way to let the application pick another encoding, or request that the I/O library uses the locale encoding. Existing code continues to work, and there are no conceptual problems (a Char is still Unicode). You have to decide what happens when the programmer tries to output a Char that is out of range for Latin-1. The current behaviour is simply to take the code point mod 0x100, but we could also decide to raise an exception in this case. Cheers, Simon

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Unicoded filenames
by Marcin 'Qrczak' Kowalczyk 15 Sep '04

15 Sep '04

Here is what happens when a language provides only narrow-char API for filenames: -------------------- Start of forwarded message -------------------- Date: Wed, 15 Sep 2004 15:18:00 +0100 From: Peter Jolly <peter(a)jollys.org> To: mattias.waldau(a)abc.se CC: caml-list <caml-list(a)inria.fr> Subject: Re: [Caml-list] How do I create files with UTF-8 file names? Mattias Waldau wrote: > I have a filename as an UTF-8 encoded string. I need to be able to > handle strange chars like accents, Asian chars etc. > > Is there any way to create a file with that name? I only need it on Win32. Windows uses UTF-16 for filenames, but provides a non-Unicode interface for legacy applications; the standard open() function that OCaml's open_out wraps appears to use the legacy interface. The precise codepage this uses is system-dependent, and AFAIK there's no way for a program to determine what it is without calling out to the Win32 API, but you can be pretty sure it won't be UTF-8. In other words, there is no reliable way to use a filename containing non-ASCII characters with OCaml's standard library. > Or should I solve this problem by talking directly to the Win32-api? This is probably the best solution. A combination of CreateFileW() and MultiByteToWideChar() should do what you want. ------------------- To unsubscribe, mail caml-list-request(a)inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners -------------------- End of forwarded message -------------------- -- __("< Marcin Kowalczyk \__/ qrczak(a)knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/

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Layered I/O
by oleg＠pobox.com 15 Sep '04

15 Sep '04

Binary i/o is not specifically a Haskell problem. Other programming systems, for example, Scheme, have been struggling with the same issues. Scheme Binary I/O proposal may therefore be of some interest in this group. http://srfi.schemers.org/srfi-56/ It is deliberately made to be the easiest to implement and the least controversial. There exist more ambitious proposals. The key feature is a layered, or stackable i/o. Enclosed is a justification message that I wrote two years ago for Haskell-Cafe, but somehow did not post. An early draft of the ambitious proposal, cast in the context of Scheme, is available here: http://pobox.com/~oleg/ftp/Scheme/io.txt More polished drafts exist, and even a prototype implementation. Unfortunately, once it became clear that the ideas are working out, the motivation fizzled. The discussion of i18n i/o highlighted the need for general overlay streams. We should be able to place a processing layer onto a handle -- and to peel it off and place another one. The layers can do character encoding, subranging (limiting the stream to the specified number of basic units), base64 and other decoding, signature collecting and verification, etc. Processing of a response from a web server illustrates a genuine need for such overlayed processing. Suppose we have established a connection to a web server, sent a GET or POST request and are now reading the response. It starts as follows: HTTP/1.1 200 Have it Content-type: text/plain; charset=iso-2022-jp Content-length: 12345 Date: Tuesday, August 13, 2002 <empty-line> To read the response line and the content headers, our stream must be in a ASCII, Latin-1 or UTF-8 encoding (regardless of the current locale). The body of the message is encoded in iso-2022-jp. This encoding may have nothing to do with the current locale. Furthermore, many character encodings cannot be reliably detected automatically. Therefore, after we read the headers we must forcibly set our stream to use the iso-2022-jp encoding. ISO-2022-JP is the encoding for Japanese characters [http://www.faqs.org/rfcs/rfc1554.html] It is a variable-length stateful encoding: the start of the specifically Japanese encoding is indicated by \e$B. After that, the reader should read _two octets_ from the input stream (and pass them to the application as they are). The server has indicated that it was sending 12345 _octets_ of data. We cannot tell offhand how many _characters_ of data we will read, because of the variable-length encoding. However, we must not even attempt to read the 12346-th octet: HTTP/1.1 connections are, in general, persistent, and we must not read more data than were sent. Otherwise, we deadlock. Therefore, our stream must be able to give us Japanese characters and still must count octets. The HTTP stream will not, in general, give EOF condition at the end of data. That is not the only complication. Suppose the web server replied: Content-type: text/plain; charset=iso-2022-jp Content-transfer-encoding: chunked We should expect to read a sequence of chunks of the format: <length> CRLF <body> CRLF where <length> is a hexadecimal number, and <body> is encoded as indicated in the Content-type. Therefore, after we read the header, we should keep our stream in the ASCII mode to read the <length> field. After that, we should switch the encoding into ISO-2022-JP. After we have consumed <length> octets, we should switch the stream back to ASCII, verify the trailing CRLF, and read the <length> of the next chunk. The ISO-2022-JP encoding is stateful and wide (a character is formed by several octets). It may well happen that a wide character is split between two chunks: one octet of a character will be in one chunk and the other octets in following chunk. Therefore, when we switch from the ISO-2022-JP encoding to ASCII and back, we must preserve the state of the encoding. This is not the end of the story however. A web server may send us a multi-part reply: a multi-part MIME entity made of several MIME entities, each with its own encoding and transfer modes. Neither of these encoding have anything to do with the current locale. Therefore, we may need to switch encodings back and forth quite a few times. Decoding of such complex streams becomes easier if we can overlay different processing layers on a stream. We start with a TCP handle, overlay an ASCII stream and read the headers, then overlay a stream that reads a specified number of units (and returns EOF when read that many). On the top of the latter we place an ISO-2022-JP decoder. Or we choose a base64-decoder overlayed with a PCS7 signed entity decoder and with a signature verification layer. OpenSSL is one package that offers i/o overlays and stream composition. Overlaying of parsers, encoders and hash accumulators is very common in that particular domain. I have implemented such a facility in two languages, e.g., to overlay an endian stream on the top of a raw stream, then a bit stream and an arithmetic compression stream. In a functional world, Ensemble system [http://citeseer.nj.nec.com/liu99building.html] supports such a stackable i/o. When using overlayed streams, we should remember that all the layers are synchronized. If we did let iso2022_stream = makeiso2022 hFile in body then the raw hFile is still available in the body. Reading the stream with hFile and iso2022_stream indiscriminately will sure wreck havoc. Clean's unique types is an excellent feature to guard against such mistakes. Exactly the same considerations apply if we were using TIFFreader or PNG reader rather than ISO-2022-JP.

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RE: [Haskell-cafe] Writing binary files?
by Simon Marlow 15 Sep '04

15 Sep '04

On 15 September 2004 12:32, ross(a)soi.city.ac.uk wrote: > On Mon, Sep 13, 2004 at 12:01:58PM +0100, Glynn Clements wrote: >> My view is that, right now, we have the worst of both worlds, and >> taking a short step backwards (i.e. narrow the Char type and leave >> the rest alone) is a lot simpler (and more feasible) than the long >> journey towards real I18N. > > This being Haskell, I can't imagine a consensus on a step backwards. > In any case, a Char type distinct from bytes and the rest is the most > valuable part of the current situation. The rest is just libraries, > and the solution to that is to create other libraries. (It's true > that the Prelude is harder to work around, but even that can be done, > as with the new exception interface.) Indeed more than one approach > can proceed concurrently, and that's probably what's going to happen: > > The Right Thing proceeds in stages: > 1. new byte-based libraries > 2. conversions sitting on top of these > 3. the ultimate I18N API > > The Quick Fix: alter the existing implementation to use the > encoding determined by the current locale at the borders. I wish I had some more time to work on this, but I implemented a prototype of an "ultimate" i18n API recently. This is derived from the API that Ben Rudiak-Gould proposed last year. It is in two layers: the InputStream/OutputStream classes provide raw byte I/O, and the TextStream class provides a conversion on top of that. The prototype uses iconv for conversions. You can make Streams from all sorts of things: files, sockets, pipes, and even Haskell arrays. InputStream and OutputStreams are just classes, so you can implement your own. IIRC, I managed to get it working with speed comparable to GHC's current IO library. Here's a tarball that works with GHC 6.2.1 on a Unix platform, just --make to build it: http://www.haskell.org/~simonmar/new-io.tar.gz If anyone would like to pick this up and run with it, I'd be delighted. I'm not likely to get back to in the short term, at least. Cheers, Simon

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Re: [Haskell-cafe] Layered I/O
by MR K P SCHUPKE 15 Sep '04

15 Sep '04

My thoughts on I/O, binary and chars can be summerised: 1) Produce a new WordN based IO library. 2) Character strings cannot be separated from their encodings (ie they must be encoded somehow - even if that encoding is ascii). I would approch this using parameterised phantom types, so for example you would have: data Ascii data Chr a = Char Then the encoding becomes explicit in the type system, but of course functions like equality on types cvan still be expressed generically: charEq :: Chr a -> Chr a -> Bool but of course, you must be comparing the same encodings (enforced by type system)! The phantom type could be used in IO, IE to define a new encodeing: data MyEncoding instance ChrToBinary MyEncoding where chrToBinary = ... instance BinaryToChr MyEncoding where binaryToChr = ... Keean.

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complete documentation like the one from Java or C++
by PRISACARIU Cristian 10 Sep '04

10 Sep '04

Hellow, can you tell me where I can find a more explicit documentation for the Haskell library and maybe for some of the extentions. GHC comes with something but is not as I want. I would like them to tell me for a function what it does not only what arguments it takes and what it returnes. And maybe some hints about how it works if not some examples. I would like something that approch the one from Java or .NET or MSDN. thank you for your time and understanding

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(no subject)
by jcp＠cs.yale.edu 10 Sep '04

10 Sep '04

From: John Peterson <peterson-john(a)cs.yale.edu> To: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Functional Reactive Programming Functional Reactive Programming is alive but in need of some new students to push the effort a bit. A lot of us have taken teaching or industrial positions so the old FRP team is a bit depleted. I don't think anyone is working on Yampa directly at the moment. Although it's stable and working well it lacks a critical mass of nice libraries to make it attractive. I'm still plugging on a wxHaskell port to Yampa (the wxFruit stuff). I've made some semantic changes to Yampa so I probably shouldn't say it's real Yampa but pretty close. I should have something to release later this fall. Aside from that, we have a student working in the hybrid modeling area. That's good stuff but not likely to produce software of interest to Joe Haskell. Another student is keeping the robotics side of things alive but it's in the context of a very specialized robotic hardware environment. So there you go! John

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Re: [Haskell] Haskell recruitment?
by Lyle Kopnicky 09 Sep '04

09 Sep '04

I think there are so few such opportunities that it would not overwhelm the list. I should also point out that you can post such openings at http://www.haskell.org/jobs.html by sending an e-mail to peterson-john(a)cs.yale.edu and olaf(a)cs.york.ac.uk. Regards, Lyle Jochen L. Leidner wrote: >Finally, would it be considered spam to post Haskell-realted job postings >in this group? Thanks in advance. > > Regards, > Jochen > > >

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cost of List.// for Ord types?
by David Roundy 08 Sep '04

08 Sep '04

Hello, I was wondering if the list diff operator \\ takes advantage of situations where the list data type is in class Ord, besides being in Eq. If it is only in Eq, then \\ must be O(n^2), while if it is in Ord, a O(nlogn) \\ can be written. Basically, I'm wondering if I should avoid using the standard library \\, and if so, whether there might be some better solution than writing my own (which would involve sorting the lists, and then the obvious picking off of the appropriate values). If there isn't a \\ that takes advantage of Ord, could there be? This being a more fundamental question: can you write a separate version of a function for the case where its argument is within a more specific class? -- David Roundy http://www.abridgegame.org

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Functional Reactive Programming
by Mark Carroll 07 Sep '04

07 Sep '04

I was wondering, how much active development is done on FRP frameworks these days. What direction is it going in, and who are the users? I haven't seen much new on Yampa lately so I wondered how that was doing, or if it was thought largely finished and maybe something else was going on. For instance, maybe more of the activity is in developing wxFruit or suchlike? -- Mark

1 0

Installing HaXML,(using Hugs and WindowsXP)
by Tom Spencer 07 Sep '04

07 Sep '04

Has anyone successfully installed HaXML to be used with winhugs in XP. Using the build.bat included with HaXML it seems that it can only be installed for GHC. Any suggestions? Thanks Tom Spencer PS. Apologies that this is pretty vague

3 2

QuickCheck - Extracting data values from tests
by Jorge Adriano Aires 04 Sep '04

04 Sep '04

Hello all, When using Quickcheck, is there some way to extract generated data values to the IO Monad? I know I can collect and print information about test cases, but that's not enough. Data may be pretty complex, and there may be no parsers for it. If a test suddenly goes wrong, just having it displayed doesn't seem that useful. I'd expect quickCheck to have type: quickCheck :: forall a. (Testable a) => a -> IO [a] Show I could just get the offending data with: please_be_empty <- quickCheck prop_foo Also, even when I'm implementing a generator, I want to see how it is working. Running a verboseCheck on some dummy property helps, but I may want to analyse the data, or some parts of it better - for instance, for many data structures I have alternative "show" functions that take parameters as arguments. Thanks in advance, J.A.

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Partially-applied type synonyms?
by Lyle Kopnicky 03 Sep '04

03 Sep '04

Hi all, I'm trying to write a monad transformer class called MonadPCont, for partial continuations, which fits in with the Control.Monad libraries. I'm having a typing problem. What I have so far looks like this: -------------------- module MonadPCont where import Control.Monad import Control.Monad.Cont import Control.Monad.Trans import Control.Monad.Reader import Control.Monad.Writer import Control.Monad.State import Control.Monad.RWS class (Monad (mc a), Monad (mc r)) => MonadPCont mc a r where shift :: ((forall b. Monad (mc b) => a -> mc b r) -> mc r r) -> mc r a reset :: mc a a -> mc r a instance MonadPCont Cont a r where shift f = Cont (\c -> runCont (f (\x -> Cont (\c' -> c' (c x)))) id) reset m = Cont (\c -> c (runCont m id)) type ContT' m r a = ContT r m a instance Monad m => MonadPCont (ContT' m) a r where shift f = ContT (\c -> runContT (f (\x -> ContT (\c' -> c x >>= c'))) return) reset m = ContT (\c -> runContT m return >>= c) -------------------- The error I get is: MonadPCont.hs:21: Type synonym `ContT'' should have 3 arguments, but has been given 1 In the instance declaration for `MonadPCont (ContT' m) i o' Failed, modules loaded: none. I guess it's not possible to partially apply a synonym for a type constructor. Essentially, I'm trying to do a 'flip', but at the type level. The underlying problem is that ContT is written to take the final result type (call it 'r') as the first parameter, and the underlying monad (call it 'm') as the second parameter, e.g. 'ContT r m a'. This is done so that 'ContT r' can be made an instance of the MonadTrans class. Unfortunately, I need 'PI r -> ContT r m', along with a and r, to be a member of the MonadPCont class (PI is the type binding operator). So I thought I'd define ContT' to take the arguments the other way around. Unfortunately, it can't be partially applied. Any ideas, or is it just not feasible to work this class into the library? Thanks, Lyle Kopnicky

3 6

Stack overflow in ghci
by Ron de Bruijn 02 Sep '04

02 Sep '04

I have an expression that gives a stack overflow in ghci (official Debian unstable CVS version)) when I evaluate it. The expression doesn't use more than 150MB of memory (I have more). How can I make sure the stack overflow doesn't happen? There are no strictness flags in my program. But I use DData Maps(I don't know whether they are implemented strict). I also don't know whether strictness/lazyness has anything do to with it. *SomeModule> test *** Exception: stack overflow (148.67 secs, 250825284 bytes) Sometimes I get a negative number of bytes. I heard of the +RTS option. I used: ghci SomeModule.hs -someoptions +RTS -K1500000, but this doesn't seem to have any effect. I think there should be someother option to change it? Thanks in advance, Ron de Bruijn __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - Send 10MB messages! http://promotions.yahoo.com/new_mail

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 02 Sep '04

02 Sep '04

>do e <- isEmptyChan ch -- is the channel empty? > case e of > True -> processFifo > False-> readChan ch >>= highPriorityOrPush >Now there is danger of blocking on the readChan. Erm, but it does not matter if the readChan blocks... This is specifically for the case where there is multiple threads writing and one reading... With multiple reading threads... well it depends on the plumbing, you could have one channel per reading thread, or one channel with a single thread looking ahead for high priority events, and then instead of a FIFO, another channel going to the worker threads that just read events: do e <- readChan ch if e highPriority then process e else writeChan ch2 e Keean.

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Implementing tryReadMVar
by Einar Karttunen 02 Sep '04

02 Sep '04

Hello Is it possible to implement an operation like tryReadMVar :: MVar a -> IO (Maybe a) in a good fashion? The semantics should be "Read the value of the MVar without taking it if it is filled, otherwise return Nothing". There are several easy and flawed implementations: tryReadMvar mv = do e <- isEmptyMVar mv case e of True -> return Nothing False-> readMVar mv >>= return . Just This does not work because there can be a thread switch between the isEmpty and readMVar. tryReadMVar mv = do mc <- tryTakeMVar mv case mc of Nothing -> return mc Just v -> putMVar mv v >> return mc Now this can block on the putMVar if there was a thread switch and someone filled the MVar behind our back. Using tryPutMVar does not help much as it just creates another race condition: tryReadMVar mv = do mc <- tryTakeMVar mv case mc of Nothing -> return mc Just c -> tryPutMVar mv v >> return mc Consider what happens if the tryPutMVar fails: -- read till we get the value with foobar in the middle loopTill mv = do foobar mc <- tryReadMVar mv case mc of Nothing -> loopTill mv Just v -> return v maybe (loopTill mv) process (tryReadMVar mv) error = do mv <- newEmptyMVar forkIO (mapM_ (\i -> putMVar mv i) [1..10]) mapM_ (\_ -> loopTill mv >>= print >> takeMVar mv >>= print) [1..10] If a tryPutMVar fails, then there will be less than ten values to read which will make the process block in takeMVar. This seems quite straightforward in C with GHC (might be wrong in the SMP case with locking?): tryReadMVarzh_fast { W_ mvar, info; /* args: R1 = MVar closure */ mvar = R1; info = GET_INFO(mvar); if (info == stg_EMPTY_MVAR_info) RET_NP(0, stg_NO_FINALIZER_closure); RET_NP(1, vStgMVar_value(mvar); } What is the best way to do this? - Einar Karttunen

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 02 Sep '04

02 Sep '04

>might be a reason to want to prefer one event over another. You can still use a single channel... If you read all pending events on the channel into a FIFO (lazy list) then you can check for high priority events on read, and then deal with the next item on the top of the FIFO... something like the following (in pseudo code) while channel not empty read next event if event high priority process now else queue event in FIFO process first event in FIFO So inbetween processing low priority events we check ahead for any high priority ones... This could be extended with multiple FIFO's to deal with multiple priority levels... but this ensures all events are dealt with sequentially (if out of order)# Keean.

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Re: [Haskell-cafe] Implementing tryReadMVar
by MR K P SCHUPKE 01 Sep '04

01 Sep '04

>tryReadMVar mv = do mc <- tryTakeMVar mv The normal reason people want tryRead is to do something like unix's 'select' function, where you want to wait on one of several signals... In my opinion it is better to do this with a _single_ channel and have one thread taking from the channel, whilst all sources of the 'events' write to the same channel... so the refactoring would be like: data Event = Even1 | Event2 | Event3 ... c <- newChan forkIO (...) a <- readChan c case a of Event1 -> ... Event2 -> ... Keean.

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Haskell Naming Conventions ?
by Koray Can 01 Sep '04

01 Sep '04

Hi, I am new to haskell. One of the things that struck me odd from the very beginning was the names of things. At my workplace (where we use imperative languages), we abide by standards such as this: http://www-106.ibm.com/developerworks/library/ws-tip-namingconv.html Such conventions are debatable, but I believe some of them are worthy of applying. "ShowS" and "shows" do not have the same type, I know, but I read them as the same word. Similarly, fractional is "Fractional", but numeric is just "Num". There are "seq", "sequence" and "sequence_", etc. In The Gentle Introduction to Haskell, the author states: "...We use the term /error/ for /_|_/: a condition which cannot be recovered from such as non-termination or pattern match failure. Exceptions, on the other hand, can be caught and handled within the I/O monad...." Then he goes on to say that the data type's name is IOError (not IOException, as I was expecting.) I can imagine once one gets used to them, they don't pose any problems. Nevertheless, while I still learn, these names are quite awkward for me to read and talk about haskell. I searched the mailing list archives for a similar discussion, but I couldn't fine one. Is there a reason why things are named in such a way that conflicts with what's being followed for other languages ? Thanks for your replies.

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RE: [Haskell-cafe] Higher-order unification
by Simon Peyton-Jones 01 Sep '04

01 Sep '04

| Yeah, here's a program which causes GHC to hang on compilation, but | causes no problem for hugs. Does this qualify as higher-order unification? | | newtype X a = X (X a -> a) | | selfapp :: X a -> a | selfapp self@(X f) = f self It's a documented bug in GHC. http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html Nothing to do with h-o unification Simon

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BBEdit syntax highlighting for Haskell
by David Menendez 31 Aug '04

31 Aug '04

If anyone else is using BBEdit 8.0 on the Mac, I've put together a simple language module for highlighting Haskell syntax. <http://www.eyrie.org/~zednenem/sw/bbedit/haskell-syntax.plist> To install, download the file above to ~/Application Support/BBEdit/Language Modules and restart BBEdit. -- David Menendez <zednenem(a)psualum.com> | "In this house, we obey the laws <http://www.eyrie.org/~zednenem> | of thermodynamics!"

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too much like Java
by enrico.santoemma＠tin.it 30 Aug '04

30 Aug '04

Hello again. I began coding the haskell prototype of the java app I told here some weeks ago. I'm finding myself writing a new module where in Java I'd have written a class, and exporting from the module only the type, the constructor and the functions. By now I'm only writing the functions' signature. The topic is CRM, and modules are stuff like orders, products, invoices... I'm asking a little help just to be sure I'm not going in the wrong direction. My doubts come from the fact that this design looks too much like if I am writing Java code. I know that when it is the time to implement the functions I'll take advantage of the functional way, but by now I'm not using it at all. The exact question is: where one has tenths of entities, is it correct to write a module (in the form of ADT) for each entity? thanks, Enrico

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Functor instance for weak pointers?
by David Roundy 29 Aug '04

29 Aug '04

Might it be possible to create a Functor instance for weak pointers? I don't really need to, but it would certainly be handy. Unfortunately, I don't see how it could be implemented, since there's no way to create a new already-tombstoned weak pointer, and without that ability, I can't see how one could do this. I'd like to write weakFmap :: (a -> b) -> Weak a -> Weak b but don't see how I can, since if deRefWeak returns Nothing, I have no way of creating a Weak b that will also return Nothing. :( A simple emptyWeak :: Weak a would be sufficient, as I could then write weakFmap f wa = unsafePerformIO $ do ma <- deRefWeak wa case ma of Nothing -> return emptyWeak Just b -> mkWeakPtr (f b) Nothing -- David Roundy http://www.abridgegame.org

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anti-memoization?
by David Roundy 28 Aug '04

28 Aug '04

I've been thinking and wondering about the possiblility of anti-memoization. Memoization reduces computational time at the expense of additional memory use. I would like to be able to do the opposite. Obviously, antimemoization is something you'd only want to do with "cheap" functions having larger outputs than inputs. In particular, I have (in darcs) a number of "parsing" functions, which use up a lot of memory, and *also* hold onto their argument. The simplest example is linesPS, which breaks a PackedString (from my FastPackedString library) into lines. The contents of the argument are not copied, so the original string is held in memory as long as its parsed version is also held. linesPS :: PackedString -> [PackedString] I'd like to be able to tell the GC that it can "throw away" the parsed lines, and replace the thunk of the [PackedString] with the original function if necesary. I think this can be approximated using weak pointers, but to do it "right" would require support in the compiler. Using weak pointers I imagine something like: antimemoize :: (a -> b) -> a -> AntiMemo a b readAntiMemo :: AntiMemo a b -> b with data AntiMemo a b = AntiMemo (a -> b) a (IORef (Weak b)) where readAntiMemo (AntiMemo f a iowb) = case unsafePerformIO (readIORef iowb >>= deRefWeak) of Nothing -> case f a of newb -> unsafePerformIO $ do wb <- mkWeakPtr newb Nothing writeIORef iowb wb return newb Just b -> b antimemoize f a = unsafePerformIO $ do wb <- mkWeakPtr (f a) Nothing iowb <- newIORef wb return (AntiMemo f a iowb) I haven't actually tested it, but to the extent that I understand weak pointers, it ought to work. Comments or suggestions are welcome. It would be really nice if this could be done at the "thunk" level, so it could be transparent to the caller. I'd really like to be able to create the function antimemoize :: (a -> b) -> (a -> b) which like "memo" would be completely transparent in the sense that it would be a pure function and wouldn't modify the output of the function it modifies, but would just make the output of that function a thunk that when evaluated stores not only the output b, but also the function itself and the input a, so that the b output could be garbage-collected. On the other hand, perhaps there's a reason why all this wouldn't work. I certainly don't understand space usage in haskell as well as I wish. But I also *really* would like to reduce the space usage of darcs. Some of this I can acheive by strategically introducing laziness, but that's a rather fragile way to reduce the memory footprint, since relatively small modifications change what gets held in memory. So it'd be nice to be able to throw away the results of a parse that I can always re-perform. -- David Roundy http://www.abridgegame.org/darcs

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file upload with Network.CGI
by Peter Simons 27 Aug '04

27 Aug '04

Has anyone managed to process the results of a "file upload" formular, such as <form action="input_file.htm" enctype="multipart/form-data"> <p>Choose a file: <input name="foobar" type="file" size="50" maxlength="100000" accept="text/*"> </p> </form> ..., with Network.CGI successfully? I have tried it, but it appears that the library can't handle the "multipart/form-data" encoding -- is that possible? I'll appreciate any suggestions how to work around (or fix) this problem. Peter P. S.: I have made a lot of changes to the CGI library already; does anyone actively maintain it? Or should I just post my diffs on the libraries list?

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Re: Extensible Serialization class with type classes
by Ralf Laemmel 27 Aug '04

27 Aug '04

[redirected to Haskell-cafe from ghc...] I think the point is not to define your class Seriliazer, but to define it as a generic function instead. After all, it is kind of show (because it is serialisation). Reading your earlier emails, it seems a remaining problem is to keep this function somewhat open as to allow to special cases afterwards. This is a nice topic. Here we go. The normal serialisation function with a serialisation result Q is of such a type serialise :: Data a => a -> Q Internally such a function would be probably defined by recursion such as serialise = ... gmapQ serialise ... In fact, some types would already be subject to special cases. So we have serialise = generic_case `extQ` specific_case1 `extQ` specific_case2 ... where generic_case = ... gmapQ serialise ... specific_case1 = ... % do something for my favourite type This is exactly the code skeleton of show. Now the question is how to make the whole business extensible. I normally do this with a type as follows: extensible_serialise :: Data a => (forall b. Data b => b -> Maybe Q) -> a -> Q That is, there is an extra parameter for the type specific cases to be ruled from the outside. Note the Maybe. Here, Nothing means that a specific type is not handled by the parameter, so the normal serialise has to be applied. Regarding your other problem, optional types, this seems to be solvable as well. Again, *don't* define this class class XSDType a with where xsdType :: a -> String xsdNS :: a -> String ...but rather define a generic function, this time is nothing but a type case of type forall x. Typeable x => x -> Maybe (String, String) Hope it helps. Conclusion: don't use classes, rather use generic functions. Ralf Simon David Foster wrote: >On Thu, 2004-08-26 at 20:52, Ralf Laemmel wrote: > > >>Hi Simon, >> >>I think you should be able to succeed with Scrap your boilerplate. >>(I am not sure that you need existentials.) >> >>For the overall XmlIsh style of type erasue and type validation see the >>XmlIsh example at: >>http://www.cs.vu.nl/boilerplate/ >> >> > >The main requirements for the XML serializer are; > >1) It should be scalable; so users should be able to import a compile >module with the basic XML infrastructure and add serializers for new >Complex and Simple (i.e. leaf node) types. > >2) It should be optionally typeable; there exists several systems for >typing an XML document, the most prominent of which is XSD. The main >requirement is that if the XML tree is to be typed, clearly each node in >the tree must be associated with mapping to an XSD name and name-space >(2 Strings) or some other data items if typing by another system. The >way I was planning on doing this was using a class; XSDType a with >functions xsdType :: a -> String and xsdNS :: a -> String. Then at >serialisation if a particular type and all it's constituent types were >XSD Typeable, extra data (i.e. an extra attribute type="nsp:name") can >(again optionally) be inserted into the tree. > >The XmlIsh example uses a number of functions for serialising different >data-types down the tree. I'm not sure how scalable doing it this way >would be; can you integrate a type-class system with it? The other >problem is dealing with Schematic extensions, such as SOAP Arrays which >have the requirement that the elements are always typeable but should >not be part of the core serialiser. > >My other question is, section 7 only mentions polymorphic types which >hold constraints Data, Typeable and Typeable1; can the new Generics deal >with other polymorphic constraints? This is the main problem; the >type-class system can deal fine with just serialisation of data-type >content, it just doesn't seem to be able to handle an optional >type-system. > >Ideally I need to produce a system where one XML namespace maps to one >module and that module provides data-types, serialisers and typers for >the given XML types, such that other modules which use those data-types >themselves should be able to import that module and its serialisers to >(heuristically) build serialisers for itself. > >-Si. > > > >_______________________________________________ >Glasgow-haskell-users mailing list >Glasgow-haskell-users(a)haskell.org >http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >

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ghc binaries for sunos, hpux, aix, irix, and osf1?
by Goetz Isenmann 23 Aug '04

23 Aug '04

Hi! Are there other binary ghc versions available besides those on http://www.haskell.org/ghc/download.html, that I can use for ghc bootstrap on sunos (5.5.1-5.9), hpux (10.20 and 11.x), aix (4.3-5.2), irix (6.5), and osf1 (4.0-5.1)? -- Goetz Isenmann

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Fun with multi-parameter type classes
by Sam Mason 22 Aug '04

22 Aug '04

Hi, I've been getting into Haskell over the past few months and have just come up against a bit of a brick wall when trying to encapsulate some of the data structures in my code nicely. Basically what I want to have, is a type class where one of the intermediate values is opaque with respect to code using its implementations. This is a simplified version of what I'm trying to accomplish: class Foo t where encode :: String -> t decode :: t -> String instance Foo Int where encode = read decode = show test = decode . encode This currently fails, because the type checker insists on trying to figure out what its type should be - even though it shouldn't be needed. Any suggestions on how to encode this sort of thing? And if it is possible, can it be extended to multiple type parameters? as this is really what I want to use it for. About the only way I can think of "fixing" it, is by turning the code inside out - sort of like the way an AST drives the compiler, but without knowing how it represents things internally. Thanks, Sam

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hugs, ghc portability
by Vadim Zaliva 20 Aug '04

20 Aug '04

Hi! I have following code: import Data.Bits xcomp :: Int -> Int -> Int xcomp a m = (xor a (complement m)) which compiles fine under GHC. However hugs does not like it, since they intentionally did not do: --instance Bits Int How can I resolve this to have this code work under GHC and hugs? Sincerely, Vadim -- "La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever." (Antoine de Saint-Exupery)

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Logic programming in Haskell
by Stefan Monnier 20 Aug '04

20 Aug '04

Has anyone already tried to embed prolog-style logic programming as a Haskell library? Kind of like Schelog but for Haskell? I know about Curry, but I'd like a Haskell library rather than another language. Stefan

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Re: [Haskell-cafe] Fun with multi-parameter type classes
by Jon Cast 19 Aug '04

19 Aug '04

> Hi, > > I've been getting into Haskell over the past few months and have just > come up against a bit of a brick wall when trying to encapsulate > some of the data structures in my code nicely. Basically what I > want to have, is a type class where one of the intermediate values > is opaque with respect to code using its implementations. This is > a simplified version of what I'm trying to accomplish: > > class Foo t where > encode :: String -> t > decode :: t -> String > > instance Foo Int where > encode = read > decode = show > > test = decode . encode > > This currently fails, because the type checker insists on trying to > figure out what its type should be - even though it shouldn't be > needed. The intermediate type /is/ needed---it's a (hidden) parameter to your `encode' and `decode' functions. Why do you think it shouldn't be? <snip> Jon Cast

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Re: [Haskell-cafe] Logic programming in Haskell
by Benderjgdefault＠aol.com 19 Aug '04

19 Aug '04

In a message dated 8/19/04 11:11:00 AM, monnier(a)iro.umontreal.ca writes: >Has anyone already tried to embed prolog-style logic programming as >a Haskell library? Kind of like Schelog but for Haskell? > >I know about Curry, but I'd like a Haskell library rather than >another language. No doubt you are looking for an implementation, not papers, but I would like to point a selection of papers dealing with embedding logic programming in Haskell (on my Haskell bibliography site): http://haskell.readscheme.org/appl.html#logic This includes both the Seres and Spivey papers, but also a couple of papers by Ralf Hinze. Also, since you mention Schelog, perhaps you would also find interesting KANREN, a more recent logic-programming-in-Scheme system by Dan Friedman, Oleg and others: http://kanren.sourceforge.net Jim

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Haskell extension/improvement
by Ron de Bruijn 18 Aug '04

18 Aug '04

I was playing with QuickCheck and I just wanted to put all of my tests in a list, but that's not possible, because of the fact that a list can only contain values of the same type. So concrete I would like to write down this: prop_RevRev xs = reverse (reverse xs) == xs where types = xs::[Int] --trivial test prop_Simple = True == True whatIwant::(Testable a)=>[a] whatIwant = [prop_RevRev, prop_Simple] testAll::IO () testAll = do sequence_ $ map quickCheck whatIwant quickCheck can work on all Testable values, so why can't a list (any datastructure) can't hold Testable values? What this gives you is, a lot more flexibility. Just like you can define your functions on interfaces, now you can use interfaces in your datastructures. I can't think of any reason why it couldn't be implemented in any Haskell compiler. I saw this page: http://www.haskell.org/hawiki/ExistentialTypes but you still have to explicitly add the relation via Obj (and that's bad). Regards, Ron P.S. If it already exist in this exact way, how is it called? _______________________________ Do you Yahoo!? Express yourself with Y! Messenger! Free. Download now. http://messenger.yahoo.com

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puzzle: prove this floorSqrt correct
by blaetterrascheln＠web.de 17 Aug '04

17 Aug '04

-- Here's the discrete version of Newton's method for finding -- the square root. Does it always work? Any literature? floorSqrt :: Integer -> Integer floorSqrt n | n>=1 = detect (iterate (\x->(x+n`quot`x)`quot`2) 1) where detect (x:xs@(y:z:_)) | x==z = min x y | otherwise = detect xs isFloorSqrt n x = x*x <= n && n < (x+1)*(x+1) main = print$ all (\n->isFloorSqrt n (floorSqrt n)) [1..1000000] ________________________________________________________________ Verschicken Sie romantische, coole und witzige Bilder per SMS! Jetzt neu bei WEB.DE FreeMail: http://freemail.web.de/?mc=021193

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Haskell extension/improvement
by MR K P SCHUPKE 16 Aug '04

16 Aug '04

See http://www.cwi.nl/~ralf/HList for a paper which shows how to implement a heterogeneous list in haskell, and how to implement operations on it (amongst other things)... Keean.

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automatic Haskell indentations
by Henning Thielemann 16 Aug '04

16 Aug '04

Looking for automatic indentation of Haskell code I found the thread http://www.haskell.org/pipermail/haskell-cafe/2003-August/005000.html It ended up with many suggestions for manual formatting of code. I wonder if there is some tool which simply formats a Haskell program. A tool like 'indent' (or 'm3pp' for Modula-3) which processes data from stdin to stdout could be nicely integrated into NEdit.

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RE: [Haskell-cafe] Hiding functions
by Simon Peyton-Jones 16 Aug '04

16 Aug '04

Yes that's true. See http://haskell.org/onlinereport/modules.html Section 5.5 S | -----Original Message----- | From: Lyle Kopnicky [mailto:lists@qseep.net] | Sent: 13 August 2004 23:37 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: Re: [Haskell-cafe] Hiding functions | | Simon, | | It also appears that if I don't call 'catch' from my module, I can | import both and not have a conflict. | | - Lyle | | Simon Peyton-Jones wrote: | | >It's an explicit Haskell 98 design choice | > http://haskell.org/onlinereport/modules.html | > | >"5.6.2 Shadowing Prelude Names | > | >The rules about the Prelude have been cast so that it is possible to use | >Prelude names for nonstandard purposes; however, every module that does | >so must have an import declaration that makes this nonstandard usage | >explicit." | > | >It's a decision one could debate, but it was an explicit choice. | > | >Simon | > | >

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Control.Monad.Error with a custom error type
by Brian Smith 15 Aug '04

15 Aug '04

I have: > data Reference = Ref [String] String > data ReferenceError = RefError > { expectedType :: String -- type of element we were looking for (e.g. "type","package") > , pointOfError :: Reference -- path to deepest parent element not found in path > } > type ReferenceMonad = Either ReferenceError I want to write functions that use "Either ReferenceError a" as as the error monad, instead of the more common "Either String a". In particular, I want to use be able to write: > type Model = [(String,Type)] > findType :: Model -> Reference -> ReferenceMonad Type > findType m -> r@(Ref [] name) = case lookup ((==name) . nameOf) m of > Nothing -> throwError r > Just x -> return x I know that I could make this work by making ReferenceError an instance of the Error class, but I cannot provide meaningful implementations of "noMsg" and "strMsg" for ReferenceError. So, it seems instead I need to make (Either ReferenceError) an instance of MonadError. However, when I try, I get: > instance MonadError (Either ReferenceError) Kind error: `Either ReferenceError' is not applied to enough type arguments When checking kinds in `MonadError (Either ReferenceError)' In the instance declaration for `MonadError (Either ReferenceError)' So, how do I get the effect I want for "findType"? Besides "throwError" I also want to use "catchError". Thanks, Brian (Haskell newbie)

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