- Haskell-Cafe - Haskell.org

Eager global IO actions (per module initialization)
by Einar Karttunen 28 Sep '06

28 Sep '06

Hello I am needing a way to run initializers defined in various modules in an eager fashion before main. I am doing this to load deserialization functions for a Typeable function. Basically I have code like: $(inferDecoderAndRegisterItOnStartup ''MyType) which defines a class instance, but additionally I want to call 'registerDecoderForType "MyType" decodeMyType' automatically on startup. Calling registerDecodeForType for all types in main gets very tedious and error-prone when doing things by hand. Thus an automated solution would be very nice. - Einar Karttunen

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Re: [Haskell] BitSyntax for Haskell
by Einar Karttunen 28 Sep '06

28 Sep '06

On 26.09 10:01, Adam Langley wrote: > >For the decoding part: > >* Provide a monadic interface > > Are you suggesting a monad to pass in the input around, or that it > returns mzero on error? The latter makes more sense to me. Yes. Also make it possible for user supplied functions to fail in better ways than to produce Either or use error. > >* Add a test part to ReadType: > > Test :: ReadType a -> (a -> Bool) -> ReadType Test > > (or a -> m ()) in the monadic case. > > Again, I'm not clear what you are thinking of here? In some protocols I am using there are some fixed bytes which I want to ignore (no Haskell value produced), but check that they are valid in the data stream. - Einar Karttunen

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Expressing seq
by Chad Scherrer 28 Sep '06

28 Sep '06

I was reading on p. 29 of "A History of Haskell" (a great read, by the way) about the controversy of adding seq to the language. But other than for efficiency reasons, is there really any new primitive that needs to be added to support this? As long as the compiler doesn't optimize it away, why not just do something like this (in ghci)? Prelude> let sq x y = if x == x then y else y Prelude> 1 `sq` 2 2 Prelude> (length [1..]) `sq` 2 Interrupted. There must be a subtlety I'm missing, right? -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: Curious Functor Class
by Ashley Yakeley 28 Sep '06

28 Sep '06

Jeremy Gibbons wrote: > I haven't assimilated the forall here, but datatypes with only one shape > of data have been called "Naperian" by Peter Hancock (because they > support a notion of logarithm), and they're instances of McBride and > Paterson's "idioms" or "applicative functors". > > http://sneezy.cs.nott.ac.uk/containers/blog/?p=14 > http://www.cs.nott.ac.uk/~ctm/Idiom.pdf > > As for uses: Bruno Oliveira and I claim an application via The Essence > of the Iterator Pattern: > > > http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/index.html#iterator Perhaps the key is that there exist types P and Q s.t. there's an isomorphism F a <=> (P -> a,Q) This seems to be intuitively Napierian: ln (P -> a,Q) = (P,ln a) | ln Q I can believe that Hoistables are in fact Idioms, though I know there are Idioms that are not Hoistables (Maybe and Either, for instance). (Also I think "Idiom" is a better class name than "Applicative".) -- Ashley Yakeley Seattle WA

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Re: [Haskell] Re: compiler-independent core libraries infrastructure
by Neil Mitchell 27 Sep '06

27 Sep '06

[Moving to cafe for follow up discussions] Hi Bulat, So, just to confirm in my mind what you are proposing: Compiler/Version specific Core: Yhc.Core, Hugs.Core, GHC.Core .... With a different version for each compiler version. Tied intimately to the compiler. The leveler: Core - which abstracts each version/variant of *.Core into one big pile. Contains versions of STM/Arrays in Pure Haskell which might not give the expected performance, but at least keep the semantics as best they can. Base: Pure Haskell which depends on Core, but NOT on *.Core. (Then after you split up base - useful, but personally not of massive importance to me ;) ) I think this is a great plan! Now you have a concrete idea of what you want to achieve in the end, how are you going to get there? It seems a rather ambitious project. Perhaps you might be best suited by getting base to compile with Cabal (and maybe even natively on Windows - without MingW), then you can go from there to the splitting in a Cabal friendly manner without hacking through makefiles? Anyway, once you have decided what you want, I'm sure the Yhc team will do their best to help you get to where you want with regards to that specific compiler. Just to try and keep your work more generally applicable, it would be nice if you kept the Hat project in the back of your mind, so that this work can be reused by them to get full haskell.org libraries. Thanks Neil

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Creating DLLs with GHC
by Jason Dagit 27 Sep '06

27 Sep '06

On 9/27/06, SevenThunders <mattcbro(a)earthlink.net> wrote: > Does cabal really work on windows? I've never had a problem with cabal on windows. I use it instead of makefiles and I'm reasonably happy with it. > Although it's installed I notice that > when I try to build my library using it, it dies on the first foreign import > statement in the first .hs source it tries to compile. It is reminiscent of > trying to import a foreign C module directly ghci. ghc.exe, on the other > hand has no problem with it. Perhaps your .cabal file is not correct or your cabal version is too old? It's very hard to say without detailed error messages and a clear description of what you're doing (with sample code). > Does this scheme address the problem of being unable to link external C > libraries (a DLL) into the Haskell DLL? Are there any other workarounds > here? I only create a DLL from haskell and link that in a C++ application. I haven't tried the other way. Probably over the next month or so the DLL I'm creating in Haskell will need to access an existing DLL. Hopefully this will work fine. > Is it possible to create static link libraries compatible with MS > VC++? Ultimately I'm trying to link my C code with Matlab. The C code stub > will then call a bunch of Haskell routines. I have no idea. I'm fairly new to using Haskell in windows. Jason

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Duplicate Instance problem
by Jason Dagit 27 Sep '06

27 Sep '06

Hello, I tried to create a type class for making instances of Show display a custom way. After using my class for a while I found that sometimes RealFloats would display as 'NaN' and this is unacceptable. So at this point I had something like: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} class Show a => StringValue a where -- | Convert a showable value to a string toString :: a -> String instance Num a => StringValue a where toString = show and I added an instance for RealFloat: instance RealFloat a => StringValue a where toString x | isNaN x = "" -- as an example | otherwise = show x Now, I get the error: Duplicate instance declarations: instance [overlap ok] (Num a) => StringValue a -- Defined at ... instance [overlap ok] (RealFloat a) => StringValue a -- Defined at ... I've also tried spliting Num into an instance for Integral and RealFloat but that doesn't work either (same error). For now I'm using an instance for RealFloat and an instance for Int and Integer (which are identical) and that seems to be okay but suboptimal. Why does the error say they are duplicates? I would have expected something about them overlapping but not duplicates. Is there a way to still do this without duplicates? Thanks, Jason

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Getting the latest
by Lyle Kopnicky 27 Sep '06

27 Sep '06

Hi folks, I am running GHC 6.4.1 on my Linux box at work, which is the latest packaged version for Ubuntu Dapper Drake. (They have a 6.4.2 package for Edgy, but I don't know how to install that. The Synaptic Package Manager seems to only want to install packages specifically labeled for Dapper Drake.) I tried downloading the latest binary snapshot. But when I try to unpack the archive, bzip2 tells me it's a bad archive. So, I downloaded the latest source version. That unpacked just fine, and I'm currently building it. My question is, when I do 'make install', will it just overwrite the version (6.4.1) I already have? Or will they go in separate places? I have no idea how it decides where to go. Right now ghc 6.4.1 is in /usr/local/bin/ghc. After I 'make install', will it be ghc 6.5? I don't want to screw up the installed package so it can't be updated later. Thanks, Lyle

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Re: [Haskell-cafe] Optimizing a title matcher
by Lyle Kopnicky 27 Sep '06

27 Sep '06

Hi folks, It turns out Haskell is vindicated. It's my algorithm that was slow. As Robert Dockins pointed out, the double nested loop is just going to take a long time. As evidence, it turns out my C++ version is just as slow as the Haskell version. So, I'm going to go back to Haskell, but be more selective about which titles from the reference table I choose to match against, for any given import title. Thanks, Lyle

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Computing lazy and strict list operations at the same time
by C Rodrigues 27 Sep '06

27 Sep '06

Here's a puzzle I haven't been able to solve. Is it possible to write the initlast function? There are functions "init" and "last" that take constant stack space and traverse the list at most once. You can think of traversing the list as deconstructing all the (:) [] constructors in list. init (x:xs) = init' x xs where init' x (y:ys) = x:init' y ys init' _ [] = [] last (x:xs) = last' x xs where last' _ (y:ys) = last' y ys last' x [] = x Now, is there a way to write initlast :: [a] -> ([a], a) that returns the result of init and the result of last, takes constant stack space, and traverses the list only once? Calling reverse traverses the list again. I couldn't think of a way to do it, but I couldn't figure out why it would be impossible.

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Re: [Haskell-cafe] Unable to profile program using Data.ByteString.Lazy
by dons＠cse.unsw.edu.au 27 Sep '06

27 Sep '06

lists: > Donald Bruce Stewart wrote: > >Probably you didn't build fps with profiling as well? You can rebuild > >fps with: > > runhaskell Setup.hs configure -p > >as the first step. > > > That worked on my Windows box at home, but on my Linux box at work, I > got "unrecognized flag -p". You're cabal version is too old then. Try updating either Cabal or GHC. -- Don

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Re: Creating DLLs with GHC
by Cyril Schmidt 27 Sep '06

27 Sep '06

Matthew, As regards the symbols that end with _closure, I believe you can resolve them by adding -package parsec to the ghc command line (as far as I can see, all the symbols you list come from the parsec package). I don't know, though, what to do with the undefined symbols from matrixstack. You can try to check two things: 1) That the options you specify with -optl really reach the linker (e.g. try to give a non-existent library in -optl option and see if it complains). 2) That matrixstack.lib is understood by the linker (that is, it must be created with dlltool) 3) That the symbols it is looking for do exist in matrixstack.lib (you can check it with objdump, for instance). Cheers, Cyril ___ Matthew Bromberg wrote: >ghc --mk-dll -o netsim.dll ExternLib.o ExternLib_stub.o dllNet.o src1.o >src1_stub.o src2.o -optl-lmatrixstack -optl-L"." >My external C library is in matrixstack.dll and it has a corresponding >static link stub library matrixstack.lib in the same directory as all >the sources. It has references in one of the sources (say src1.hs). >Unfortunately I get a host of undefined references to basically all the >functions in matrixstack.dll and also some undefined references of the form >Parsefile.o:ghc2996_0.hc:(.text+0x130): undefined reference to >`TextziParserCombinatorsziParsecziError_show_closure'

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Re: [Haskell] MR details (was: Implicit type of numeric constants)
by Bernie Pope 27 Sep '06

27 Sep '06

On 23/09/2006, at 4:33 AM, Christian Sievers wrote: > Hello, > > I don't take my advice to go to haskell-cafe :-) I will take your advice :) > > The discussion continued outside the mailing list, and now I have > two questions myself: > > 1. Why do the rules of the monomorphism restriction explicitly mention > *simple* pattern bindings? > Where is the difference, especially as there is a translation to > simple pattern bindings? > Why should > > p | "a"=="b" = 2 > | otherwise = 3 > > be treated different than > > p = if "a"=="b" then 2 else 3 They are the same (both are simple pattern bindings). The report says in section 4.4.3.2 that the first can be translated into the second. A simple pattern binding is one where the lhs is a variable only. If a pattern binding is not simple, it must have a data constructor on the lhs, therefore it cannot be overloaded. So the (dreaded) MR only applies to simple pattern bindings. Cheers, Bernie.

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Re: Haskell DLL crashes Excel
by Cyril Schmidt 26 Sep '06

26 Sep '06

A few people recently asked how to pass a string between a Haskell DLL and Excel. I attach the proof-of-concept code that I wrote a while ago; it demonstrates passing a string from Excel to Haskell and the other way. Most of the C++ code is taken from code examples at http://msdn.microsoft.com Beware, though, that the code has never been thoroughly tested. I ran it, and it did not crash -- that's almost all I can say. I even do not know if it leaks any memory. Use it at your own peril. The tar file contains a sample Excel sheet, and the sources of two DLLs: a COM DLL which interfaces with Excel, and a Haskell DLL which is invoked by the COM DLL. You will need Visual Studio 7.1 to build the COM DLL. The Haskell DLL exports one function, hString2String, which takes a C-style string, and returns the reverse of it. To build the Haskell DLL, run build.bat. To build (and register) the COM DLL, open Excel2Haskell.sln in Visual Studio, and hit Build. Once the two libraries are built, open Worksheet.xls and see how the function is invoked. Note that the COM DLL can be used not only by Excel, but by any other program or script that understands COM. For example, VBA scripts in Word, or VB and Javascript run by Wscript or Cscript. Hope this helps. Regards, Cyril ___

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Haskell.org down
by dons＠cse.unsw.edu.au 26 Sep '06

26 Sep '06

Just in case it has gone unnoticed, haskell.org seems to have been down for a few hours now. Do we have an admin looking into this? -- Don

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Haskell.org down (again)
by dons＠cse.unsw.edu.au 26 Sep '06

26 Sep '06

Something's going on. Haskell.org seems to be down again. That's the 3rd time in 4 days. -- Don

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haskell.org down again
by Pasqualino 'Titto' Assini 26 Sep '06

26 Sep '06

_____ From: HostTracker Notifier [mailto:noreply@host-tracker.com] Sent: 26 September 2006 01:19 To: tittoassini(a)gmail.com Subject: Error Alert Hello, The following url is down: http://haskell.org the error detected is: Http error:Http_client.No_reply Error was detected at 2006-09-26 01:14:41 Your login:titto -- Best regards, http://host-tracker.com/ support team uid:957058 tid:188350 eid:188350-209-0-0-4eee7afa9e44102989dd000c7651f68b

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Curious Functor Class
by Ashley Yakeley 26 Sep '06

26 Sep '06

Is there anything useful about the class of functors which foralls can move across? In other words, functors f, for which for any function g there is this isomorphism f (forall a. g a) <=> forall a. f (g a) In this Haskell snippet I've called the class Hoistable and the isomorphism is (hoist,unhoist): newtype All g = MkAll (forall a. g a); class (Functor f) => Hoistable f where { hoist :: forall g. (f (All g) -> (forall a. f (g a))); hoist = fmap (\(MkAll ga) -> ga); unhoist :: forall g. ((forall a. f (g a)) -> f (All g)); }; Functors that can be made instances of Hoistable: data Singleton a = MkSingleton newtype Identity a = MkIdentity a (->) p -- forall p. data Pair a = MkPair a a data Extra p a = MkExtra p a -- forall p. data Compose p q a = MkCompose (p (q a)) -- forall p q. (Hoistable p,Hoistable q) => On the other hand, Maybe and Either cannot. The key seems to be in only having one "form", i.e. no "|"s in the definition. Does this class actually have a use, or is it merely a curiosity? -- Ashley Yakeley Seattle WA

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Re: Haskell DLL crashes Excel
by Cyril Schmidt 25 Sep '06

25 Sep '06

Nikunj, The symptoms that you describe hint at some incorrect pointer handling. I believe the culprit is that pesky BSTR. I will try to dig up the code that I wrote a while ago to pass a BSTR to Haskell. Meanwhile, another hint. The GHC FAQ 1.4.1 says: "For utterly horrible reasons, programs that use more than 128Mb of heap won't work when compiled dynamically on Windows (they should be fine statically compiled)." (See http://haskell.org/haskellwiki/GHC:FAQ#My_program_that_uses_a_really_large_….) I have no clue what those horrible reasons might be, but you can limit the heap used by the Haskell DLL by setting the environment variable GHCRTS=-M128m Regards, Cyril ___ > | From: Gupta, Nikunj (London) [mailto:Nikunj_Gupta@ml.com] > | Sent: 25 September 2006 10:31 > | To: Simon Peyton-Jones > | Subject: RE: [Haskell-cafe] Haskell DLL crashes Excel > | > | Hello Simon, > | > | Nice to hear from you. I haven't really been working with Haskell but > I > | posted the message to that list since that was the only list I was > able > | to locate even remotely referring to the problem I was facing. > | I have a c++ code which I try to compile as a dll. When debugging the > | code works fine but as soon as I compile it for release and launch an > | independent process not attached to VC++ it crashes. Do you have any > | suggestions with regards to the project settings that might be causing > | the same. > | > | Any sort of help would be much appreciated > | Regards > | Nikunj >

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Writing forum software in Haskell
by David House 25 Sep '06

25 Sep '06

Hi all. The recent thread on email vs. forums inspired a bit of interest to get some forum software on haskell.org. Were we to go ahead with this, I think it'd be great to have this software written in Haskell itself. I'd also love to be involved in a collaboration should one be drummed up to write this kind of thing. Therefore, my questions: * What kind of forum are we aiming at? In my eyes, it'd be less like the bloated phpBBs out there full of oversized signatures and avatars, and more like the minimalistic bbPress on show at, for example, the WordPress support forums [1]. * What would be a compulsory feature list? * What would be a list of things to avoid like the plague? * Which web platform (HAppS, Hope, ...) would make the most appropriate target? Appreciate thoughts and opinions. -- -David House, dmhouse(a)gmail.com

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job writing security software in Haskell
by Andrew Pimlott 25 Sep '06

25 Sep '06

Planning Systems, Inc. has a job opportunity for Haskell programmers. My synopsis is below, and the official description is at http://www.plansys.com/careers/job_details.cfm?JobID=28 Write Haskell code for a US government contractor. Projects include a high-assurance authorization system build around Soutei: http://okmij.org/ftp/Prolog/#Soutei Job is located in Monterey, CA, Slidell, LA, or Portsmouth, VA; working remotely is probably not an option. US citizenship required. Desirable experience: - Haskell - Logic programming - Formal methods - Network security - Web services I welcome you to ask me for more information, and please pass this along to Haskellers looking for a job who can meet the citizenship and (re-)location requirements. Andrew

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Haskell DLL crashes Excel
by Andreas Marth 25 Sep '06

25 Sep '06

Hi everybody! As you might now already know I try to let VBA call Haskell via a DLL. The function returns a String. Everything works fine now if I call the function only once. If I call it more often Excel crashes soon. Does any body have any idea what is going wrong or how I can find out. (I am a medium skilled Haskell user but no VBA programmer.) My code for Haskell (Calculate.hs): module Calculate where import Foreign.C.String (CString, peekCString, newCString) import Data.Word (Word8, Word16, Word32) import Data.Bits (shiftR, (.&.)) import Foreign.Marshal.Array (newArray) import Foreign.Ptr (Ptr, plusPtr) type BSTR8 = Ptr Word8 createBSTR8 :: String -> IO BSTR8 createBSTR8 s = do let len :: Word32 = fromIntegral (length s) low_l :: Word8 = fromIntegral (len .&. 0xFFFF) low_h :: Word8 = fromIntegral (shiftR len 8 .&. 0xFFFF) high_l :: Word8 = fromIntegral (shiftR len 16 .&. 0xFFFF) high_h :: Word8 = fromIntegral (shiftR len 24 .&. 0xFFFF) arr <- newArray ([low_l,low_h,high_l,high_h] ++ map (fromIntegral . fromEnum) s ++ [0]) return $! plusPtr arr 4 testL :: Int -> CString -> IO BSTR8 testL n cs = do s <- peekCString cs createBSTR8 $ concat $ take n $ repeat s foreign export stdcall testL :: Int -> CString -> IO BSTR8 --------------------------------------------------------------- My VBA code (in Excel 2000): Option Explicit Dim h1 As String Dim a As Long Dim b As Long Private Declare Function testL Lib "P:\Daten\Code\Calculate.dll" (ByVal n As Long, ByVal str As String) As String Sub Test() a = 0 b = 10 h1 = "" Do While a <= 15 h1 = testL(b, "What is going on here?") a = a + 1 Debug.Print a Debug.Print h1 Loop End Sub ----------------------------------------------------------- And my DllMain.c: #include <windows.h> #include <Rts.h> extern void __stginit_Calculate(void); static char* args[] = { "ghcDll", NULL }; /* N.B. argv arrays must end with NULL */ BOOL STDCALL DllMain ( HANDLE hModule , DWORD reason , void* reserved ) { if (reason == DLL_PROCESS_ATTACH) { /* By now, the RTS DLL should have been hoisted in, but we need to start it up. */ startupHaskell(1, args, __stginit_Calculate); return TRUE; } if (reason == DLL_PROCESS_DETACH) { shutdownHaskell(); return TRUE; } return TRUE; } ---------------------------------------------------------------- And I compile the DLL with: ghc -c --make -fglasgow-exts -static Calculate.hs ghc -c -fglasgow-exts -static *.c ghc --mk-dll -o Calculate.dll *.o -static -optdll--def -optdllCalculate.def ------------------------------------------------------------------ Nearly forgot Calculte.def: LIBRARY Calculate EXPORTS testL = testL@8 ----------------------------------------------------------------- The DLL is stable regarding the length of the input string and thelength of the return string (determined with b). But crashes for larger(?) a's. At my computer it crashes with a >9. There seems to be a connection between b and a. If b is 10 then a>9 crashes a<10 is fine. If b=1000 then a<5 is okay a>4 crashes. If b =100 then a<5 is okay, a>5 crashes and a=5 crashes with an error "Statement in 0x778cb032 points to 0x00000000 the function 'written' could not be done ..." And a second one "Statement in 0x778cc441 points to 0x00000000 the function 'written' could not be done ..." (My translation of the german errors.) If anybody knows what is going wrong or can give me a hint how to find out what is going wrong it would be very helpful. Thanks Andreas

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Re: [Haskell] BitSyntax for Haskell
by Einar Karttunen 25 Sep '06

25 Sep '06

On 23.09 15:00, Adam Langley wrote: > Erlang's bit syntax[1] is a great for building and breaking up binary > structures. I've knocked up something similar (although a little > clumsy) for Haskell: > > http://www.imperialviolet.org/binary/bitsyntax/ > http://www.imperialviolet.org/binary/bitsyntax/BitSyntax.hs > > I'm sure that this isn't the best possible way to do this, but it > suffices at this stage for many problems. This looks very nice. Here are some feature wishes: BitBlock: add a way to encode length prefixed ByteStrings. For the decoding part: * Provide a monadic interface * Add a test part to ReadType: Test :: ReadType a -> (a -> Bool) -> ReadType Test (or a -> m ()) in the monadic case. * Add a way to limit the size of a LengthPrefixed: e.g. [Unsigned 4, LengthPrefixed] is very unsafe, the app should have a way to control the maximum length. - Einar Karttunen

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System threads?
by Lyle Kopnicky 25 Sep '06

25 Sep '06

Hi folks, I'm working on a project for which the solution is highly parallelizable. I've been writing it so far for GHC as a single-threaded app. I'd like to be able to split the job into multiple pieces, and spawn different system threads for each piece, so they will run on separate CPUs. Either each thread needs to write to the same IO handle, or else the results need to be collected when the threads are merged, so the results can all be printed. I'm working on Linux. What's the easiest way to accomplish this? Glasgow Parallel Haskell? Is there a Posix.Threads library? I understand that Haskell threads (Control.Concurrent) all run in one system thread, so would not take advantage of SMP. I suppose in the worst case I can write a shell program that divides up the problem, runs several worker processes to output intermediate results to files, then combines the results to the desired filehandle. Thanks, Lyle

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Unable to profile program using Data.ByteString.Lazy
by Lyle Kopnicky 25 Sep '06

25 Sep '06

Hi folks, I wrote a program that uses some of the Data.ByteString libraries. I'm using GHC 6.4.1 and FPS 0.7. The program compiles and works just fine. But when I try to profile it, by compiling with -prof, I get: Failed to load interface for `Data.ByteString.Lazy': Could not find module `Data.ByteString.Lazy': locations searched: Data/ByteString/Lazy.hi Data/ByteString/Lazy.hi-boot /f/g/lib/fps-0.7/Data/ByteString/Lazy.p_hi Why can it find the module when it's compiling without -prof, but not when it's compiling with it? I would really like to get profiling to work. Thanks, Lyle

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FFI and LD_PRELOAD -> segfault
by Stephan Walter 24 Sep '06

24 Sep '06

Hi, I'm trying to use a shared lib written in Haskell to overload C functions via LD_PRELOAD. You might think this is a bit silly, but hey, why not? I want to overload connect() from sys/socket.h. I'll document what I've written so far; unfortunately there doesn't seem to be a lot of documentation about this topic. This is my haskell code (in testffi.hs): > module Socks where > > import Foreign.C.Types > import Foreign.Ptr > > newtype S_sockaddr = S_sockaddr () > > foreign export ccall "connect" connect :: > CInt -> Ptr (S_sockaddr) -> CUInt -> IO CInt > > connect :: CInt -> Ptr (S_sockaddr) -> CUInt -> IO CInt > connect _ _ _ = return (-1::CInt) Here's how I compile it: ghc -Wall -c -fffi testffi.hs ghc -Wall -optl "-shared" -optl "-Wl,-soname,libtestffi.so" \ -o libtestffi.so testffi.o testffi_stub.o I'm not at all sure about these compiler and linker options, but this is the best I could come up with (i.e., no errors or warnings) Then I run a test program which calls connect(). I won't include the C source of that program, but it basically connects to the IP address given as the first argument (at port given by third argument) and sends a string. Basic error checking is done, i.e. the program does test the return value of connect(): LD_PRELOAD=./libtestffi.so ./conntest 127.0.0.1 "HELO" 1234 This aborts with a segfault in scheduleWaitThread() from ./libtestffi.so The test program doesn't use threads, so I'm wondering what I did wrong? Any help is appreciated. Greetings, Stephan Walter

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haskell.org down
by Pasqualino 'Titto' Assini 24 Sep '06

24 Sep '06

Haskell.org has been down for over 7 hours (see attached log). The Web master might use host-tracker.com (or a similar free tracking service) to keep an eye on it. Regards, titto _____ From: HostTracker Notifier [mailto:noreply@host-tracker.com] Sent: 24 September 2006 10:41 To: tittoassini(a)gmail.com Subject: Operation Restored Alert Hello, http://haskell.org is now back to normal. Operation restored at 2006-09-24 10:36:55. Url was down as a resultat of: Http error:Http_client.No_reply Error was detected at 2006-09-24 03:36:16 Downtime total 7 hour(s) 39 sec(s). Check failures total: 14. Your login:titto -- Best regards, http://host-tracker.com/ support team uid:957058 tid:188350 eid:188350-61-0-0-669df3829d00102989dd000c7651f68b ffeid:188350-47-0-0-a2bced5e9cc5102989dd000c7651f68b

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[newbie] How to test this function?
by Bruno Martínez 23 Sep '06

23 Sep '06

I've written a function that looks similar to this one getList = find 5 where find 0 = return [] find n = do ch <- getChar if ch `elem` ['a'..'e'] then do tl <- find (n-1) return (ch : tl) else find n First, how do I fix the identation of the if then else? Second, I want to test this function, without hitting the filesystem. In C++ I would use a istringstream. I couldn't find a function that returns a Handle from a String. The closer thing that may work that I could find was making a pipe and convertind the file descriptor. Can I simplify that function to take it out of the IO monad? How? I thought about getContents, but that eats all input. Thanks in advance, Bruno

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Haskell Program
by Tim Newsham 23 Sep '06

23 Sep '06

The company I work for was gracious enough to allow me to release a haskell program I wrote for them: http://www.isecpartners.com/file_fuzzers.html There's a python version and a rewrite in haskell. I used some looping constructs discussed earlier on this list to keep the haskell code similar to its original python code. Don Stewart was kind enough to help me optimize some of the code using ByteString and a faster PRNG (the System.Random class seems to be a bit on the slow side!). Prior to using ByteString and optimizing the code it was a bit slower than the python version, but with optimizations it is quite a bit faster. (In practice, the external program being invoked is usually the performance bottleneck). Tim Newsham http://www.thenewsh.com/~newsham/

3 2

Polymorphism/monomorphism (and the restriction)
by Aaron McDaid 22 Sep '06

22 Sep '06

Hi, I think the following might help a little in understanding the monomorphic restriction (which I don't fully understand myself). I'm a bit of a newbie so apologies in advance if I've made a mistake or if my description isn't as useful to others as it seems to me. I've been following a thread on haskell(a)haskell.org and I think the below might help. I used GHCi, version 6.2.2 (it fails in hugs but that seems to be because of hugs non-compliance with the standard in this case). First off, I'm guessing that I'm getting Haskell98 behaviour here and not some GHCi extension. Please tell me if this is not the case. Run the code listing at the bottom of this email and you should get the output which I've also listed below. This code experiments with "Int", "Float" and "(Num a) => a", and I tried to print x*2 and x/2 for each. (4::Int)/2 isn't allowed because / isn't defined for Ints. You can see that kN :: (Num a) => a took two different types depending on what method ( / or * ) was applied to it. kN / 2 = 2.0 kN * 2 = 8 kN/2 is a Float (it can't use Int as / isn't defined for Int, so it uses Float, for which / is defined). kN*2 is an Int. The above outputs demonstrates polymorphism, doesn't it? i.e. Not only has the compiler got a variety of types to choose from, but a variety of types can be used at runtime? The output for kI and kF is obvious. The interesting thing is that k behaves as a Float in both cases. This is monomorphism isn't it? i.e. the compiler may have a variety of types to choose from, but it picks one and sticks to it for every usage. In summary, k didn't give the same outputs as kN. And the monomorphism restriction is a rule which means that sometimes things are forced to a monomorphic type (like k as Float here) when it could have given it a polymorphic type like "kN :: (Num a) => a" I'm fairly new to these lists, so apologies if I'm covering old ground again. My first aim is to understand exactly what polymorphism and monomorphism is and demonstrate corresponding results, before thinking about the restriction. Thanks, Aaron -- The code kI :: Int kI = 4 kF :: Float kF = 4 kN :: (Num a) => a kN = 4 k = 4 main = do p "kI * 2" $ kI * 2 p "kF / 2" $ kF / 2 p "kF * 2" $ kF * 2 p "kN / 2" $ kN / 2 p "kN * 2" $ kN * 2 p "k / 2" $ k / 2 p "k * 2" $ k * 2 p :: (Show a) => String -> a -> IO () p s = putStrLn.(s++).(" = "++).show -- the output - remember kI / 2 is not possible. kI * 2 = 8 kF / 2 = 2.0 kF * 2 = 8.0 kN / 2 = 2.0 kN * 2 = 8 k / 2 = 2.0 k * 2 = 8.0 -- PS: If you delete the k / 2 line from the program, then k * 2 becomes simply 8 (not 8.0). It uses Int if possible, and Float if that's not available.

2 1

Why am I not allowed to use CStringLen in foreign export?
by Andreas Marth 22 Sep '06

22 Sep '06

Hi! I try to export a Haskell function to VBA. (At the moment without COM.) Because VBA seems to expect a String with length information I tried to return a CStringLen as defined in Foreign.C.String. But if I try to compile it I get an "unacceptable argument type in foreign declaration: CStringLen". My reduced test program now is a strange "Hallo world!" program: >module Test where >import Foreign.C.String > >foreign export stdcall hello :: IO CStringLen >hello :: IO CStringLen >hello = do newCStringLen ("Hallo world!") Do I do some thing wrong here? (If I use CString instead of CStringLen and accordingly newCString instead of newCStringLen everything compiles fine but VBA crashes.) Any help is appreciated, Andreas

3 6

Trying to write a TCP proxy
by John Ky 22 Sep '06

22 Sep '06

Hello, I'm trying to use haskell to put together a TCP proxy I can put between my browser and my webserver. This is as far as I got. The webserver isn't returning my request: > listen = withSocketsDo $ do > putStrLn "Listening..." > socket <- listenOn $ PortNumber 8082 > (handleToClient, hostName, portNumber) <- accept socket > putStrLn "Connected to client" > contents <- hGetContents handleToClient > putStrLn "Received from client:" > putStrLn contents > handleToServer <- connectTo "127.0.0.1" (PortNumber 8080) > putStrLn "Connected to server" > hPutStrLn handleToServer contents > putStrLn "Waiting for server to respond" > hPutStrLn handleToServer "\r\n\r\n" > putStrLn "Waiting for server to respond" > result <- hGetContents handleToServer > putStrLn result > hClose handleToServer > hClose handleToClient > sClose socket Am I doing something wrong? Thanks, -John

6 9

Re: Haskel DLL crashes Excel
by Cyril Schmidt 22 Sep '06

22 Sep '06

Andreas Marth wrote: > As you might now already know I try to let VBA call Haskell via a DLL. I had to do this a while ago; the best I could come up with was: 1. Make a Haskell DLL that exports plain C-style interface. 2. With Visual Studio, make a COM DLL that exports the interfaces to be used by VBA and passes the calls to the Haskell DLL. This COM DLL, among other things, handles the conversion between C-style strings and BSTR (there are examples on msdn.microsoft.com of doing this). Hope this helps. Cheers, Cyril

1 0

Haskore
by David Curran 22 Sep '06

22 Sep '06

Hi I have been trying to learn haskell (tip over the vending machine) for a while and eventually decided the Haskore music library might be a good way to start understating the language. I am using windows and hugs98. The IOExtensions.hs file will not work under windows. Any ideas on how to make it work or is this library *nix only? Regards David ----------------------------------------------------------------------------- -- -- Replacement for IOExtensions.hs of hugs for ghc -- -- Implements only the functions necessary for Haskore! -- -- Does not work under Windows (where there is a difference between -- binary and text files) -- -- Suitable for use with Hugs 98 ----------------------------------------------------------------------------- module IOExtensions( writeBinaryFile, openBinaryFile, readBinaryFile ) where import System( getArgs ) import IO( Handle, IOMode(WriteMode), openFile, hPutStr, hClose ) import IOExts openBinaryFile :: FilePath -> IOMode -> IO Handle openBinaryFile path mode = openFileEx path (BinaryMode mode) writeBinaryFile :: FilePath -> String -> IO () writeBinaryFile = writeFile -- this does not work on Windows!!! readBinaryFile :: FilePath -> IO String readBinaryFile = readFile -- this does not work on Windows!!! -- It chews up lies and spits out the gristle of truth http://liveatthewitchtrials.blogeasy.com/

5 4

Trouble with simple embedding of shell in haskell
by George Brewster 22 Sep '06

22 Sep '06

I'm just tried writing a function to allow convenient embedding of shell commands, but I'm running into behavior I don't really understand somewhere at the intersection of lazy evaluation, IO, and threading. The function in question is: sh :: String -> String -> IO String sh cmd = \input -> do (stdin, stdout, _, pid) <- runInteractiveCommand cmd forkIO $ hPutStr stdin input >> hClose stdin -- get exit status to prevent zombie accumulation forkIO $ waitForProcess pid >> return () hGetContents stdout Simple uses like this work (prints "hi" and returns): return "hi" >>= sh "cat" >>= putStrLen However, this blocks: return "hi" >>= sh "cat" >>= sh "cat" >>= putStrLen this prints "hi\nblar" and returns return "hi" >>= sh "cat" >>= (\s -> putStrLn "blar" >> sh "cat" s) >>= putStrLn and this always returns but sometimes prints nothing and sometimes prints "hi\n" sh "echo hi" "there" >>= sh "cat" >>= putStrLn All these were tried with ghc 6.4.1 compiled with -threaded Just wondering if anybody has a good explanation of this range of behavior and/or a working rewrite of this function. -George

3 4

Non-existant existential?
by Jim Apple 22 Sep '06

22 Sep '06

Consider the following: data SimpExist a = SimpExist (forall x . x -> a) f :: SimpExist Bool f = SimpExist (const True) g = SimpExist id What is the type of g? In a similar example, GHC tells me it is of type SimpExist c. Yet, I can't unify it with any other SimpExist c'. It seems to me that this is something like exists x . SimpExist x, and is similar to: data ExistWrap = forall a . ExistWrap (forall x . x -> a) Jim

2 3

Re: Haskell web forum
by Albert Lai 21 Sep '06

21 Sep '06

As requested, I continue here the thread on the proposal for a web forum. You will soon enough find out what I think of web forums. Let me first show you what this mailing list looks like to me using software of my choice: ! [ 192: Mark Carroll ] [Haskell-cafe] Card trick ! [ 37: Thomas Conway ] Re: [Haskell-cafe] How can we detect and fix E [ 19: Albert Lai ] Re: [Haskell-cafe] Either e Monad E [ 19: Deokhwan Kim ] [ 62: Bas van Dijk ] E [ 47: apfelmus@quantentunn] [Haskell-cafe] Re: Optimization problem [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: apfelmus@quantentunn] [Haskell-cafe] Re: Optimization prob [ 14: Ross Paterson ] [ 154: Jan-Willem Maessen ] Re: [Haskell-cafe] Traversing a graph in STM [ 52: Josef Svenningsson ] [ 18: Bulat Ziganshin ] Re[2]: [Haskell-cafe] Traversing a graph [ 26: Sebastian Sylvan ] [ 22: Ashley Yakeley ] [Haskell-cafe] Re: Wiki contact An elaboration of the cool features I have always enjoyed is in order: On browsing threads, both inter-thread jumps and intra-thread navigation, the screenshot speaks for itself. (If for any perverted reason I want the messages listed chronologically rather than well-organized, the software can do it too.) The messages marked with "E" are messages I have just read. With this mark, they will not show up next time I browse this mailing list again. Take note that I can mark and unmark any subset of messages, unlike certain people's linear-time idea that one single timestamp distinguishes "already read" from "new" across the board. (I have always heard that non-linear thinking is superior to linear thinking. I think I believe it for at least this application.) (Is this an example of what is meant when someone said that a web forum requires "less technical knowledge", i.e., programmers on a web forum will not need to know about subsets?) (Speaking of which, is the thread-browsing part also an example, i.e., programmers on a web forum will not need to know about trees and forests?) Though the marked messages will not show up next time, they are not thrown away yet. They are kept on my disk for quite a while. (I get to set how many days they stay.) The software offers several ways to show them, but I think the most useful one is this. Suppose I look at apfelmus's message on "Optimization problem" (two are shown, I'm referring to the first one), and wonder, "gee, what is it replying to?" To find out, I position myself to that message, then press a button, then the software will show the desired parent message. In fact, the updated screen looks like this (I just include the thread in question): E [ 18: Ross Paterson ] Re: [Haskell-cafe] Re: Optimization problem E [ 47: apfelmus@quantentunn] [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: apfelmus@quantentunn] [Haskell-cafe] Re: Optimizatio [ 14: Ross Paterson ] So even if apfelmus's message quotes nothing from Ross's, I can still find out, provided it is recently enough to be still on my disk. Take note that, on a linear-thinking web forum, there could be any number of intervening messages in the same thread between Ross's and apfelmus's, and even though it is true that Ross's is immortalized in the database of the forum, the real question is how to fish for it. (ObRant again about how web forums are "friendly" to "programmers" who have no clue about trees.) Here is a feature unrelated to threading, and in fact it is much more fundamental and pervasive (since almost all email software, not just mine, provides this), and in fact it has much to do with pervasive computing too. Here in Canada, in cafes one can get wireless Internet access, but it has to be paid by the minutes. So suppose I want to visit haskell-cafe when I visit a Starbucks cafe, and there are 30 messages I want to read, and I can read them at the rate of half a minute per message. If haskell-cafe were on a web forum, I would end up keeping my wireless connection for 15 minutes. (Could I disconnect and reconnect between messages? First of all that's really a pain. Secondly, let's say I do that, the billing is still such that I connect for 2 seconds and I'm still charged for 1 minute.) But since haskell-cafe is on a mailing list, I can just connect for a few seconds to download the messages, then disconnect and read offline; I'm charged for at most 1 minute. (Alternatively but equivalently, suppose I want to read haskell-cafe while riding subway - underground trains. Here in Toronto the subway doesn't provide wireless Internet access yet, and probably won't for another decade. If haskell-cafe were on a web forum, I'd be SOL. But since it is on a mailing list, I can pull the same trick as above.) If you can find a single implementation of web forums that provides the features I have always taken for granted... but wait, there is one more feature, actually it's a meta-feature, a principle, a belief of mine. I am an individualist and non-conformist. I believe in each individual choosing and customizing his/her software and human-computer interface. There must be a common protocol for the exchange of content and meta-data, of course, but that should be where the commonality ends. How this content is presented to me, how I navigate and manipulate this content, what human-computer interface I use, and whose software I use, should be entirely up to me. The very idea of a web forum is an antithesis to this. In practice, because a web forum has to include some human-computer interface, and because someone has to write code for that, and because that "someone" is at best a few people with only a few hours per week for this, the web forum is forced to include little or no choice in its human-computer interface and its expressive power. And since you always have to worry about members who "have less technical knowledge such as professional programmers who don't understand trees and subsets", its expressive power cannot be too great, and so the mandatory web interface must suck. In other words, in theory you can always say "let's code up a superior, even diverse, web forum", but in practice who's going to do that? In contrast, superior software for mailing lists and newsgroups already exist, and if you have Mozilla or Thunderbird or Evolution or Outlook Express, you probably already have decent or half-decent such software. So let me make a daring counter-proposal. We go back to newsgroups. We set up our own news server for this; use just one server so everyone is always synced to it, and we do not pose a heavy load on it by today's standard anyway. Here are the benefits. First and foremost, all my favourite features are provided. (I'm allowed to be a bit self-centred for just a little while, ain't I? :) ) Everyone can use his/her favourite software. For example Mozilla can do newsgroups and news servers, and everyone knows how to use Mozilla, and if someone doesn't yet, the GUI is easy to pick up. It's a no-brainer. Normally news servers are set up to throw away old messages, but we can tell ours to keep everything forever. You can always consult the server for old messages for context or archaeology, in case you don't keep local copies of your own. Your favourite software can search. There may even be a way to get Google and other search engines to do the search. There is a valid concern raised about the ritual overhead on the part of a new member in subscribing and unsubscribing to a multitude of mailing lists, and it is suggested that a web forum mitigates this. A web forum mitigates this by requiring just one registration and you can subsequently post in any sub-forum. The newsgroup way answers this beautifully: it is the same number of registration and you can subsequently post in any newsgroup. Last but not least, for those who are excited about the very idea of a one-size-fits-all web interface: we can put up a web interface as a front-end to the news server too! As programmers, we are very individualist about programming environments: we like to pick our own editors, colours, key bindings... Actually we don't mind all sorts of compromises on those fronts, but here is one thing all of us insist: we require our code repository to be highly concurrent and distributed. We expect to work offline. In light of this, I find it rather odd that a programmer would propose the exact opposite for our discussion repository - a web forum, which not only superficially limits customizations (which, like I said, we can live with), but also fundamentally expects us to work online. More ironically, highly concurrent distributed discussion repositories have always existed, and they are now dismissed under the false impression of "hard to use". (They are certainly easier to use than highly concurrent distributed code repositories, since you post new messages rather than mutate existing messages, and so there is no need to learn about conflicts, merging, forking, and all that.) It makes me wonder: what's next, are we going to move our IDEs to the web too, and henceforth must we debug our code through the web?

9 10

what is a difference between existential quantification and polymorhic field?
by Bulat Ziganshin 21 Sep '06

21 Sep '06

Hello haskell-cafe, now i'm reading Haskell' proposals and found that these two things considered as different: http://hackage.haskell.org/trac/haskell-prime/wiki/ExistentialQuantification http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents can you please explain me what is the difference between data Ex = forall a. Num a => Ex a and data Po = Po (forall a. Num a => a) ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

4 5

Re[2]: [Haskell-cafe] [newbie] How to test this function?
by Bulat Ziganshin 21 Sep '06

21 Sep '06

Hello Bruno, Thursday, September 21, 2006, 6:16:49 PM, you wrote: >>> Second, I want to test this function, without hitting the filesystem. >>> In >>> C++ I would use a istringstream. I couldn't find a function that >>> returns >>> a Handle from a String. >> >> http://haskell.org/haskellwiki/Library/Streams > I'll give this a try. btw, although this may be considered as bug, it's possible to use Streams to combine getContents and other I/O on the same stream: h <- openFD "test" ReadMode str <- vGetContents h print (take 5 str) c <- vGetChar h print с print (take 5 str) :) so it's possible to use Streams to simplify your program. of course, you should understand how it works in order to use it in safe manner and that means that you should understand how unsafeInterleaveIO works. look at my today message in haskell list and expalanation of unsafePerformIO in "IO inside" -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

Re: Haskell web forum
by Aaron Denney 21 Sep '06

21 Sep '06

Moved to haskell-cafe from haskell-general. On 2006-09-20, Niklas Broberg <niklas.broberg(a)gmail.com> wrote: > On 9/20/06, Aaron Denney <wnoise(a)ofb.net> wrote: >> And I disagree with you. Web forums are usenet reinvented poorly. >> It's impossible to keep track of what's new, threading is either poor or >> nonexistent. Mailing lists with searchable archives work well. gmane >> provides a nice usenet interface to mailing lists. > > I don't recognize the forums I frequent in your description at all. I > have absolutely no problem keeping up with what's new. In fact I find > it a lot easier since many of the topics take place in subforums that > I know don't interest me, so I don't even need to go there to check. And half of the time messages are miscategorized, because conversations are miscategorized, so I do need to go check. > With this mailing list, I have to manually "mark as read" about 2/3 of > all incoming mails because they don't (from the title) interest me, > and I have to read a few that don't interest me because I couldn't > tell from the title. Where as an entire page of messages under a particular "thread" (it's not a thread, because they almost never have the messages showing the exact post they're responding to, just the general list of messages in this topic.) Then I need to go locate the tiny link that gets me to the next page. They're nearly unuseable. > Threading also works as well as could be expected, better than > threading in gmail for instance, so I don't see the problem here > either. That has never been my experience. Threads branch annd mutate and drift. And this is a good thing. The rigid categorizations can't handle this, whereas I can easily ignore entire threads and subthreads with a client designed for it. >> > It's all there, all the time. >> >> Yes, that's part of the problem. > > How is that? You mean because all the old stuff gets in the way of the > new? Then you're just using a bad forum software that can't properly > point out the new stuff for you. I agree that not all forums are good, > but there are definitely those that are. Exactly. but the problem with forums is that everynone must use the same forum software because it's sitting on a website. With usenet or mailing lists, everyone gets their own software that provides the features they want, with the interface they want. >> > It is also easy to create sub-groups/forums for specific projects, >> >> This is the one semi-useful thing. Of course, what you end up with is >> another not-so-useful forum. > > This is probably the one most useful thing yes. I wouldn't call it > semi-useful though. Just remember all the responses that the HCAR gets > each year of the form "wow how many cool projects are out there that I > had no clue existed". What if all (or many of) those projects were > actually there, on the forums, where everyone knew where to look? Forums are another place to look, instead of on the wiki, on the mailing lists, etc. It fragments the community. > And your last sentence, I just don't understand. "not-so-useful" just > because it's a forum, or did you mean somethine else as well? Yes, because it's a forum. Almost any feature can be implemented in a web-forum. And many will, but almost none will be done well, because (a) it's hard to get something that will please everyone and it has to please everyone because everyone must use the same software (b) The experience isn't there. Mail & usenet software has been worked on for over 25 years. It works. Your mail client sucks? Get a new one. There are lots, and one should surely work for you. I don't have the option of using different web-forum software when something about it annoys the hell out of me. The biggest thing I see for them is web accessibility. But we already have that. We have archives. We have gmane. Tons of other gatewaying software exist, and could be easily set up, because mailing lists are designed to be used by multiple clients. Web forums are only "pull". I have to go check each individual one, on a semi-regular basis. Yes, there is RSS. It's another hacky workaround for something that mail inherently has. Mailiing lists can be either pull or push. I can dump the list mail in my inbox and get it regularly, or segregate it out to another mailbox, or several mailboxes, or whatever. -- Aaron Denney -><-

1 0

Either e Monad
by Deokhwan Kim 20 Sep '06

20 Sep '06

Dear all, Where is the Monad instance declaration of Either e? >From the description of Control.Monad.Error, I deduce that Either e is an instance of Monad. http://haskell.org/ghc/docs/latest/html/libraries/mtl/Control-Monad-Error.h… class Monad m => MonadError e m | m -> e where ... Error e => MonadError e (Either e) But, I cannot find the Monad instance declaration of Either anywhere. Thanks. -- Deokhwan Kim

3 3

MonadList?
by Michael Shulman 20 Sep '06

20 Sep '06

In another thread, I wrote: > main = (>> return ()) $ runListT $ do > arg <- ListT getArgs > n <- ListT $ return [1..3] > liftIO $ putStrLn ((show n) ++ ") " ++ arg) The frequent occurence of "ListT $ return" in my code when I use the ListT monad transformer has made me wonder why there isn't a standard typeclass `MonadList', like those for the other monad transformers, encapsulating the essence of being a "list-like" monad -- in this case, the ability to select from a list of things. I quickly wrote one for myself: class MonadList m where option :: [a] -> m a instance MonadList [] where option = id instance (Monad m) => MonadList (ListT m) where option = ListT . return Has anyone else thought about or done something like this? Mike

4 6

Re: Numeric type classes
by Henning Thielemann 20 Sep '06

20 Sep '06

On Mon, 11 Sep 2006, Ross Paterson wrote: > On Mon, Sep 11, 2006 at 04:26:30PM +0200, Henning Thielemann wrote: > > On Sat, 9 Sep 2006, Ross Paterson wrote: > > > I think that a finer grain numeric hierarchy, while retaining Num, etc, > > > is feasible without changing the language: unlike the case of monads, > > > the people who will be defining instances of numeric classes are the > > > very ones who are inconvenienced by the current hierarchy. The main > > > impact on clients of the classes is that some functions would have > > > more general types. > > > > There are many Num instances around in libraries where people wrap to > > external libraries: functionalMetapost, CSound wrapper in Haskore, > > SuperCollider (GSL too?). What about Num (algebraically Ring) instances of > > polynomials, residue classes and other such advanced mathematical objects? > > And what do abs and signum mean for Haskore's orchestra expressions, > polynomials, residue classes, vectors, matrices, functions, etc? For clarification: Haskore does not define any arithmetic for music, but CSound provides some arithmetic and Haskell wraps it with Num instances. > The people who define those wish they were defining Ring, but they must > define Num. It seems we are at a point, where we have to define what is a 'number'. More precisely: Can you tell me the difference between numbers and "more complex mathematical objects"? Is a complex number a number? Is a quaternion a number? Is a residue class a number? We can calculate with integers modulo some other integer like with integers - is that considered computation with numbers? Shall we distinguish between matrices of numbers and matrices of more complex mathematical objects? In signal theory matrices of polynomials are very common.

14 30

Java or C to Haskell
by Carajillu 20 Sep '06

20 Sep '06

I'm trying to write in Haskell a function that in Java would be something like this: char find_match (char[] l1, char[] l2, char e){ //l1 and l2 are not empty int i = 0; while (l2){ char aux = l2[i]; char[n] laux = l2; while(laux){ int j = 0; if(laux[j] = aux) laux[j] = e; j++; } if compare (l1, laux) return aux; else i++; } return ''; } compare function just compares the two lists and return true if they are equal, or false if they are not. it is really a simple function, but I've been thinking about it a lot of time and I can't get the goal. It works like this: find_match "4*h&a" "4*5&a" 'h' ----> returns '5' (5 matches with the h) find_match "4*n&s" "4dhnn" "k" ----> returns '' (no match at all - lists are different anyway) I'm trying to use map for scanning the list l2, then using some function for substitute the char, and finally something like zipWith x == y laux l1 for comparing the lists, but I can't find a way for putting all this together!! -- View this message in context: http://www.nabble.com/Java-or-C-to-Haskell-tf2303820.html#a6403589 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

7 14

ICFP programming contest 2006 results: video stream
by dons＠cse.unsw.edu.au 20 Sep '06

20 Sep '06

Malcolm Wallace has recorded the ICFP programming contest results announcement as video, straight from the ICFP conference in Portland. He's posted it to Google Video, and it's available to download (120M) or stream from Google video, here: http://video.google.com/videoplay?docid=6419094369756184531 Thanks Malcolm! -- Don

1 0

Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by Vivian McPhail 20 Sep '06

20 Sep '06

> Vivian McPhail wrote: > > > > class Forkable a where > > > > fork :: String -> a -> a -> a > .... > > What I would like to be able to do is > > differentiate between Forkable (m a -> > > b) and Forkable (<function type> -> b). > > Have you tried this combination of instances? > > instance Forkable (IO a) where ... > -- and similarly for all the concrete > -- monad types you will use fork with > > instance (Forkable a, Forkable b) => > Forkable (a -> b) where ... > > Alternatively, since the fork function seems to be all about > propagating a value (the String), would Control.Monad.Reader > serve your purpose? The value that gets 'forked' is not actually the string, it is the result of a monadic computation. > > Regards, > Tom > I have tried: > instance Forkable (USM NRef) where... Which is my Monad The problem lies with > instance (Forkable a, Forkable b) => Forkable (a -> b) where > fork n a1 a2 a = fork n (a1 a) (a2 a) because I need the arg a to be evaluated before it gets passed to a1 and a2. This definition does the right thing when type 'a' is a function type, because it is not a value, but with something like 'm a -> (m a -> m a) -> m a' with Forkable (a -> b) the first arg gets evaluated twice, to be more concrete: With (and golden white) eggs I want the 'eggs' that is passed to 'golden' to be the same as the 'eggs' that is passed to 'white', i.e. -> and1 (golden2 eggs3) (white4 eggs3) and not -> and1 (golden2 eggs3) (white4 eggs5) So to do this I need to be able to recognise the case where the 'a' of (a -> b) is of type 'm a' so that I can evaluate it -- doesn't typecheck instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where fork n a1 a2 a = do a' <- a fork n (a1 $ return a') (a2 $ return a') Tom suggested that I might be able to use the Reader monad, but I'm not clear as to how I could do this. Cheers, Vivian -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.405 / Virus Database: 268.12.5/451 - Release Date: 19/09/2006

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sections for record settors
by Tim Newsham 20 Sep '06

20 Sep '06

Hi, When you create a record in haskell, it automatically generates gettor functions for accessing the record fields. It also provides you with a record update syntax: rec {field=val} but there is no settor function. It would be nice if there was some sort of section-like syntax to access the settor function, like: {field=} -- val rec {field=val} -- rec or even if there was a name for that function (such as field_settor). Without such a syntax, one often has to write lambdas like: \r v -> r {field=v} \r -> r {field=3} Consider for example an cmd line parser that collects flags in a record. Instead of writing: opt "l" (NoArg {c_loop=False}) "dont loop", opt "o" (optStr {c_out=} "dir") "output dir", opt "t" (optInt {c_maxtime=} "mins") "maximum time" we would have to write it out as: options = [ opt "l" (NoArg (\c -> c {c_loop=False})) "dont loop", opt "o" (optStr (\v c -> c {c_out=v}) "dir") "output dir", opt "t" (optInt (\v c -> c {c_maxtime=v}) "mins") "maximum time" ] where opt cs d s = Option cs [] d s optStr f name = ReqArg f name optInt f name = ReqArg (f . read) name getOptions :: String -> [String] -> IO Config getOptions prog argv = let header = "Usage: " ++ prog ++ " [OPTIONS...] files..." in let usage s = ioError (userError (s ++ "\n" ++ usageInfo header options)) in let (updates, files, errs) = getOpt2 options argv in let config = updates defConfig {c_files=files} in case () of _ | not (null errs) -> usage (concat errs) | null (c_files config) -> usage "no files specified" | otherwise -> return config Tim Newsham http://www.thenewsh.com/~newsham/

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Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by tpledger＠ihug.co.nz 20 Sep '06

20 Sep '06

Vivian McPhail wrote: > > > class Forkable a where > > > fork :: String -> a -> a -> a .... > What I would like to be able to do is > differentiate between Forkable (m a -> > b) and Forkable (<function type> -> b). Have you tried this combination of instances? instance Forkable (IO a) where ... -- and similarly for all the concrete -- monad types you will use fork with instance (Forkable a, Forkable b) => Forkable (a -> b) where ... Alternatively, since the fork function seems to be all about propagating a value (the String), would Control.Monad.Reader serve your purpose? Regards, Tom

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[OT] Yet Another program written in Haskell: a language tag parser
by Stephane Bortzmeyer 20 Sep '06

20 Sep '06

The program is written in Haskell, hence my shameless ad here: http://www.bortzmeyer.org/gabuzomeu-parsing-language-tags.html GaBuZoMeu is a set of programs to parse and check language tags (see the RFC 4646 produced by the IETF Working Group LTRU - Language Tag Registry Update). Language tags are used by several protocols (like HTTP) or languages (like XML) to indicate the human language you want or have. Examples of language tags are "fr" (French), "apa" (Apache), "se-FI" (Swedish, as practiced in Finland) or "uk-Latn" (ukrainian in the latin script). An IANA registry stores the registered values (http://www.iana.org/assignments/language-subtag-registry) A language tag can be well-formed (syntactically correct) or not. This can be tested without access to the registry. A langauge tag can be valid or not (all its subtags registered). This depends on the copy of the registry you use (since the registry changes). GaBuZoMeu includes the following programs: * check-wf checks the well-formedness of tag(s). Example of use: "check-wf fr en-AU". * check-valid checks the validity of tag(s). Same use. GaBuZoMeu is written in Haskell (http://www.haskell.org/) so you'll need an Haskell compiler such as ghc (http://haskell.org/ghc/) To compile, just type "make". To check that everything is OK, you can type "make test". You should get zero "Errors" and zero "Failures". GaBuZoMeu is distributed as free software, under the GNU General Public Licence. Remarks, patches and bug reports are welcome. Written and maintained by Stephane Bortzmeyer <bortzmeyer(a)nic.fr>.

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map (-2) [1..5]
by Tamas K Papp 19 Sep '06

19 Sep '06

The code in the subject generates an error. I understand why this is (- is treated as part of the number), but I don't know how to solve it, ie how to tell Haskell that - is a function/binary operator? Thanks, Tamas

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