- Haskell-Cafe - Haskell.org

Problems with nested Monads
by Job Vranish 15 Jul '09

15 Jul '09

I'm trying to make a function that uses another monadic function inside a preexisting monad, and I'm having trouble. Basically my problem boils down to this. I have three monadic functions with the following types: f :: A -> M B g :: B -> N C h :: C -> M D (M and N are in the monad class) I want a function i where i :: A -> M (N D) the best I can come up with is: i :: A -> M (N (M D)) i a = liftM (liftM h) =<< (return . g) (f a) I'm starting to feel pretty sure that what I'm going for is impossible. Is this the case?

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ANN: darcs 2.3 beta 4
by Petr Rockai 15 Jul '09

15 Jul '09

Hi again! We have decided to delay the release cycle slightly for 2.3 and release another beta. The primary reason for this is our Windows support -- I would like to invite all Windows users to install darcs-beta and give it a ride. If you have a working cabal-install, all you need to do is run: $ cabal update $ cabal install darcs-beta (this works on all platforms; you may need to use -f-curl, if you don't have the cURL headers available -- this is often the case on Windows). If you do not have cabal-install, please follow the instructions near the end of this message, "Installing on Windows". In addition to using cabal-install, you can also download a tarball from http://repos.mornfall.net/darcs/darcs-2.2.98.4.tar.gz and build manually (see the build instructions in README inside the tarball). Feedback -------- To make the darcs 2.3 release a good one, we still need testing feedback: please drop a note to darcs-users@ if you have installed darcs-beta, or failed to install it. If you run into any bugs, we need to know about them. Thanks! The question of GHC 6.8 ----------------------- Using GHC 6.10.3 or newer is *strongly recommended*. You may compile darcs with GHC 6.8, but there are several caveats. If you are using 6.8.2 or older, please disable mmap support (pass -f-mmap to cabal install or runghc Setup configure below). Note that the GHC 6.8.2 that ships with Debian Lenny is not affected and it should be safe to keep mmap enabled. It is also recommended to disable use of Hackage zlib when compiling with GHC 6.8.2 (including the Debian Lenny version): pass -f-zlib to cabal. When using zlib, we have seen occasional crashes with error messages like "openBinaryFile: file locked" -- this is a known GHC 6.8.2 bug (and is fixed in GHC 6.8.3). Last, if you are using a 64-bit system, darcs may hang when you exit a pager when compiled with GHC older than 6.10.3. Although this is harmless, it is quite inconvenient. Overall, the status of GHC 6.8 is "semi-supported": for many cases, things will work just fine, especially if you take a little extra caution with compilation flags. Installing on Windows --------------------- To install darcs on Windows systems from scratch, please download the Haskell Platform and MSYS: * http://hackage.haskell.org/platform/2009.2.0.1/HaskellPlatform-2009.2.0.1-s… * http://sourceforge.net/projects/mingw/files/MSYS+Base+System/MSYS-1.0.11-rc… After installing both, you should have an "MSYS" icon: run MSYS and in the terminal window type: $ cabal update $ cabal install darcs-beta -f-curl This should download, compile and install all required dependencies and darcs-beta itself. The resulting darcs executable will be placed into the Haskell Platform executables folder, and should be accessible from the MSYS shell (just type "darcs --version" to check). Yours, Petr.

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Build 32-bit apps on 64-bit Windows?
by Lyle Kopnicky 15 Jul '09

15 Jul '09

If I am running GHC on 64-bit Windows, do I have a choice of building a 32-bit or 64-bit app? On a cursory glance through the command-line options, I didn't find anything. - Lyle

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Circular pure data structures?
by John Ky 15 Jul '09

15 Jul '09

Hello, Is it possible to create a circular pure data structure in Haskell? For example: a :: Data let b = getNext a let c = getNext b c == a -- Gives True Thanks, -John

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Re: iteratee enumHandle (and parsing)
by John Lato 15 Jul '09

15 Jul '09

Hello, This explanation is entirely correct. I'm not happy with this situation, because I think it's confusing for people just starting with iteratee. However, I do want something that will be unicode (and other possible encodings) aware. Currently, I'm waiting to see how unicode support in the base libraries works out. What seems the best possibility at this time is writing codecs that could be used with mapStream or convStream to handle this appropriately, but I'd prefer to re-use core unicode support if possible. I'm open to suggestions on this issue. At the very least, I should probably include the ASCII encoding in the iteratee package. I'd like to make the common case of reading ASCII text easier than it currently is. Until then, one approach is using mapStream as suggested below. Another is using a wrapped ByteString instead of the List chunk. The ByteString chunk forces the element to be an 8-bit ASCII character instead of using the Char storable instance. If the full polymorphism of the List chunk is required, the conversion can be done with either mapStream or convStream. convStream is probably more efficient for this at the moment. Regarding using Parsec with iteratees: unfortunately I don't really do much in the way of parsing myself so I'm not much help here. I think that Oleg intended for parsers to be written directly using the iteratee primitives, in particular the "heads" function would be useful (at least he found it so). I don't see an efficient way to generate a continuation on a partial parse with Parsec, using iteratees or not. I think you could just load data into a parser with setInput, but if you don't have enough data for a complete parse you'll just need to hold onto all the data until you've loaded enough to run it. If you tell me what functions you find lacking from iteratee, I'll look into implementing them. I'd rather not re-implement all of Parsec, but the text parsers should be simple enough, as well as some of the combinators. Cheers, John Lato > From: Echo Nolan <hellish(a)comcast.net> > > Hi Paolino. > > What's happening is reading [Char] uses the Storable instance for Char > which is 32-bit. Thus, you get gibberish. The below does what you want, > by reading Word8s and converting them. > > import Control.Exception > import Data.Char > import Data.Iteratee.IO > import Data.Iteratee.Base > import Data.Word > import System.IO > > main :: IO () > main = do > h <- openFile "mamma23" ReadWriteMode > hPutStr h "ciao" > hSeek h AbsoluteSeek 0 > l <- enumHandle h readString >>= run > print $ assert (l == "ciao") () > > -- This is declared on its own so I can give a type signature without making > -- any of the above lines unmanageably long. > readString :: IterateeG [] Word8 IO String > readString = joinI $ mapStream (chr . fromIntegral) stream2list > > This only works for ASCII, of course. Someone should write some > enumerators for the other encodings. > > Regards, > Echo Nolan

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How to "Read" this?
by Magicloud Magiclouds 15 Jul '09

15 Jul '09

Hi, I have a data structure, which shows like this: AttrBgColor {bgColor = Color 0 0 0} And the following is my Read code. But it failed parsing.... 31 instance Read Attribute where 32 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) <- lex str 33 , (color, rest) <- if (isPrefixOf " {bgColor = " rest1) 34 || (isPrefixOf " {fgColor = " rest1) 35 then case reads $ drop 12 rest1 of 36 [(color_, rest_)] -> [(color_, rest_)] 37 _ -> [(Color 0 0 0, rest1)] 38 else [(Color 0 0 0, rest1)] ] 39 where mkAttr "AttrFgColor" color = AttrFgColor color 40 mkAttr "AttrBgColor" color = AttrBgColor color 41 mkAttr "AttrInverse" _ = AttrInverse 42 mkAttr "AttrWeak" _ = AttrWeak 43 mkAttr "AttrUnderline" _ = AttrUnderline -- 竹密岂妨流水过山高哪阻野云飞

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Re: [Haskell] ANNOUNCE: StateVar 1.0.0.0
by Felipe Lessa 14 Jul '09

14 Jul '09

On Tue, Jul 14, 2009 at 07:16:41PM +0200, Sven Panne wrote: > To further modularize the OpenGL/OpenAL packages, a new > StateVar package has been released to Hackage. It contains > state variables, which are references in the IO monad, like > IORefs or parts of the OpenGL state. That's just great, thanks a lot! I was just going to ask if you had any plans of releasing it in the near future. Now, do you have any plans of releasing OpenGL using OpenGLRaw in the near future :)? I'm asking only because using StateVar right now would actually give more headaches than solve them as we would have two different versions of get and ($=) in scope. Thanks for your work, -- Felipe.

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Goldman Sachs - your home for OCaml and Erlang?
by Max Cantor 14 Jul '09

14 Jul '09

I know that this is a bit off topic, but thought it would interest several readers. Apparently, GS, which. AFAIK, doesn't make a lot of noise in the FP space compared to some other banks, does use some FP: http://www.zerohedge.com/article/aleynikov-code-dump-uncovered

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Re: [Haskell-cafe] Haskell as a first language?
by Gareth Smith 14 Jul '09

14 Jul '09

> On Tue, 2009-07-14 at 03:01 -0700, Michael Vanier wrote: > > Haskell is a wonderful language (my favorite language by far) but it is > > pretty difficult for a beginner. In fact, it is pretty difficult for > > anyone to learn in my experience, because it has so many advanced > > concepts that simply don't exist in other languages, and trying to > > absorb them all at once will likely be overwhelming. Then Duncan said: > As a contrary data-point, at Oxford we teach functional programming > (using Haskell) as the first course at the very beginning of the > computer science degree. I know several other universities also use FP > and Haskell very early on in their CS courses. At Imperial College, Haskell's also the first language the undergrads touch and people seem to like it. I think it's easy to forget how reassuring it is to be surrounded by lots of other beginners when one is learning something completely new - like programming. A lot of people here seem to be suggesting that haskell is an advanced language for advanced programmers. I think it more likely that people whose first experience of haskell is online are likely to come into contact with mostly advanced programmers in here and on #haskell. This is great, because they can teach us things - but it can also be off-putting, because they can seem so far beyond us. Imagine trying to learn basic arithmetic in a university common room full of category theorists. Personally, I think that haskell is a great beginners language. Just don't feel that you have to be able to keep up with all the #haskellers. It's absolutely fine to not understand what a monad transformer is. G

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Example for formatted show in HStringTemplate
by Kemps-Benedix Torsten 14 Jul '09

14 Jul '09

Hello, is there a working example of how to use the format clause with HStringTemplate, e.g. for Data.Time.Day? I think, if there is a parameter $day$, a reasonable template might contain e.g.: $day;format="%d.%b.%Y"$ But I only get "toModifiedJulianDay: [54960]" as the result which corresponds to the unformatted show. Regards, Torsten

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