July 2009 - Haskell-Cafe

Re: iteratee enumHandle (and parsing)
by John Lato 14 Jul '09

14 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 14 Jul '09

14 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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What to say about Haskell?
by Patai Gergely 14 Jul '09

14 Jul '09

Hello all, I was asked to give a one-hour 'introductory' seminar on Haskell. The audience is a bunch of very clever people with a wider than usual perspective on programming and mathematics, and my talk should be rather informational than evangelistic. Which topics do you think I should touch by all means given the short time? Gergely -- http://www.fastmail.fm - Same, same, but different...

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Re: [Haskell-cafe] Program with profiling runs faster than without
by Grigory Sarnitskiy 14 Jul '09

14 Jul '09

14.07.09, 15:16, "Malcolm Wallace" <malcolm.wallace(a)cs.york.ac.uk>: > This is only a guess, but maybe there is a context-qualified CAF-like > value that is being re-evaluated multiple times in the non-profiling > case, but is appropriately monomorphised in the profiling case, so it > is only evaluated once? > Regards, > Malcolm And I guess you are quite right. I knew there is a poorly optimized place in my program, but I didn't touch it for some ideological reasons. But this morning I get how to optimize it in an ideological friendly manner, so now without profiling the program ~ 2 times faster. The speed of profiled version hasn't changed, hence I've manually done some work the compiler does during profiling.

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Program with profiling runs faster than without
by Grigory Sarnitskiy 14 Jul '09

14 Jul '09

Hello! I can't understand why the following happens. ghc --make -fforce-recomp -O2 -fexcess-precision -fvia-C -optc-O2 Run.lhs -o Run -prof -auto-all and time ./Run TestSim seems to be much faster (I got ~4 times faster indeed) than just ghc --make -fforce-recomp -O2 -fexcess-precision -fvia-C -optc-O2 Run.lhs -o Run and time ./Run TestSim with ghc 6.8.2 on pentium 4 2.4 GHz under Ubuntu I was on my way to optimize the program when I've discovered this. The program is rather complicated and badly optimized but still I think it is quite strange to the profiled version to run faster. All necessary files I send with this letter (Tann.7z).

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Re: [Haskell-cafe] Are GADTs what I need?
by Chris Eidhof 14 Jul '09

14 Jul '09

Then you could add a specific constructor for String. The main point is: the case construct only works for values, not for types. There is no typecase construct. If you want to have certain restrictions on the 'a', such as the Show class, you could also do something like this: > data Value where > VWrapper :: (Show a) => a -> Value If you could elaborate a bit on what you're trying to accomplish (from a higher viewpoint) then maybe we can help you some more. -chris On 13 jul 2009, at 17:42, Kev Mahoney wrote: > Thanks, that helps. > > I was hoping to not have to parametrize Value as there is a fair bit > of code to change, and it cascades down through the data structures > (maybe a forall a . Value a will help here?) > > I will have a go at this approach. In case anyone is interested the > code is at http://github.com/KMahoney > > > 2009/7/13 Chris Eidhof <chris(a)eidhof.nl>: >> Hey Kev, >> >> The types are "thrown away" during compile time. Therefore, if you >> have a >> constructor "VWrapper :: a -> Value" nothing is known about that >> "a" when >> you scrutinize it. >> >> What you could do, however, is something like this: >> >>> data Value a where >>> VInt :: Integer -> Value Integer >>> ... >>> VWrapper :: a -> Value a >> >> And then you can write a function doSomething: >> >>> doSomething :: Value String -> String >>> doSomething (VWrapper s) = s >> >> HTH, >> >> -chris >> >> On 13 jul 2009, at 12:41, Kev Mahoney wrote: >> >>> Hi there, >>> >>> I'm currently writing an interpreter that I would like to be able to >>> use with other haskell programs. I would like to be able to pass >>> along >>> arbitrary types though the interpreter. I've seen hints that GADTs >>> can >>> do this, but I am having trouble understanding them. >>> >>> So far, I've learnt you can do this: >>> >>> data Value where >>> VInt :: Integer -> Value >>> ... >>> VWrapper :: a -> Value >>> >>> which can let you encode arbitrary 'dynamic' types into Value. I was >>> hoping to be able to pattern match to get the value out again e.g. >>> >>> doSomething :: Value -> .... >>> doSomething (VWrapper String s) = ..... >>> >>> Also, anything that can help me out with GADTs in general will be >>> much >>> appreciated. >>> >>> Thanks, >>> Kevin. >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >>

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