- Haskell-Cafe - Haskell.org

Language.C++ ?
by Jacques Carette 27 Nov '10

27 Nov '10

Is anyone working on a Language.C++ module, similar to the wonderful Language.C ? Right now just the syntax and a pretty-printer would be really nice to have... Jacques

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Re: Template Haskell: exchanging information between 'splicers'
by oleg＠okmij.org 27 Nov '10

27 Nov '10

Jean-Christophe Mincke wrote: > Does anyone know a clean solution to pass information between 2 executions > of splicers. > > $(splicer ....) -- first invocation gather and store some data > .... > $(splicer ...) -- second one use the data gathered above. Just the other day I wrote such code; I'm not sure it qualifies as a `clean solution' as it was meant for a nefarious purpose. > e2 = runIO $ do > varref <- newIORef undefined > let put v = runIO $ do > putStrLn "put" > writeIORef varref v > runQ [|$v + 1|] > let get v = runIO $ do > putStrLn "get" > vold <- readIORef varref > runQ [|$v + $vold|] > > c1 <- runQ [|\x -> $(put [|x|]) + 1::Int|] > c2 <- runQ [|\x -> $(get [|x|]) + 1::Int|] > runQ $ [| ($(return c1),$(return c2)) |] > > show_code cde = runQ cde >>= putStrLn . pprint > te2 = show_code e2 One may even try to splice e2 at the top level (in a different module). The code does exchange data between two splicers -- one may argue, it exchanges too much data.

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version of findIndex that works with a monadic predicate
by José Romildo Malaquias 27 Nov '10

27 Nov '10

Hello. I need a function findIndexM, similar to findIndex from the standard module Data.List, but which works with a monadic predicate to test list elements. findIndex :: (a -> Bool) -> [a] -> Maybe Int findIndexM :: (Monad m, Num a) => (t -> m Bool) -> [t] -> m (Maybe a) findIndexM p xs = go 0 xs where go _ [] = return Nothing go n (x:xs) = do res <- p x if res then return (Just n) else go (n+1) xs How can this function be rewritten using combinators? Romildo

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Plot Haskell functions in TikZ
by Henning Thielemann 27 Nov '10

27 Nov '10

I'd like to draw function graphs in a LaTeX document. I'm using PGF/TikZ that allows to interact with gnuplot, but it does not use gnuplot as plotter but just for function evaluation. And gnuplot is not very good at this, it does not even know 'min' or 'max'. pgfmanual.pdf says, that in the TikZ you can write in a tikzpicture environment: --plot[local options] function {gnuplot formula} This command does two things: 1. It writes a gnuplot script containing the formula to a file like plot-name.gnuplot . 2. It checks for a data table in plot-name.table and displays the contained curve. Outside of LaTeX, say in a Makefile, you call gnuplot in order to run plot-name.gnuplot and receive plot-name.table: %.table: %.gnuplot gnuplot $< Now you can easily guess, what I want to do. I like to write a Haskell formula in '--plot' and get a Haskell program, that renders the table. The LaTeX macro invocation should look like: --plot[local options] haskellfunction {haskell formula} The Make rule should look like: %.table: %.hs runhaskell $< Has anyone tried to write such a '--plot' command for TikZ?

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Template Haskell: exchanging information between 'splicers'
by jean-christophe mincke 27 Nov '10

27 Nov '10

Hello, Does anyone know a clean solution to pass information between 2 executions of splicers. Ex. $(splicer ....) -- first invocation gather and store some data .... $(splicer ...) -- second one use the data gathered above. Thank you Regards J-C

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How to instance class with functional dependency?
by Magicloud Magiclouds 26 Nov '10

26 Nov '10

Hi, In System.Posix.IOCtl, there is such a class: class Storable d => IOControl req d | req -> d where ioctlReqSource :: req -> CInt How to instance it? I do it as: data TIOCGWINSZ = TIOCGWINSZ #starttype struct winsize #field ws_row , CUShort #field ws_col , CUShort #field ws_xpixel , CUShort #field ws_ypixel , CUShort #stoptype instance IOControl TIOCGWINSZ C'winsize where ioctlReq _ = #const TIOCGWINSZ Then I got: topIO.hs:19:0: Couldn't match expected type `C'winsize' against inferred type `Int' When using functional dependencies to combine IOControl (Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> C'winsize) Int, arising from a use of `ioctl'' at topIO.hs:21:35-60 IOControl (Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> Foreign.C.Types.CUShort -> C'winsize) C'winsize, arising from a use of `ioctl'' at topIO.hs:20:12-37 When generalising the type(s) for `main' -- 竹密岂妨流水过山高哪阻野云飞

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Eq instance for Chan
by Mitar 25 Nov '10

25 Nov '10

Hi! Why is there no Eq instance for Chan? There is Eq for MVar so it is quite possible to define also Eq for Chan? What I would like to do is keep track how many consumers I have for each Chan so duplicating them with dupChan as necessary. So I was thinking of storing a list of Chans which already have a consumer and then duplicating it if another (and every additional) consumer would be added. But without Eq it is not possible to check if there is Chan already in a list (and I hope Eq would compare just Chan and not its content). Is there some other way to achieve this? Mitar

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Precedence of "if then else"
by Sean Leather 25 Nov '10

25 Nov '10

This may be a silly, but I occasionally run into a situation where I'm wondering what the effective precedence of "if then else" is. I was finally motivated to do a few experiments, and it seems like the precedence level is reset (below 0) after "if", "then", and "else". λ> if not $ False then "here" else "x" "here" λ> if False then (++ "y") $ "x" else "here" "here" λ> if True then "here" else (++ "y") $ "x" "here" I suppose I knew this was the case with "if", but I had not given it much thought for "then" or "else". Of course, it makes sense. It's the same for "case" and many other syntactic constructs. I was wondering where this was defined in the Language Report. I had looked for prose, but found nothing. Now, I look at it again (with more motivation) and see it (clearly) in the following excerpt of the grammar (Ch. 3 Expressions): exp → infixexp :: [context =>] type (expression type signature) | infixexp infixexp → lexp qop infixexp (infix operator application) | - infixexp (prefix negation) | lexp lexp → \ apat1 … apatn -> exp (lambda abstraction, n ≥ 1) | let decls in exp (let expression) | if exp [;] then exp [;] else exp (conditional) | case exp of { alts } (case expression) ... Plainly and simply, this says that all infix expressions (infixexp) are expressions (exp), and these can be found in "if then else" and "case" among others. So, while I was looking for something regarding precedence, it is actually defined straightforwardly in the grammar. In case others happen to search for the same thing I did, maybe this will provide them with an answer of sorts. Regards, Sean

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Monadic function purity
by Gregory Propf 25 Nov '10

25 Nov '10

I have a pretty basic question. I've been wondering about whether monadic functions that do NOT us IO can be pure or not. There seems to be some confusion on this topic on the web. I'm especially interested in whether they can be memoized. It seems to me that something like a function in the State monad should be pure provided the same initial state and same function arguments are present. Likewise with the list monad and most other monads in fact.

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Equality constraint "synonyms"
by Hugo Pacheco 25 Nov '10

25 Nov '10

Dear Haskellers, When programming with type functions, I often find myself with a set of invariants that are frequent in my programs and can be expressed as equality constraints on the context of the functions. I wonder if there is any way in current GHC to express some kind of synonyms for equality constraints, to avoid a fixed set of constraints for a smaller expression. The natural way for type classes would be to create a superclass containing all the desired instances class (A a, B a) => C a where {} but as from http://hackage.haskell.org/trac/ghc/ticket/2715, equality constraints are currently not supported in class contexts, what might be reasonable. However, in fact I am just looking for some kind syntactic sugar. Would this be a desired feature for other people? Regards, hugo -- www.di.uminho.pt/~hpacheco

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ANN: tskiplist-0.0.0
by Peter Robinson 25 Nov '10

25 Nov '10

This package [1] provides an implementation of a skip list using STM. A skip list is a probabilistic data structure with Data.Map-like operations. In contrast to a balanced tree, a skip list does not need any (expensive) rebalancing, which makes it particularly suitable for concurrent programming. You can find some documentation on how to use it here: http://darcs.monoid.at/tskiplist/dist/doc/html/tskiplist/Control-Concurrent… Feedback appreciated! Peter [1] http://hackage.haskell.org/package/tskiplist-0.0.0

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Женская логика
by Vovan 25 Nov '10

25 Nov '10

Хотите понимать женщин? Знать реальные их мысли, когда они пытаются спрятать их за совершенно отвлеченными речами? На практических тренингах Академии Знакомств вы в реальных условиях научитесь быть предупредительным и понимающим мужчиной для женщин. http://samec.org.ua/?p=202 Не верите? Попробуйте, вам понравится. Желаю вам больших научных успехов.

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Пикап
by Vovan 25 Nov '10

25 Nov '10

Пикап блог один из лучший информативных сайтов о в Украине, который помогает мужчинам в разы улучшить их личную и сексуальную жизнь дает ответы на вопрос как познакомиться с девушкой? , и решить множество проблем, найти ответы которые помогут Вам стать настоящими мужчинами!!!

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Re: [Haskell-cafe] Confused about missing data constructor
by Antoine Latter 24 Nov '10

24 Nov '10

I meant that 'pop' and 'push' should have been written with 'modify', 'get', and 'set' instead of the raw constructor, not as a drop-in replacement. I can show you examples later if this isn't clear, unless I'm not understanding your code above. Antoine On Nov 24, 2010 10:46 AM, "Adam Miezianko" <adam(a)theorylounge.org> wrote: > On Wed, 24 Nov 2010 06:17:47 -0800, Antoine Latter <aslatter(a)gmail.com> > wrote: > >> On Tue, Nov 23, 2010 at 6:58 PM, Adam Miezianko <adam(a)theorylounge.org> >> wrote: >>> I'm working through Learn You a Haskell for Great Good [1] and getting >>> a compiler error while playing around with some of the code. I have >>> this: >>> >>> pop :: State Stack Int >>> >>> But when I try to load it into ghci I get the following errors: >>> >>> /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' >>> >>> Now, I'm not exactly sure how to read the documentation for >>> Control.Monad.State [2] but it seems that newtype State s a = State >>> {...} defines a constructor, or am I wrong on that point too? So, >>> what am I missing here? In case it matters, I am using mtl-2.0.1.0 >>> and ghci 6.12.3. >>> >>> [1] http://learnyouahaskell.com/for-a-few-monads-more >>> [2] >>> http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html >>> >> >> It looks like your documentation doesn't match the library you're using. >> >> The documentation for mtl-2.x is here: >> http://hackage.haskell.org/package/mtl > > Thanks! > > My documentation didn't have any version identification, so discovering > that there was a breaking interface change is no surprise now. Next time > hoogle doesn't return me what I want I'll first suspect it's been removed > before questioning the accuracy of the search results. > > On a somewhat related note, I also found the link you provide, but I found > hackage.haskell.org to be intermittently timing out for me this last week. > >> However you might be better served using the 'modify' function. > > Looking at the type signature of modify, I'm unsure how to use it. It > does not look like I can just drop it in as a replacement for State > (StateT, state). Could someone provide an example? The tutorials seem to > mostly use the State constructor that's now gone. > > -- > Adam Miezianko

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Another Quasi-Quotation question
by jean-christophe mincke 24 Nov '10

24 Nov '10

Hello, I am still playing with template-haskell... I wonder, is there any reason why a quasiquoter cannot create haskell statements and declarations in addition to expressions and patterns? Or more generally create any legal Haskell syntax tree. I.e Suppose I would like to create a quasiquoter for the C language (please, imagine that it could be useful). I could write sth such as : [$c| int a = 6; int b = 7; int c = a +b; struct S { int x; int y} |] It could be nice to generate the appropriate haskell code: a = 6 b= 7 c= a + b data S = S { x::Int; y:Int} etc That would allow to really embed any kind of language in a more or less easy way into haskell, provided that code can be translated into legal haskell. Is there anything that prevent these features. Thank you Regards J-C

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Confused about missing data constructor
by Antoine Latter 24 Nov '10

24 Nov '10

On Tue, Nov 23, 2010 at 6:58 PM, Adam Miezianko <adam(a)theorylounge.org> wrote: > I'm working through Learn You a Haskell for Great Good [1] and getting > a compiler error while playing around with some of the code. I have > this: > > -- BEGIN state.hs > import Control.Monad.State > > type Stack = [Int] > > pop :: State Stack Int > pop = State $ \(x:xs) -> (x,xs) > > push :: Int -> State Stack () > push x = State $ \xs -> ((), x:xs) > > stackManip :: State Stack Int > stackManip = do > push 3 > pop > pop > -- END state.hs > > But when I try to load it into ghci I get the following errors: > > Prelude> :load "/home/admi/.pe/state.hs" > [1 of 1] Compiling Main ( /home/admi/.pe/state.hs, interpreted ) > > /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' > > /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State' > Failed, modules loaded: none. > Prelude> > > Now, I'm not exactly sure how to read the documentation for > Control.Monad.State [2] but it seems that newtype State s a = State > {...} defines a constructor, or am I wrong on that point too? So, > what am I missing here? In case it matters, I am using mtl-2.0.1.0 > and ghci 6.12.3. > > [1] http://learnyouahaskell.com/for-a-few-monads-more > [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html > It looks like your documentation doesn't match the library you're using. The documentation for mtl-2.x is here: http://hackage.haskell.org/package/mtl Starting in version 2.0, the mtl package no longer exports a constructor called 'State' - but there is a function 'state' which servers the same purpose. However you might be better served using the 'modify' function. I hope that helps, Antoine > -- > Adam Miezianko > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Confused about missing data constructor
by Adam Miezianko 24 Nov '10

24 Nov '10

I'm working through Learn You a Haskell for Great Good [1] and getting a compiler error while playing around with some of the code. I have this: -- BEGIN state.hs import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push x = State $ \xs -> ((), x:xs) stackManip :: State Stack Int stackManip = do push 3 pop pop -- END state.hs But when I try to load it into ghci I get the following errors: Prelude> :load "/home/admi/.pe/state.hs" [1 of 1] Compiling Main ( /home/admi/.pe/state.hs, interpreted ) /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State' Failed, modules loaded: none. Prelude> Now, I'm not exactly sure how to read the documentation for Control.Monad.State [2] but it seems that newtype State s a = State {...} defines a constructor, or am I wrong on that point too? So, what am I missing here? In case it matters, I am using mtl-2.0.1.0 and ghci 6.12.3. [1] http://learnyouahaskell.com/for-a-few-monads-more [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html -- Adam Miezianko

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ANNOUNCE: Graphalyze-0.11.0.0 and SourceGraph-0.7.0.0
by Ivan Lazar Miljenovic 24 Nov '10

24 Nov '10

I am pleased to announce the 0.7.0.0 release of my graph-theoretic source code analysis tool SourceGraph [1], and the library it uses Graphalyze 0.11.0.0 [2]. [1]: http://hackage.haskell.org/package/SourceGraph [2]: http://hackage.haskell.org/package/Graphalyze Changes in SourceGraph (apart from those resulting from improvements to Graphalyze): * Edge widths are now log-based rather than being the number of function calls; this avoids having absurdly thick edges. * Source files that were not able to be parsed are now listed both to stderror and in the resulting report (thus helping you work out why you're missing some code) * Also builds with Cabal-1.10; whilst I do not currently have GHC 7 installed anywhere and thus can't test this, I know of no reason why SourceGraph cannot be built with GHC 7 (unless it uses a library that isn't buildable on GHC 7 that I have no control over). Changes in Graphalyze: * Legends can now contain text-only fields rather than just graphs; also, the title of each legend is now displayed first rather than what it describes. * To fix a bug spotted by Han Joosten, the reports currently use Posix-style path separators even on Windows (otherwise, HTML documents produced on Windows were not in a state to be uploaded elsewhere). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: graphviz-2999.11.0.0
by Ivan Lazar Miljenovic 24 Nov '10

24 Nov '10

I am pleased to announce version 2999.11.0.0 of graphviz [1], my Haskell bindings to the Graphviz suite of graph visualisation tools [2]. [1]: http://hackage.haskell.org/package/graphviz [2]: http://graphviz.org/ This release is mostly backwards-compatible (unless you dealt directly with Point and LayerList values); the changes are: * Improved documentation, including examples of how to use `GraphvizParams` values (since several people indicated that they found them confusing) * Addition of the `Labellable` class (and its method `toLabel`) to make it easier to construct labels. * Backslashes (i.e. the `\` character) are now escaped/unescaped properly (bug spotted by Han Joosten). As part of this: - Dot-specific escapes such as `\N` are now also handled correctly, so the slash does not need to be escaped. - Newline (`'\n'`) characters in labels, etc. are escaped to centred-newlines in Dot code, but not unescaped. * `Point` values can now have the optional third dimension and end in a `!` to indicate that that position should be used (as input to Graphviz). * `LayerList` uses `LayerID` values, and now has a proper `shrink` implementation in the test suite. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANN: dnscache - caching DNS resolver
by Ertugrul Soeylemez 24 Nov '10

24 Nov '10

Hello everybody, (second try, since the first mail somehow didn't arrive) I've just uploaded a caching DNS resolver library, which also includes a command line utility for quick mass DNS resolution: http://hackage.haskell.org/package/dnscache Although I've tested it throroughly it's still in beta phase. Feel free to play around with it. Feedback is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Re: [Haskell-cafe] Derived type definition
by Miguel Mitrofanov 24 Nov '10

24 Nov '10

Well, you can resort to functional dependencies, I guess... {-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, UndecidableInstances #-} module FunDeps where data Rec a r = Rec a r data RecNil = RecNil data Wrapper a = Wrapper a class Wrapped r w | r -> w where i :: r -> w instance Wrapped RecNil RecNil where i RecNil = RecNil instance Wrapped r w => Wrapped (Rec a r) (Rec (Wrapper a) w) where i (Rec a r) = Rec (Wrapper a) (i r) type TTest = Rec Int (Rec String RecNil) type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) a :: TTest a = Rec 1 (Rec "a" RecNil) f :: TTestWrapped -> (Int, String) f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) r = f (i a) ...but that would be just an awkward way to do the same thing, so, my advice: don't. Type families are much nicer. On the other hand, you could do your "Rec" type polimorphic in "wrapper"; assuming your real-life Wrapper is not just an identity, this would be worth considering: {-# LANGUAGE KindSignatures #-} module PolyM where data Rec a r w = Rec (w a) (r w) data RecNil (w :: * -> *) = RecNil data Wrapper a = Wrapper a -- in reality it should be something else newtype Identity a = Identity a class Wrapped r where i :: r Identity -> r Wrapper instance Wrapped RecNil where i RecNil = RecNil instance Wrapped r => Wrapped (Rec a r) where i (Rec (Identity a) r) = Rec (Wrapper a) (i r) type TTest = Rec Int (Rec String RecNil) Identity type TTestWrapped = Rec Int (Rec String RecNil) Wrapper a :: TTest a = Rec (Identity 1) (Rec (Identity "a") RecNil) f :: TTestWrapped -> (Int, String) f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) r = f (i a) 24.11.2010 12:24, kg пишет: > Ok, it's exactly what i hoped. > > And I would like to know (for fun) if it's possible to do it without type family extension. > I've tried ... without success. > > Thx. > > On 11/22/2010 10:46 PM, Miguel Mitrofanov wrote: >> Sure, it's possible with TypeFamilies. The following compiles OK: >> >> {-# LANGUAGE TypeFamilies #-} >> module TypeCalc where >> data Rec a r = Rec a r >> data RecNil = RecNil >> data Wrapper a = Wrapper a >> class TypeList t where >> type Wrapped t >> i :: t -> Wrapped t >> instance TypeList RecNil where >> type Wrapped RecNil = RecNil >> i RecNil = RecNil >> instance TypeList r => TypeList (Rec a r) where >> type Wrapped (Rec a r) = Rec (Wrapper a) (Wrapped r) >> i (Rec a r) = Rec (Wrapper a) (i r) >> type TTest = Rec Int (Rec String RecNil) >> type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) >> a :: TTest >> a = Rec 1 (Rec "a" RecNil) >> f :: TTestWrapped -> (Int, String) >> f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) >> r = f (i a) -- so, "i a" is of the type TTestWrapped. >> >> >> On 22 Nov 2010, at 23:43, kg wrote: >> >>> Hi, >>> >>> I've tried to simplify as much as possible my problem. Finally, I think I can resume it like that: >>> >>> Suppose these following data types : >>> data Rec a r = Rec a r >>> data RecNil = RecNil >>> data Wrapper a = Wrapper a >>> >>> Then, we can build the following type: >>> type TTest = Rec Int (Rec String RecNil) >>> or this type: >>> type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) >>> >>> Is it possible to build TTestWrapped from TTest ? >>> >>> >>> Thx in advance, >>> Antoine. >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-beginners] Problem "grouping" a list
by Aai 24 Nov '10

24 Nov '10

I don't know what this tuple is representing, but if you want to group you'll have to specify on 'what': - the tuple, - the fst or - the snd Here's a possibility with grouping on the fst import Data.List import Data.Ord import Data.Function groupAtoms :: (Float -> Bool) -> [(Float,Integer)] -> ([[(Float,Integer)]],[[(Float,Integer)]]) groupAtoms p = partition (p.sum. map fst). groupBy ((==)`on`fst). sortBy (comparing fst) Use: groupAtoms (>=1.0) myList If this is what you want: proper lists are in the fst of the result. Hallo João Paulo Pizani Flor, je schreef op 23-11-10 18:23: > Hello dear Haskellers! > > I've been a user and big fan of Haskell for a while, but only now I'm writing > my first "big" project in Haskell (some thousands of lines of code perhaps). > It's an interpreter for a programming language, the source code is music! Yes, > sheet music! :D > > OK, so my specific problem goes like this: I have a list of tuples > :t myList > [ (Float, Integer) ] > > And I want to "group" this list into a nested list > groupAtoms :: [ (Float,Integer) ] -> [ [(Float,Integer)] ] > > Of course, I want that the concatenation of the groups yield me the original > list, i.e, (foldl (++) [] . groupAtoms == id), and the criterion that defines > a group is that: > "The sum of the first elements in the tuples comprising the list must be > greater than or equal to 1.0". That is, given a list of tuples, the boolean > predicate deciding whether this list is a PROPER group (True) or TOO SMALL > (False) is: > \g -> sum (map fst g) >= 1.0 > > > Although the criterion is very clear, I've tried hard until now and couldn't > come up with a function for producing the nested list based on this grouping > criterion. I am sure that the Haskell Hierarchical Libraries have something > to help me, but only now I see I'm still a big noob :P > > Could someone please help me writing this function? > > > My best regards from Brazil, > > João Paulo Pizani Flor > joaopizani(a)gmail.com <mailto:joaopizani@gmail.com> > Federal University of Santa Catarina > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners -- Met vriendelijke groet, =@@i

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ANN: dnscache - caching DNS resolver
by Ertugrul Soeylemez 24 Nov '10

24 Nov '10

Hello everybody, I've just uploaded a caching DNS resolver library, which also includes a command line utility for quick mass DNS resolution: http://hackage.haskell.org/package/dnscache Although I've tested it throroughly it's still in beta phase. Feel free to play around with it. Feedback is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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save the dates: SF Bay Area Haskell Hackathon: Feb 11~13, 2011
by Mark Lentczner 24 Nov '10

24 Nov '10

Bryan O'Sullivan and I are hosting a SF Bay Area Haskell Hackathon at the Hacker Dojo in Mountain View, California. Details are all sketchy at this point, but we plan on two components: 1) Haskell Project Hackathon 2) Learn Haskell Workshop See: http://wiki.hackerdojo.com/w/page/Haskell-Hackathon-2011 At this stage we're trying to gauge interest, size, and volunteers to help with organizing and running it. Either add yourself to that wiki page, or e-mail me. - Mark Lentczner Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

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Another Quasi Quotation question
by jean-christophe mincke 24 Nov '10

24 Nov '10

Hello, I am still playing with template-haskell... I wonder, is there any reason why a quasiquoter cannot create haskell statements and declarations in addition to expressions and patterns? Or more generally create any legal Haskell syntax tree. I.e Suppose I would like to create a quasiquoter for the C language (please, imagine that it could be useful). I could write sth such as : [$c| int a = 6; int b = 7; int c = a +b; struct S { int x; int y} |] It could be nice to generate the appropriate haskell code: a = 6 b= 7 c= a + b data S = S { x::Int; y:Int} etc That would allow to really embed any kind of language in a more or less easy way into haskell, provided that code can be translated into legal haskell. Is there anything that prevent these features. Thank you Regards J-C

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Musings on type systems
by Andrew Coppin 23 Nov '10

23 Nov '10

OK, so how do types actually work? Musing on this for a moment, it seems that declaring a variable to have a certain type constrains the possible values that the variable can have. You could say that a type is some sort of "set", and that by issuing a type declaration, the compiler statically guarantees that the variable's value will always be an element of this set. Now here's an interesting thought. Haskell has "algebraic data types". "Algebraic" because they are sum types of product types (or, equivilently, product types of sum types). Now I don't actually know what those terms mean, but proceeding by logical inferrence, it seems that Either X Y defines a set that could be described as the union or "sum" of the types X and Y. So is Either what is meant by a "sum type"? Similarly, (X, Y) is a set that could be considered the Cartesian product of the sets X and Y. It even has "product" in the name. So is this a "product type"? So not only do we have "types" which denote "sets of possible values", but we have operators for constructing new sets from existing ones. (Mostly just applying type constructors to arguments.) Notionally (->) is just another type constructor, so functions aren't fundamentally different to any other types - at least, as far as the type system goes. Now, what about type variables? What do they do? Well now, that seems to be slightly interesting, since a type variable holds an entire type (whereas normal program variables just hold a single value), and each occurrance of the same variable is statically guaranteed to hold the same thing at all times. It's sort of like how every instance of a normal program variable holds the same value, except that you don't explicitly say what that value is; the compiler infers it. So what about classes? Well, restricting ourselves to single-parameter classes, it seems to me that a class is like a *set of types*. Now, interestingly, an enumeration type is a set of values where you explicitly list all possible values. But once defined, it is impossible to add new values to the set. You could say this is a "closed" set. A type, on the other hand, is an "open" set of types; you can add new types at any time. I honestly can't think of a useful intuition for MPTCs right now. Now what happens if you add associated types? For example, the canonical class Container c where type Element c :: * We already have type constructor functions such as Maybe with takes a type and constructs a new type. But here we seem to have a general function, Element, which takes some type and returns a new, arbitrary, type. And we define it by a series of equations: instance Container [x] where Element [x] = x instance Container ByteString where Element ByteString = Word8 instance (Ord x) => Container (Set x) where Element (Set x) = x instance Container IntSet where Element IntSet = Int ... Further, the inputs to this function are statically guaranteed to be types from the set (class) "Container". So it's /almost/ like a regular value-level function, just with weird definition syntax. Where *the hell* do GADTs fit in here? Well, they're usually used with phantom types, so I guess we need to figure out where phantom types fit in. To the type checker, Foo Int and Foo Bool are two totally seperate types. In the phantom type case, the set of possible values for both types are actually identical. So really we just have two names for the same set. The same thing goes for a type alias (the "type" keyword). It's not quite the same as a "newtype", since then the value expressions do actually change. So it seems that a GADT is an ADT where the elements of the set are assigned to sets of different names, or the elements are partitioned into disjoint sets with different names. Hmm, interesting. At the same type, values have types, and types have "kinds". As best as I can tell, kinds exist only to distinguish between /types/ and /type functions/. For type constructors, this is the whole story. But for associated types, it's more complicated. For example, Element :: * -> *, however the type argument is statically guaranteed to belong to the Container set (class). In other news... my head hurts! >_< So what would happen if some crazy person decided to make the kind system more like the type system? Or make the type system more like the value system? Do we end up with another layer to our cake? Is it possible to generalise it to an infinite number of layers? Or to a circular hierachy? Is any of this /useful/ in any way? Have aliens contacted Earth in living member? OK, I should probably stop typing now. [So you see what I did there?] Also, I have a sinking feeling that if anybody actually replies to this email, I'm not going to comprehend a single word of what they're talking about...

12 16

About "Fun with type functions" example
by Arnaud Bailly 23 Nov '10

23 Nov '10

Hello, I am trying to understand and use the Nat n type defined in the aforementioned article. Unfortunately, the given code does not compile properly: Here is the code: module Naturals where data Zero data Succ a class Nat n where toInt :: n -> Int instance Nat Zero where toInt _ = 0 instance (Nat n) => Nat (Succ n) where toInt _ = 1 + toInt (undefined :: n) type One = Succ Zero type Two = Succ One And here is the error: D:\projets\sequencer>ghc Naturals.hs Naturals.hs:16:18: Ambiguous type variable `n' in the constraint: `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 Probable fix: add a type signature that fixes these type variable(s) I use 6.12.3 on Windows XP. I am most probably missing an option but does not know which one. Thanks in advance for any advice, Arnaud

11 20

Cabal-install is broken
by Permjacov Evgeniy 23 Nov '10

23 Nov '10

current cabal-install (0.8.2) cannot be compiled with ghc-7.0.1 set of boot libraries. It requires cabal 1.8.* wich fails to compile. Does anyone worked this out ?

3 2

Why does cabal unnecessarily reinstall dependencies?
by Martijn van Steenbergen 23 Nov '10

23 Nov '10

Hello cafe, When I want to locally install my own package through "cabal install" it tries to reinstall dependencies convertible-1.0.9.1, HDBC-2.2.6.1 and HDBC-mysql-0.6.3 even though they are already installed (and work fine). Why does it do this? cabal-install version 0.8.2 using version 1.8.0.2 of the Cabal library Mac OS Leopard Thanks, Martijn.

5 4

VNC server in Haskell
by C K Kashyap 23 Nov '10

23 Nov '10

Hi, I've implemented a VNC server (very incomplete) - What it can do currently is display a blue screen (100x100 screen). It's very trivial to try out - https://github.com/ckkashyap/Chitra I look forward to some feedback/comments on this. -- Regards, Kashyap

1 0

Wrong Package-URL in regex-tdfa.cabal
by José Romildo Malaquias 23 Nov '10

23 Nov '10

Hello. The cabal file for the regex-tddfa-1.1.6 package seems to have an invalid URL for the Package-URL field: Package-URL: http://darcs.haskell.org/packages/regex-unstable/regex-tdfa/ It should be fixed. Regards. Romildo

1 0

Build problems with HDBC-mysql on the Mac
by Martijn van Steenbergen 23 Nov '10

23 Nov '10

Dear cafe, Following the instructions at http://matt.freels.name/2010/hdbc-mysql-os-x.html I successfully installed HDBC-mysql on my Mac! However when doing a "cabal install" in a local package which depends on HDBC-mysql, cabal proceeds to reinstall HDBC-mysql (is this a bug? see my other email from a few moments ago), this time unsuccessfully because it is using the unpatched cached version downloaded from hackage. This breaks my installation: ghc-pkg list colors the package red. Now comes the weird part. :-) I try to reinstall HDBC-mysql with my unpacked, patched version. It fixes my installation of HDBC-again, even though it exits prematurely with some errors! I've included the output at the end of this email. I wonder: Where do these errors suddenly come from? The first time the installation went without trouble. How come it fixes my installation even though it reports errors? Thank you in advance, Martijn. Output of "cabal install --extra-lib-dirs=/usr/local/lib:" in a local version of HDBC-mysql, patched according to http://matt.freels.name/2010/hdbc-mysql-os-x.html: Resolving dependencies... [1 of 1] Compiling Main ( Setup.lhs, dist/setup/Main.o ) Setup.lhs:8:0: Warning: In the use of `findProgramOnPath' (imported from Distribution.Simple.Program): Deprecated: "use findProgramLocation instead" Linking ./dist/setup/setup ... Configuring HDBC-mysql-0.6.3... Preprocessing library HDBC-mysql-0.6.3... Connection.hsc:23:19: error: mysql.h: No such file or directory Connection.hsc: In function ‘main’: Connection.hsc:166: error: ‘MYSQL_FIELD’ undeclared (first use in this function) Connection.hsc:166: error: (Each undeclared identifier is reported only once Connection.hsc:166: error: for each function it appears in.) Connection.hsc:170: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:170: error: expected ‘;’ before ‘)’ token Connection.hsc:170: error: expected statement before ‘)’ token Connection.hsc:171: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:171: error: expected ‘;’ before ‘)’ token Connection.hsc:171: error: expected statement before ‘)’ token Connection.hsc:172: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:172: error: expected ‘;’ before ‘)’ token Connection.hsc:172: error: expected statement before ‘)’ token Connection.hsc:173: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:173: error: expected ‘;’ before ‘)’ token Connection.hsc:173: error: expected statement before ‘)’ token Connection.hsc:174: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:174: error: expected ‘;’ before ‘)’ token Connection.hsc:174: error: expected statement before ‘)’ token Connection.hsc:175: error: expected specifier-qualifier-list before ‘MYSQL_FIELD’ Connection.hsc:175: error: expected ‘;’ before ‘)’ token Connection.hsc:175: error: expected statement before ‘)’ token Connection.hsc:203: error: ‘MYSQL_BIND’ undeclared (first use in this function) Connection.hsc:210: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:210: error: expected ‘;’ before ‘)’ token Connection.hsc:210: error: expected statement before ‘)’ token Connection.hsc:211: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:211: error: expected ‘;’ before ‘)’ token Connection.hsc:211: error: expected statement before ‘)’ token Connection.hsc:212: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:212: error: expected ‘;’ before ‘)’ token Connection.hsc:212: error: expected statement before ‘)’ token Connection.hsc:213: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:213: error: expected ‘;’ before ‘)’ token Connection.hsc:213: error: expected statement before ‘)’ token Connection.hsc:214: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:214: error: expected ‘;’ before ‘)’ token Connection.hsc:214: error: expected statement before ‘)’ token Connection.hsc:215: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:215: error: expected ‘;’ before ‘)’ token Connection.hsc:215: error: expected statement before ‘)’ token Connection.hsc:216: error: expected specifier-qualifier-list before ‘MYSQL_BIND’ Connection.hsc:216: error: expected ‘;’ before ‘)’ token Connection.hsc:216: error: expected statement before ‘)’ token Connection.hsc:228: error: ‘MYSQL_TIME’ undeclared (first use in this function) Connection.hsc:232: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:232: error: expected ‘;’ before ‘)’ token Connection.hsc:232: error: expected statement before ‘)’ token Connection.hsc:233: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:233: error: expected ‘;’ before ‘)’ token Connection.hsc:233: error: expected statement before ‘)’ token Connection.hsc:234: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:234: error: expected ‘;’ before ‘)’ token Connection.hsc:234: error: expected statement before ‘)’ token Connection.hsc:235: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:235: error: expected ‘;’ before ‘)’ token Connection.hsc:235: error: expected statement before ‘)’ token Connection.hsc:236: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:236: error: expected ‘;’ before ‘)’ token Connection.hsc:236: error: expected statement before ‘)’ token Connection.hsc:237: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:237: error: expected ‘;’ before ‘)’ token Connection.hsc:237: error: expected statement before ‘)’ token Connection.hsc:242: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:242: error: expected ‘;’ before ‘)’ token Connection.hsc:242: error: expected statement before ‘)’ token Connection.hsc:243: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:243: error: expected ‘;’ before ‘)’ token Connection.hsc:243: error: expected statement before ‘)’ token Connection.hsc:244: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:244: error: expected ‘;’ before ‘)’ token Connection.hsc:244: error: expected statement before ‘)’ token Connection.hsc:245: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:245: error: expected ‘;’ before ‘)’ token Connection.hsc:245: error: expected statement before ‘)’ token Connection.hsc:246: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:246: error: expected ‘;’ before ‘)’ token Connection.hsc:246: error: expected statement before ‘)’ token Connection.hsc:247: error: expected specifier-qualifier-list before ‘MYSQL_TIME’ Connection.hsc:247: error: expected ‘;’ before ‘)’ token Connection.hsc:247: error: expected statement before ‘)’ token Connection.hsc:368: error: ‘MYSQL_TYPE_NULL’ undeclared (first use in this function) Connection.hsc:381: error: ‘MYSQL_TYPE_VAR_STRING’ undeclared (first use in this function) Connection.hsc:385: error: ‘MYSQL_TYPE_LONGLONG’ undeclared (first use in this function) Connection.hsc:389: error: ‘MYSQL_TYPE_TINY’ undeclared (first use in this function) Connection.hsc:397: error: ‘MYSQL_TYPE_DOUBLE’ undeclared (first use in this function) Connection.hsc:401: error: ‘MYSQL_TYPE_LONG’ undeclared (first use in this function) Connection.hsc:422: error: ‘MYSQL_TYPE_DATETIME’ undeclared (first use in this function) Connection.hsc:438: error: ‘MYSQL_TYPE_TIME’ undeclared (first use in this function) Connection.hsc:492: error: ‘UNSIGNED_FLAG’ undeclared (first use in this function) Connection.hsc:510: error: ‘MYSQL_TYPE_STRING’ undeclared (first use in this function) Connection.hsc:512: error: ‘MYSQL_TYPE_SHORT’ undeclared (first use in this function) Connection.hsc:513: error: ‘MYSQL_TYPE_INT24’ undeclared (first use in this function) Connection.hsc:514: error: ‘MYSQL_TYPE_YEAR’ undeclared (first use in this function) Connection.hsc:515: error: ‘MYSQL_TYPE_ENUM’ undeclared (first use in this function) Connection.hsc:516: error: ‘MYSQL_TYPE_DECIMAL’ undeclared (first use in this function) Connection.hsc:518: error: ‘MYSQL_TYPE_NEWDECIMAL’ undeclared (first use in this function) Connection.hsc:520: error: ‘MYSQL_TYPE_FLOAT’ undeclared (first use in this function) Connection.hsc:521: error: ‘MYSQL_TYPE_DATE’ undeclared (first use in this function) Connection.hsc:522: error: ‘MYSQL_TYPE_TIMESTAMP’ undeclared (first use in this function) Connection.hsc:523: error: ‘MYSQL_TYPE_NEWDATE’ undeclared (first use in this function) Connection.hsc:524: error: ‘MYSQL_TYPE_BLOB’ undeclared (first use in this function) Connection.hsc:544: error: ‘MYSQL_DATA_TRUNCATED’ undeclared (first use in this function) Connection.hsc:545: error: ‘MYSQL_NO_DATA’ undeclared (first use in this function) Connection.hsc:607: error: ‘NOT_NULL_FLAG’ undeclared (first use in this function) Connection.hsc:629: error: ‘MYSQL_TYPE_VARCHAR’ undeclared (first use in this function) Connection.hsc:630: error: ‘MYSQL_TYPE_BIT’ undeclared (first use in this function) Connection.hsc:633: error: ‘MYSQL_TYPE_SET’ undeclared (first use in this function) Connection.hsc:634: error: ‘MYSQL_TYPE_TINY_BLOB’ undeclared (first use in this function) Connection.hsc:635: error: ‘MYSQL_TYPE_MEDIUM_BLOB’ undeclared (first use in this function) Connection.hsc:636: error: ‘MYSQL_TYPE_LONG_BLOB’ undeclared (first use in this function) Connection.hsc:640: error: ‘MYSQL_TYPE_GEOMETRY’ undeclared (first use in this function) compiling dist/build/Database/HDBC/MySQL/Connection_hsc_make.c failed command was: /usr/bin/gcc -c -m32 -D__GLASGOW_HASKELL__=612 -I-L/usr/local/Cellar/readline/6.1/lib -I-arch -Ii386 -I-L/usr/local/Cellar/mysql/5.1.51/lib/mysql -I-lmysqlclient -I-lz -I-lm -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/time-1.1.4/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/old-time-1.0.0.5/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/bytestring-0.9.1.7/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/base-4.2.0.2/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include -I/Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/include/ dist/build/Database/HDBC/MySQL/Connection_hsc_make.c -o dist/build/Database/HDBC/MySQL/Connection_hsc_make.o cabal: Error: some packages failed to install: HDBC-mysql-0.6.3 failed during the building phase. The exception was: ExitFailure 1

1 0

Derived type definition
by kg 23 Nov '10

23 Nov '10

Hi, I've tried to simplify as much as possible my problem. Finally, I think I can resume it like that: Suppose these following data types : data Rec a r = Rec a r data RecNil = RecNil data Wrapper a = Wrapper a Then, we can build the following type: type TTest = Rec Int (Rec String RecNil) or this type: type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) Is it possible to build TTestWrapped from TTest ? Thx in advance, Antoine.

2 1

looking for tagsoup-parsec examples
by José Romildo Malaquias 22 Nov '10

22 Nov '10

Hello. I am looking for some examples of code using Text.HTML.TagSoup.Parsec, from the tagsoup-parsec package. Romildo

1 1

GUI Designer
by c8h10n4o2 22 Nov '10

22 Nov '10

There's a tutorial on how to use qtDesigner with qtHaskell ? Or how to use XRC files with wxHaskell? -- View this message in context: http://haskell.1045720.n5.nabble.com/GUI-Designer-tp3271441p3271441.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 2

Berlin Haskell Meeting
by Sönke Hahn 22 Nov '10

22 Nov '10

The next haskell meeting in Berlin will be this wednesday: Date: Wednesday, November 24th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin Hope to see you there, Sönke

1 0

Yesod.Form: Password fields?
by Ertugrul Soeylemez 22 Nov '10

22 Nov '10

Hello everybody, how can I create a password field using Yesod.Form? It doesn't seem to support them. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

1 0

Same compiled program behaving differently when called from ghci and shell
by Bruno Damour 22 Nov '10

22 Nov '10

Hello, I have a very strange (for me) problem that I manage to reduce to this : I have a small program that reads a file with 1 only character (è = e8) The program is ftest2.hs : import IO import Data.Maybe tfind s = lookup (head s) $ zip ['\xe8', '\xde'] "12" main = do h<- readFile "g:\\CODE\\rlib\\test.txt" putStrLn h print $ tfind h I compile it from command line : ghc --make ftest2.hs Now the weird results : 1/ cmd line: ftest2.exe è Just '2' 2/ ghci Prelude> :!ftest2.exe è Just '2' 3/WinGHci Prelude> :! ftest2.exe è Just '1' I tested different variants, there is always a difference. Any idea to help me trace this behaviour ?

3 8

OT: Mailing list for testing email
by Alexander Solla 22 Nov '10

22 Nov '10

Hi everybody This is kind of off topic, but mailing list related. I've been messing around with my email server, to help fix some issues with the new version of Postfix (with regards to my old configuration). I seem to be receiving email intermittently. Does anybody know of a mailing list for email testing? I mean, I would like to be able to "spam" the mailing list with test messages and receive other people's test messages, until I'm sure my configuration is working right, without bothering the Cafe (or any other topic specific list). I know there used to be usenet groups for this purpose, but that's usenet, not email. I guess it would be nice if somebody sent me a quick pong in response to my ping, but I don't want to make too much noise here. Thanks

1 0

Catching up on Time and Directory
by Jon Fairbairn 22 Nov '10

22 Nov '10

I'm probably terribly out of date with this, so I wonder if anyone can save me the bother of working out what the /preferred/ libraries are for (a) determining the last-modified-time of a file or directory and (b) manipulating the resulting time datum. I can find System.Directory.getModificationTime and Data.Time.formatTime, but using them together seems unduly awkward. -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2010-09-14)

7 9

Codec.Crypto.RSA question
by Charles-Pierre Astolfi 21 Nov '10

21 Nov '10

Hi -cafe, I have a question about Codec.Crypto.RSA: how to enforce that (informally) decrypt . encrypt = id Consider this code: encrypt2 :: String -> ByteString encrypt2 = fst . encrypt (mkStdGen n) pubKey encode decrypt2 :: ByteString -> String decrypt2 = toString . decrypt privKey Since decrypt2 takes a bytestring which size is a multiple of the modulus, I've guessed that the encrypted bytestring is padded with '\NUL' characters. But then some strange stuff shows up: decrypt2 $ encrypt2 "haskell" returns "\NUL\NUL\NUL\NUL\EOThaskell" decrypt2 $ encrypt2 "foobar" returns "\NUL\NUL\NUL\NUL\65533foobar" I may use dropWhile to get rid of the NUL-padding, but can I assume that I'll always get a random character between my string and the padding? I tried to pad myself with NUL characters but I got almost the same results (the "random char" went to the head of the string) -- Cp

4 6

Making HList's field access operators (#) and (.!.) left-associative?
by Josh Triplett 21 Nov '10

21 Nov '10

Hello, In experimenting with HList's very impressive extensible records, I discovered that the field access operators (#) and (.!.) have right associativity; this means I can't write (r # f1 # f2) to access field f2 of the record stored in field f1 of r. If (#) and (.!) had left-associativity instead, then I could write such nested record operations. As far as I can tell, the right-associative binding would only make sense if attempting to store field labels inside a record, which seems much more unlikely. I wrote to the author of HList about this issue, and he agreed that this sounded reasonable, but he wanted me to confirm via haskell-cafe that nobody had code which relied on the existing behavior. Comments? Objections? Thanks, Josh Triplett

2 1

Tooltips on tree view rows
by José Romildo Malaquias 21 Nov '10

21 Nov '10

Hello. I need the gtk_tree_view_set_tooltip_column from Gtk+, but it seems tht it has not been ported to gtk2hs. Is it the case? How can I easily add tooltips to rows in a TreeView? Regards, Romildo

1 0

trouble with HDBC-mysql on Mac OS X
by Mark Lentczner 21 Nov '10

21 Nov '10

I'm having trouble with HDBC-mysql on Mac OS X (10.6, Snow Leopard). It compiles and builds fine, but when loaded (into ghci), the dynamic loader complains about not being able to find libmygcc.dylib. The issue is, MySQL's libmygcc is a static lib, not a dynamic one, so of course it won't find it. I've checked the build sequence that HDBC-mysql uses, and it calls upon mysql_config to generate the lib options. I've checked those and they are correct, expressly listing libmygcc statically (!). I've tried building against both the 64bit and 32bit versions of MySQL libs. (Suspect that only the 32bit should work.) Anyone have HDBC-mysql running on 10.6? Any ideas for me to try would be appreciated. - Mark Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

2 1

ghc panic when compiling blaze-builder
by José Romildo Malaquias 21 Nov '10

21 Nov '10

When compiling blaze-builder-0.2.0.1 with ghc-7.0.1 on my ~amd64 gentoo system, I am getting the shown below. Any clues? Romildo [...] Building blaze-builder-0.2.0.1... [1 of 8] Compiling Blaze.ByteString.Builder.Internal ( Blaze/ByteString/Builder/Internal.hs, dist/build/Blaze/ByteString/Builder/Internal.o ) ghc: panic! (the 'impossible' happened) (GHC version 7.0.1 for x86_64-unknown-linux): dsLet: unlifted AbsBinds [b{tv aUH} [sk]] [] [pe{v axd} [lid] <= [b{tv aUH} [sk]] pe{v aUG} [lid]] pe{v axd} [lid] :: forall b{tv aUH} [sk]. base:GHC.Ptr.Ptr{tc 33A} b{tv aUH} [sk] [LclId] { {273:19-54} {273:19-54} !((pe{v aUG} [lid] :: base:GHC.Ptr.Ptr{tc 33A} b{tv aUH} [sk])) = {273:30-54} {273:33-41} (base:GHC.Ptr.plusPtr{v r4X} [gid]) @ base:GHC.Word.Word8{tc 32U} @ b{tv aUH} [sk] pf{v axc} [lid] firstBufSize{v arR} [lid] } EvBinds{{}} base:GHC.Base.>>={v 01P} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aUI} [lid] @ blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildSignal{tc rre} @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} (step0{v arY} [lid] pf{v axc} [lid] (pe{v axd} [lid] @ base:GHC.Word.Word8{tc 32U})) (\ (next{v axT} [lid] :: blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildSignal{tc rre}) -> let { ds_d15b{v} [lid] :: blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildSignal{tc rre} [LclId] ds_d15b{v} [lid] = next{v axT} [lid] } in case ds_d15b{v} [lid] {ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}} of (wild_B1{v} [lid] :: blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildSignal{tc rre}) { blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.Done{d rrk} (rb_d15l{v} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33}) -> let { pf'{v axU} [lid] :: base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U} [LclId] pf'{v axU} [lid] = base:GHC.Ptr.Ptr{v rcP} [gid[DataCon]] @ base:GHC.Word.Word8{tc 32U} rb_d15l{v} [lid] } in let { fail_d15c{v} [lid] :: ghc-prim:GHC.Prim.State#{(w) tc 32q} ghc-prim:GHC.Prim.RealWorld{(w) tc 31E} -> ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} [LclId] fail_d15c{v} [lid] = \ (ds_d15d{v} [lid] :: ghc-prim:GHC.Prim.State#{(w) tc 32q} ghc-prim:GHC.Prim.RealWorld{(w) tc 31E}) -> base:GHC.Base.${v 019} [gid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (base:GHC.Base.return{v 01T} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aUN} [lid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.$WChunk{v reF} [gid[DataConWrapper]] (mkbs{v axe} [lid] @ base:GHC.Word.Word8{tc 32U} pf'{v axU} [lid]) k{v arT} [lid]) } in case base:GHC.Classes.=={v 01L} [gid[ClassOp]] @ (base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U}) $dEq{v aUK} [lid] pf'{v axU} [lid] pf{v axc} [lid] {ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}} of (wild_B1{v} [lid] :: ghc-prim:GHC.Bool.Bool{(w) tc 3c}) { ghc-prim:GHC.Bool.False{(w) d 68} -> fail_d15c{v} [lid] ghc-prim:GHC.Prim.realWorld#{(w) v 0o} [gid]; ghc-prim:GHC.Bool.True{(w) d 6u} -> base:GHC.Base.return{v 01T} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aUL} [lid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} k{v arT} [lid] }; blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BufferFull{d rri} (rb_d15n{v} [lid] :: ghc-prim:GHC.Prim.Int#{(w) tc 3G}) (rb_d15o{v} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33}) (nextStep{v axX} [lid] :: blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildStep{tc rrc}) -> let { newSize{v axV} [lid] :: ghc-prim:GHC.Types.Int{(w) tc 3J} [LclId] newSize{v axV} [lid] = ghc-prim:GHC.Types.I#{(w) v 6d} [gid[DataCon]] rb_d15n{v} [lid] } in let { pf'{v axW} [lid] :: base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U} [LclId] pf'{v axW} [lid] = base:GHC.Ptr.Ptr{v rcP} [gid[DataCon]] @ base:GHC.Word.Word8{tc 32U} rb_d15o{v} [lid] } in let { l{v aUP} [lid] :: ghc-prim:GHC.Types.Int{(w) tc 3J} [LclId] l{v aUP} [lid] = base:GHC.Ptr.minusPtr{v r4U} [gid] @ base:GHC.Word.Word8{tc 32U} @ base:GHC.Word.Word8{tc 32U} pf'{v axW} [lid] pf{v axc} [lid] } in case l{v aUP} [lid] {ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}} of (l{v aUP} [lid] :: ghc-prim:GHC.Types.Int{(w) tc 3J}) { __DEFAULT -> let { l{v axY} [lid] :: ghc-prim:GHC.Types.Int{(w) tc 3J} [LclId] l{v axY} [lid] = l{v aUP} [lid] } in base:GHC.Base.${v 019} [gid] @ blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildStep{tc rrc} @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (fillNewBuffer{v arW} [lid] (base:GHC.Classes.max{v rG} [gid[ClassOp]] @ ghc-prim:GHC.Types.Int{(w) tc 3J} $dOrd{v aUQ} [lid] (base:GHC.Num.+{v r24} [gid[ClassOp]] @ ghc-prim:GHC.Types.Int{(w) tc 3J} $dNum{v aY6} [lid] l{v axY} [lid] newSize{v axV} [lid]) minBufSize{v arQ} [lid])) (\ (pfNew{v axZ} [lid] :: base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U}) (peNew{v ay0} [lid] :: base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U}) -> base:GHC.Base.>>{v 01Q} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aY7} [lid] @ ghc-prim:GHC.Unit.(){(w) tc 40} @ blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildSignal{tc rre} (base:Foreign.Marshal.Utils.copyBytes{v r4c} [gid] @ base:GHC.Word.Word8{tc 32U} pfNew{v axZ} [lid] pf{v axc} [lid] l{v axY} [lid]) (nextStep{v axX} [lid] (base:GHC.Ptr.plusPtr{v r4X} [gid] @ base:GHC.Word.Word8{tc 32U} @ base:GHC.Word.Word8{tc 32U} pfNew{v axZ} [lid] l{v axY} [lid]) peNew{v ay0} [lid])) }; blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.ModifyChunks{d rrg} (rb_d15q{v} [lid] :: ghc-prim:GHC.Prim.Addr#{(w) tc 33}) (bsk{v ay2} [lid] :: bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} -> bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (nextStep{v ay3} [lid] :: blaze-builder-0.2.0.1:Blaze.ByteString.Builder.Internal.BuildStep{tc rrc}) -> let { pf'{v ay1} [lid] :: base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U} [LclId] pf'{v ay1} [lid] = base:GHC.Ptr.Ptr{v rcP} [gid[DataCon]] @ base:GHC.Word.Word8{tc 32U} rb_d15q{v} [lid] } in let { fail_d15g{v} [lid] :: ghc-prim:GHC.Prim.State#{(w) tc 32q} ghc-prim:GHC.Prim.RealWorld{(w) tc 31E} -> ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} [LclId] fail_d15g{v} [lid] = \ (ds_d15h{v} [lid] :: ghc-prim:GHC.Prim.State#{(w) tc 32q} ghc-prim:GHC.Prim.RealWorld{(w) tc 31E}) -> base:GHC.Base.${v 019} [gid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (base:GHC.Base.return{v 01T} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aYc} [lid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.$WChunk{v reF} [gid[DataConWrapper]] (mkbs{v axe} [lid] @ base:GHC.Word.Word8{tc 32U} pf'{v ay1} [lid]) (bsk{v ay2} [lid] (base:GHC.Base.${v 019} [gid] @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} (bytestring-0.9.1.8:Data.ByteString.Internal.inlinePerformIO{v riV} [gid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (fillNewBuffer{v arW} [lid] bufSize{v arP} [lid] nextStep{v ay3} [lid])))) } in case base:GHC.Classes.=={v 01L} [gid[ClassOp]] @ (base:GHC.Ptr.Ptr{tc 33A} base:GHC.Word.Word8{tc 32U}) $dEq{v aY9} [lid] pf'{v ay1} [lid] pf{v axc} [lid] {ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}} of (wild_B1{v} [lid] :: ghc-prim:GHC.Bool.Bool{(w) tc 3c}) { ghc-prim:GHC.Bool.False{(w) d 68} -> fail_d15g{v} [lid] ghc-prim:GHC.Prim.realWorld#{(w) v 0o} [gid]; ghc-prim:GHC.Bool.True{(w) d 6u} -> base:GHC.Base.${v 019} [gid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (base:GHC.Base.return{v 01T} [gid[ClassOp]] @ <nt>ghc-prim:GHC.Types.IO{tc 32I} $dMonad{v aYa} [lid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (bsk{v ay2} [lid] (base:GHC.Base.${v 019} [gid] @ (ghc-prim:GHC.Types.IO{tc 32I} bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5} (bytestring-0.9.1.8:Data.ByteString.Internal.inlinePerformIO{v riV} [gid] @ bytestring-0.9.1.8:Data.ByteString.Lazy.Internal.ByteString{tc rg5}) (fillNewBuffer{v arW} [lid] bufSize{v arP} [lid] nextStep{v ay3} [lid]))) } })

2 1

Interactive OpenGL-based graphics and ghci?
by Conal Elliott 21 Nov '10

21 Nov '10

I'm trying to find some way to do interactive, OpenGL-based graphics in Haskell on Mac OS X. Does anyone here use GLUT or SDL on Mac OS X with ghci, or maybe an alternative library? Using ghci is very important to me, as my programs are pretty high-level and are often half-liners. I'm using gtk2hs for now, but I want something that works more natively, i.e., without the awkwardness of going through the X server. Thanks, - Conal

3 2

Downloading web page in Haskell
by José Romildo Malaquias 21 Nov '10

21 Nov '10

In order to download a given web page, I wrote the attached program. The problem is that the page is not being full downloaded. It is being somehow intettupted. Any clues on how to solve this problem? Romildo

7 9

category-extras clash with transformers
by Tony Morris 21 Nov '10

21 Nov '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have installed mtl-1.1.1.0 so that xmonad-contrib-0.9.1 would compile with GHC 6.12.1. http://permalink.gmane.org/gmane.comp.lang.haskell.xmonad/10603 Then I tried to installed category-extras-0.53.5, which clashed with transformers-0.2.2.0 for the Applicative/Monad instances for Either. Is there any way out of this problem? Thanks for any pointers. $ ghc-pkg list | grep mtl mtl-1.1.1.0 mtl-2.0.1.0 $ ghc-pkg list | grep transformers transformers-0.2.2.0 $ cabal install category-extras ... [39 of 99] Compiling Control.Monad.Either ( src/Control/Monad/Either.hs, dist/build/Control/Monad/Either.o ) src/Control/Monad/Either.hs:44:9: Duplicate instance declarations: instance Monad (Either e) -- Defined at src/Control/Monad/Either.hs:44:9-24 instance Monad (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error src/Control/Monad/Either.hs:49:9: Duplicate instance declarations: instance Applicative (Either e) -- Defined at src/Control/Monad/Either.hs:49:9-30 instance Applicative (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error src/Control/Monad/Either.hs:53:9: Duplicate instance declarations: instance MonadFix (Either e) -- Defined at src/Control/Monad/Either.hs:53:9-27 instance MonadFix (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error cabal: Error: some packages failed to install: category-extras-0.53.5 failed during the building phase. The exception was: ExitFailure 1 - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkznxgQACgkQmnpgrYe6r60dkACfZQkYKbMOQuGfaVpFb2MfhJWD asAAn1/hoX+m/YpUOch3r4NsR99y2htz =IsZc -----END PGP SIGNATURE-----

4 5

RegEx versus (Parsec, TagSoup, others...)
by Michael Litchard 20 Nov '10

20 Nov '10

I've been working on a project that requires me to do screen scraping. When I first started this, I worked off of other people's examples. Not one used regex. By luck I found someone at work to help me along this project. His clues and hints don't use regex either. I was at a point where I had to make a decision concerning design, so I asked the guy sitting next to me at work. He's very experienced, and comes from a Perl perspective. I let him into what I was doing, and he opined I should be using pcre. So now I'm second guessing my choices. Why do people choose not to use regex for uri parsing?

7 6

Re: [Haskell-cafe] Quasi quotation question
by Oscar Finnsson 20 Nov '10

20 Nov '10

Sure. Just use http://hackage.haskell.org/package/haskell-src-meta-0.0.6. You are probably interested in parsePat or parseExp. I've used parseExp in a package. -- Oscar

1 0