January 2011 - Haskell-Cafe

Clear screen in ghci in haskell-mode in emacs?
by JETkoten 30 Jan '11

30 Jan '11

I'm looking for a way to clear the screen in ghci within emacs. I saw a feature request from a couple of years back for :clear, but it was deleted saying something along the lines of "Ctrl-L works fine for me". This is true in my system terminal, however, in emacs, Ctrl-L is already bound to something else. Is there a way to clear the ghci screen that doesn't involve Ctrl-L?

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Haddock : Synopsis box too big for monitor.
by aditya siram 30 Jan '11

30 Jan '11

Hi all, The new Synopsis box in the API docs expands beyond the monitor for functions that have long signatures. An example is the TreeView documentation [1] in the gtk package. I have only tested Firefox. -deech [1] http://hackage.haskell.org/packages/archive/gtk/0.12.0/doc/html/Graphics-UI…

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Code from Haskell School of Expression hanging.
by michael rice 30 Jan '11

30 Jan '11

I got Hudak's Haskell School of Expression today and just tried a couple of the graphics programs from the software I downloaded at the book site. Both seem to hang on opening a window. Is there a fix for this problem? I'm on a Fedora 14 Linux (32-bit) box running GHC version 6.12.3. Michael ============ *SimpleGraphics> :l Snowflake.lhs [1 of 2] Compiling SOE ( SOE.hs, interpreted ) [2 of 2] Compiling Snowflake ( Snowflake.lhs, interpreted ) Ok, modules loaded: Snowflake, SOE. *Snowflake> main ============ This code was automatically extracted from a .lhs file that uses the following convention: -- lines beginning with ">" are executable -- lines beginning with "<" are in the text, but not necessarily executable -- lines beginning with "|" are also in the text, but are often just expressions or code fragments. > module Snowflake where > import SOE > m = 81 :: Int -- multiple of 3 for triangle size > x = 250 :: Int -- x and y coordinates of > y = 250 :: Int -- center of snowflake > colors = [ Magenta, Blue, Green, Red, Yellow ] > snowflake :: Window -> IO () > snowflake w = do > drawTri w x y m 0 False -- draw first triangle w/flat top > flake w x y m 0 True -- begin recursion to complete job > flake :: Window -> Int -> Int -> Int -> Int -> Bool -> IO () > flake w x y m c o = do > drawTri w x y m c o -- draw second triangle > let c1 = (c+1)`mod`5 -- get next color > if (m<=3) then return () -- if too small, we're done > else do > flake w (x-2*m) (y-m) (m`div`3) c1 True -- NW > flake w (x+2*m) (y-m) (m`div`3) c1 True -- NE > flake w x (y+2*m) (m`div`3) c1 True -- S > flake w (x-2*m) (y+m) (m`div`3) c1 False -- SW > flake w (x+2*m) (y+m) (m`div`3) c1 False -- SE > flake w x (y-2*m) (m`div`3) c1 False -- N > drawTri :: Window -> Int -> Int -> Int -> Int -> Bool -> IO () > drawTri w x y m c o = > let d = (3*m) `div` 2 > ps = if o > then [(x,y-3*m), (x-3*m,y+d), (x+3*m,y+d)] -- side at bottom > else [ (x,y+3*m), (x-3*m,y-d), (x+3*m,y-d)] -- side at top > in drawInWindow w > (withColor (colors !! c) > (polygon ps)) > main > = runGraphics ( > do w <- openWindow "Snowflake Fractal" (500,500) > drawInWindow w (withColor White > (polygon [(0,0),(499,0),(499,499),(0,499)])) > snowflake w > spaceClose w > ) > spaceClose :: Window -> IO () > spaceClose w > = do k <- getKey w > if k==' ' || k == '\x0' > then closeWindow w > else spaceClose w

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ANN: nanoparsec 0.1.1
by Maciej Piechotka 30 Jan '11

30 Jan '11

Nanoparsec is currently simply a port of attoparsec on the ListLike (the abstraction of lists used by iteratee). It allows to achive in parsing a near-attoparsec levels of speed (benchmarks from attoparsec library shown a 0.450 ± 0.028 for attoparsec, 0.479 ± 0.043 for nanoparsec and 1.532 ± 0.084 for parsec 3) combining the flexibility of stream of parsec 3 with the iterative approach and speed of attoparsec. Changes since version 0.1: - Lowered conditions on base to allow using GHC 6.12.x (haven't been tested) - Added IsString instance and (.*>) (<*.) helper functions Regards

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"Cabal ignores ld-options?" - 2011 remix
by Ryan Newton 30 Jan '11

30 Jan '11

Hi cafe, I am trying to link a ".a" file built by a separate makefile into my library. GHC has no problem with it, but I need to convince cabal to do it for the package to be hackage-friendly. There's a thread about this back in 2007: http://www.haskell.org/pipermail/cabal-devel/2007-April/000507.html Is there a method for this now, or is the state of affairs the same as before? Thanks, -Ryan

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Re: [Haskell-cafe] Fwd: web-routes and forms
by Jasper Van der Jeugt 29 Jan '11

29 Jan '11

Hello, As Jeremy said, the HTML returned by formHtml is meant to be placed inside the a <form> tag: it does not include a <form> tag. You should use it like this: > H.form ! A.enctype (H.stringValue $ show enctype) > ! A.method "POST" ! A.action "/" $ do > html -- The HTML rendered by formHtml > H.input ! A.type_ "submit" ! A.value "Submit" (with or without the submit button). Cheers, Jasper On Jan 27, 2011 10:54 AM, "Corentin Dupont" <corentin.dupont(a)gmail.com> wrote: > Hello Jasper, > Do you have an idea? > > Thanks, > Corentin > > ---------- Forwarded message ---------- > From: Jeremy Shaw <jeremy(a)n-heptane.com> > Date: Thu, Jan 27, 2011 at 2:22 AM > Subject: Re: web-routes and forms > To: Corentin Dupont <corentin.dupont(a)gmail.com> > Cc: haskell <haskell-cafe(a)haskell.org> > > > On Wed, Jan 26, 2011 at 4:33 PM, Corentin Dupont > <corentin.dupont(a)gmail.com> wrote: > >> Now turning to digestive functors, I don't see where do goes the "A.action >> actionURL" part that was in traditionnal forms? >> It seems I need it for routing the result of the form. > > I think you will find formHtml is returning you the stuff that goes > inside the <form> tag, but does not actually include the form tag > itself ? > > I am not sure how to modify the attrs using blaze-html. I think that > is a missing feature of the digestive-functors-blaze package. In > digestive-functors-hsp there is a function: > > > setAttrs :: (EmbedAsAttr x attr, XMLGenerator x, Monad m, Functor m) => > Form m i e [HSX.GenXML x] a > -> attr > -> Form m i e [HSX.GenXML x] a > setAttrs form attrs = mapView (map (`set` attrs)) form > > > You probably need something similar for blaze. > > - jeremy

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tricky recursive type instance method
by Frank Kuehnel 29 Jan '11

29 Jan '11

Hi folks, how do I make this work: I want a division algebra over a field k, and I want to define the conjugation of complex numbers, i.e. conj (C 1 2) but also the conjugation of tensors of complex numbers conj (C (C 1 2) (C 1 4)) ghci load that stuff butt barfs on a simple > conj (C 1 2) with instance Real a => DAlgebra a a -- Defined at Clifford.hs:20:10-31 instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r here's the code: -- for a normed division algebra we need a norm and conjugation! class DAlgebra a k | a -> k where -- need functional dependence because conj doesn't refer to k conj :: a -> a abs2 :: a -> k -- real numbers are a division algebra instance Real a => DAlgebra a a where conj = id abs2 x = x*x -- Complex numbers form a normed commutative division algebra data Complex a = C a a deriving (Eq,Show) instance Num a => Num (Complex a) where fromInteger a = C (fromInteger a) 0 (C a b)+(C a' b') = C (a+a') (b+b') (C a b)-(C a' b') = C (a-a') (b-b') (C a b)*(C a' b') = C (a*a'-b*b') (a*b'+b*a') instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r where conj (C a b) = C a (conj (-b)) abs2 (C a b) = (abs2 a) + (abs2 b) Thanks for you help!

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Playing sounds in Haskell
by Daniel Díaz 29 Jan '11

29 Jan '11

Hi, cafe, I'm interested in write a program that plays sounds in real time. There is a library with functions able to play sounds, from a file? I'm on Windows, and I don't need special features, only play sounds in a comfortable way. Thanks in advance. -- Daniel Díaz

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Using IsString with attoparsec
by Yitzchak Gale 29 Jan '11

29 Jan '11

Using the IsString instance for Parser in attoparsec is really nice, but unfortunately, you can't use it out of the box for the most common case. One would like to be able to write parsers in this style: "(" *> stuff <* ")" But the types of *> and <* are too general - there is no way for the type checker to figure out which instance of IsString to use for the discarded parameter. I suggest adding the following type-specialized variants to Data.Attoparsec.Char8: (<*.) :: Applicative f => f a -> f ByteString -> f a (<*.) = (<*) (.*>) :: Applicative f => f ByteString -> f a -> f a (.*>) = (*>) Once attoparsec-text gets its IsString instance, I would add the same thing to Data.Attoparsec.Text, with ByteString replaced by Text. I have been using those for both attoparsec and attoparsec-text, and I find them extremely useful. Thanks, Yitz

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ANNOUNCE: Parsec 3.1.1
by Antoine Latter 29 Jan '11

29 Jan '11

Parsec is a monadic combinator library that is well-documented, simple to use, and produces good error messages. Parsec is not inherently lazy/incremental and is not well-suited to handling large quantities of simply formatted data. Parsec 3 added to Parsec the ability to use Parsec as a monad transformer and generalizes the input Parsec accepts. This is a minor point release over Parsec 3.1.0. The changes in this release are: - A bug introduced in 3.1.0 which reduced the quality of error messages when failing to parse has been fixed - Typos in some of the Haddock documentation have been fixed - The package 'syb` is no longer a dependency Please note that there are a few changes which have not yet made it to the up-stream source repository, as code.haskell.org is experiencing a temporary outage. Link on Hackage: http://hackage.haskell.org/package/parsec-3.1.1 I'd like to thank Derek Elkins for prior his maintainer-ship of the package and for getting the Summer-Of-Code work released as parsec-3.0.

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