May 2010 - Haskell-Cafe

Mystery Parse Error in LHS file
by Iæfai 06 May '10

06 May '10

I have an lhs file, 'starsystem.lhs' that is not compiling because of a parse error. Specifically: $ cabal build Preprocessing executables for starsystem-0.1... Building starsystem-0.1... [1 of 4] Compiling Render ( Render.lhs, dist/build/ starsystem/starsystem-tmp/Render.o ) [2 of 4] Compiling Passive ( Passive.lhs, dist/build/ starsystem/starsystem-tmp/Passive.o ) [3 of 4] Compiling Active ( Active.lhs, dist/build/ starsystem/starsystem-tmp/Active.o ) [4 of 4] Compiling Main ( starsystem.lhs, dist/build/ starsystem/starsystem-tmp/Main.o ) starsystem.lhs:29:0: parse error on input `main' The location is at the first instance of main: > data Action = Action (IO Action) The main program is mostly book-keeping such as initializing OpenGL and GLFW, creating window, setting up viewport, etc. \begin{code} main :: IO () -- ** line 29 main = do c_unbundled GLFW.initialize -- open window I cannot figure out what the problem here is. I have tried it in a regular hs file and it works. I have posted full source here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=25352#a25352 Any thoughts would be welcome.

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ANNOUNCE: wai-handler-fastcgi 0.0.0
by Michael Snoyman 06 May '10

06 May '10

Hi all, I finally scratched an itch I've had for a while, and put together wai-handler-fastcgi. It is built on top of Dan Knapp's wonderful direct-fastcgi[1] package, so it is free from dependencies on C libraries. This package allows WAI[2] applications to be run on a FastCGI-supporting server. The WAI, for those not familiar, is a standard interface between web servers and web applications, allowing you to write your web applications once and deploy in multiple manners. Michael [1] http://hackage.haskell.org/package/direct-fastcgi [2] http://hackage.haskell.org/package/wai

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forall (What does it do)
by John Creighton 06 May '10

06 May '10

I've seen forall used in a few places related to Haskell. I know their is a type extension call "explicit forall" but by the way it is documnted in some places, the documentation makes it sound like it does nothing usefull. However on Page 27 of Haskell's overlooked object system: >We define an existential envelope for shape data. >data OpaqueShape = forall x. Shape x => HideShape x >“Opaque shapes are (still) shapes.” Hence, a Shape instance: >nstance Shape OpaqueShape >where > readShape f (HideShape x) = readShape f x > writeShape f (HideShape x) = HideShape $ writeShape f x > draw (HideShape x) = draw x >When building the scribble list, we place shapes in the envelope. >let scribble = [ HideShape (rectangle 10 20 5 6) >, HideShape (circle 15 25 8) " It seems that forall can be used as a form of up casting. That is if we have a parametric type we can treat the parametric types as homogeneous: The paper did not seem to regard this feature too highly: >By contrast, the explicit constraint for the existential envelope cannot be eliminated. >Admittedly, the loss of type inference is a nuance in this specific example. In >general, however, this weakness of existentials is quite annoying. It is intellectually >dissatisfying since type inference is one of the added values of an (extended) Hindley/ >Milner type system, when compared to mainstream OO languages. Worse than >that, the kind of constraints in the example are not necessary in mainstream OO >languages (without type inference), because these constraints deal with subtyping, >which is normally implicit. >We do not use existentials in OOHaskell http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf but can't we just use the read shape function above if we want to be able to infer based on types. I understand that haskers tend to like things well typed but surely their are times when we would like to be able to flatten the type of our data somewhat. On another note, I also wonder how forall relates to type families. I know their are certain restrictions on where we can use type families in haskell. I wonder if we can get around these somewhat using forall.

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lhs2TeX - lhs2TeX.fmt missing
by Ozgur Akgun 06 May '10

06 May '10

Hi all, I am trying to get lhs2TeX to work. I installed the package using cabal, and now I try to run it on a very simple *.lhs file. But it blames me and says user error, cannot find lhs2TeX.fmt: lhs2TeX: user error (File `lhs2TeX.fmt' not found in search path: And, when I check the search path, there really is no lhs2TeX.fmt file. Should I download it separately or something like that? PS: Thanks for the great package to the authors! Best, Ozgur Akgun

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A query to users of graphviz
by Ivan Lazar Miljenovic 06 May '10

06 May '10

For anyone who uses my graphviz library: who uses the graphNodes and functions? Note that this also applies to anyone who uses augmentation functions as well (more specifically, when they define global attributes for use with an augmentation function). If so, would it be a problem if I switched it from returning a list to returning a Set? The rationale being: Dot allows nodes to be "defined" twice (to specify different attributes) and also that some nodes might only be defined because they appear in an edge. Here is an example containing both problems: ,---- | digraph { | a [label="hello"] | subgraph { rank=same a b } | b -> c | } `---- Currently, graphNodes ignores the first problem and doesn't even consider the second (though I thought I had implemented support for this); however, I myself am needing this functionality soon, hence why I'm considering this (note that this isn't a problem for graphEdges, as multiple edges are OK). Currently, graphNodes doesn't take this into account (for edges, multiple definitions means the edge is drawn multiple times, so this isn't a problem for graphEdges); as such, graphviz should really (but doesn't) use nub on this. A better solution would be to use Sets and merge the results by merging together the applied attributes (and adding the global attributes for nodes at the same time). However, the problem with using Sets is that it means node type has to be an instance of Ord. I can't think of many types which you might use as a node type that _wouldn't_ have an applicable Ord instance, but I suppose it's possible. As such, there are three possible options: 1) Leave as-is (just get it working properly with hack-y nubs, etc.): this will be extremely inefficient. 2) Use Sets (or a Map n Attributes, which will make merging Attributes easier); this will require an Ord constraint on the label type for the DotRepr class (i.e. it will be needed _everywhere_). It may also not be as lazy as the current implementation, and the node order will be changed. 3) As with 2), but convert the result back to a list so that the current interface remains unchanged (with the added benefit of not requiring a major version bump). With whichever option is chosen, then there is also the consideration of whether any global attributes should be considered and added to the list of attributes (this goes for graphEdges as well). I personally am thinking of going with 3) and adding in global attributes as well. What do other people think? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Mixing mtl- and transformers-using libraries
by Joachim Breitner 06 May '10

06 May '10

Hi, I have a rather large project here that uses standard mtl stuff. This program now wants to use salvia, which in turn uses the transformers library. The result is this: X.hs:163:29: Overlapping instances for Monad (Either String) arising from a use of `neuePersonCommit' at X.hs:163:29-48 Matching instances: instance (transformers-0.1.4.0:Control.Monad.Trans.Error.Error e) => Monad (Either e) -- Defined in transformers-0.1.4.0:Control.Monad.Trans.Error instance (Control.Monad.Error.Class.Error e) => Monad (Either e) -- Defined in Control.Monad.Error Is there any way of using salvia without having ghc consider instances from transformers? Thanks, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Is anyone using Haddock's support for frames?
by David Waern 06 May '10

06 May '10

Hi Since version 2.4.0 Haddock has generated HTML output that uses frames (index-frames.html) in addition to the normal output. We'd like to deprecate this feature unless there is a significant amount of users. The reason is two-fold: * We probably want to replace the frames with something more modern (like a sidebar on the same page) in the future * We are rewriting the HTML backend and it would be nice to avoid unnecessary work So if you're using this feature and want to keep it, please speak up! cc:ing cvs-ghc@ in case they have any users of the frames due to the size of the GHC code base. (This might have been the the original motivation for the feature). Thanks, David

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Why cabal forces reinstallation of happstack-utils?
by Maciej Piechotka 06 May '10

06 May '10

I try to configure happstack with parsec 3.1. It seems to fail due to cabal: I installed happstack-util editing happstack-util.cabal by hand: % grep parsec ~/.ghc/x86_64-linux-6.12.2/package.conf.d/happstack-util-0.5.0-6e27d5d3ba1c07f259d463ee3036c92b.conf parsec-3.1.0-5842597f447f82b210228391f371cff1 % cabal install --constraint 'parsec > 3' --dry-run happstack Resolving dependencies... cabal: cannot configure happstack-util-0.5.0. It requires parsec <3 For the dependency on parsec <3 there are these packages: parsec-2.0, parsec-2.1.0.0 and parsec-2.1.0.1. However none of them are available. parsec-2.0 was excluded because of the top level dependency parsec >3 parsec-2.1.0.0 was excluded because of the top level dependency parsec >3 parsec-2.1.0.1 was excluded because of the top level dependency parsec >3 % ghc-pkg list | grep happstack-util happstack-util-0.5.0 Regards

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class Arbitrary in quickcheck
by Tim Docker 06 May '10

06 May '10

I've notice a behaviour of quickcheck that is unexpected to me. With this code: import Test.QuickCheck main = check myconfig ((\v -> v == v) :: (Maybe Double,Maybe Double) -> Bool) myconfig = defaultConfig{configMaxTest=100000, configEvery = \n args -> show n ++ ":\n" ++ unlines args} I am relying on the default Arbitrary instances to generate pairs of maybe doubles. But to my surprise, all of the pairs have these patterns: (Just _ Just _) (Nothing, Nothing) I never see patterns: (Just _, Nothing) (Nothing, Just _) Why is this the case? Thanks, Tim

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Haskell XML Parsers
by R Senington 06 May '10

06 May '10

Dear all, I have been looking at using XML for a little program I have been writing. The file I am currently trying to load is about 9MB, and I have now tried to use HaXml and HST. Without any of my own code, just a simple call to the basic parsers, they both use huge amount of memory. HST is the worst and about 7GB and climbing. HaXml uses 1.3Gb. The code I am using is HST xml <- readFile file_name_here;k<-runX (parseXmlDocument True) xml;print k and for HaXml x<-readFile file_name_here let (Document _ _ e _) = xmlParse "t" x let t = myFilter $ CElem e print $ length t I have seen on previous posts to the cafe that other people have run into this problem with HST. Is this a general problem with XML in Haskell (I know that XML parsing is a slow and bulky process but this seems excessive)? Is there a known solution? Does anyone have any advice? Cheers RS

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