April 2010 - Haskell-Cafe

Why does the transformers/mtl 'Error' class exist?
by John Millikin 21 Apr '10

21 Apr '10

Both the transformers[1] and mtl[2] define a class named 'Error', for use with MonadError and ErrorT. This class is required for the instance of Monad (and therefore MonadTrans, MonadIO, etc). However, I can't figure out why this class exists. Its presence means that instead of something like: ------------------------------------- data NameError = ErrorFoo | ErrorBar validateName :: Monad m => Text -> m (Either Error Text) validateName x = runErrorT $ do when (some condition) $ throwError ErrorFoo when (other condition) $ throwError ErrorBar return x ------------------------------------- I have to define this, which is more verbose, no more useful, and adds a "fake" class to the Haddock docs (with a warning not to use it): ------------------------------------- data Error = ErrorFoo | ErrorBar instance Error NameError where strMsg = error -- validateName ... ------------------------------------- Is there any good reason why the 'Error' class can't just be removed? [1] http://hackage.haskell.org/packages/archive/transformers/0.2.0.0/doc/html/C… [2] http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Mo…

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ANNOUNCE: hamlet 0.0.0
by Michael Snoyman 21 Apr '10

21 Apr '10

Hi all, I'd like to announce the first release of Hamlet[1], a templating system which is fully compile-time checked. Templates are parsed via quasi-quoting, giving you greater confidence in the validity of your templates. The syntax is inspired by Haml[2]; however, it is most definitely its own language, avoiding embedding of arbitrary code in favor of a more strict view-logic separation. Finally, it has a goal of allowing high performance uses; to facilitate this, it allows calling monadic code from inside the template, and can use enumerators for traversing elements. While these features could be abused to allow templates to perform unintended side-effects, the performance gains allowed by them are sometimes essential. I have started a documentation site for all Yesod-related packages[3], and in particular there is a Hamlet section[4]. I've also put up a small blog post[5] on this release, which explains how Hamlet fits into the Yesod ecosystem. (For those not in the know, Yesod is a web framework I'm working on.) Michael [1] http://hackage.haskell.org/package/hamlet-0.0.0 [2] http://haml-lang.com/ [3] http://docs.yesodweb.com/ [4] http://docs.yesodweb.com/hamlet/ [5] http://www.snoyman.com/blog/entry/hamlet-is-born/

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Profiling
by rodrigo.bonifacio 21 Apr '10

21 Apr '10

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DLL on Windows
by Serguey Zefirov 20 Apr '10

20 Apr '10

I tried the way described in http://www.haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html and i got this error message: -------------------------------------------------------------------------------------------- ghc -shared -o test.dll --make test.hs [1 of 1] Compiling Test ( test.hs, test.o ) ghc.exe: panic! (the 'impossible' happened) (GHC version 6.10.3 for i386-unknown-mingw32): link: GHC not built to link this way: LinkDynLib Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug -------------------------------------------------------------------------------------------- How can I fix it?

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ANN: Rika engine - alpha :)
by Jinjing Wang 20 Apr '10

20 Apr '10

Dear list: ANN: Rika Engine Source: http://github.com/nfjinjing/rika What is it: A 3D renderer, with some basic scripting ability, extremely hacky, not practical. Why: Example GLSL renderer? Deferred shading implementation ( though heavily cheated, you will see why .. ) ? This is one. Have fun folks :) -- jinjing

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newbie question how to pass data
by Mujtaba Boori 20 Apr '10

20 Apr '10

Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) ->Bool) -> Float -> ((Float,Float) -> Bool) I am trying to make calculation in this type ((Float,Float) ->Bool) with Float and then pass the information to ((Float,Float) -> Bool) Thank again appreciated.

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memory needed for SAX parsing XML
by Daniil Elovkov 20 Apr '10

20 Apr '10

Hello haskellers! I'm trying to process an xml file with as little footprint as possible. SAX is alright for my case, and I think that's the lightest way possible. So, I'm looking at HaXml.SAX I'm surprised to see that it takes about 56-60 MB of ram. This seems constant relative to xml file size, which is expected. Only slightly depends on it as I recursively traverse the list of sax events. But it seems like too much. The size of the file is from 1MB to 20MB. The code is something like this main = do (fn:_) <- getArgs h <- openFile fn ReadMode c <- hGetContents h let out = proc $ fst $ saxParse fn c putStrLn out getChar -- 'f' is tail-recursive -- 'i' is used only to ensure that the list is completely traversed proc es = show $ f es 0 where f :: [SaxElement] -> Int -> Int f (SaxElementOpen name _ : rest) i = f rest i f (_ : rest) i = f rest i f [] i = i Thanks for you thoughts! -- Daniil Elovkov

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Re: Move MonadIO to base
by Anders Kaseorg 20 Apr '10

20 Apr '10

On Sun, 18 Apr 2010, wren ng thornton wrote: > lift ma = morph (\k -> join (fmap (k . return) ma)) Monad laws simplify that to lift ma = morph (\k -> ma >>= k . return) > The type of morph requires us to Church-encode things needlessly; what > we mean to say is: morph (fmap return ma). Hmm. If I understand this (and your other emails) correctly, you’re saying my interface class Monad m => MonadMorphIO m where morphIO :: (forall b. (m a -> IO b) -> IO b) -> m a should be equivalent to a simpler interface class Monad m => MonadJoinIO m where joinIO :: IO (m a) -> m a via some isomorphism like morphIO f = joinIO (f return) joinIO m = morphIO (\w -> m >>= w) I would be very happy to get the simpler interface to work, because it’s Haskell 98. However, if I write joinIO m = morphIO (\w -> m >>= w) morphIO' f = joinIO (f return) and define catch using morphIO' instead of morphIO: m `catch` h = morphIO $ \w -> w m `Control.Exception.catch` \e -> w (h e) m `catch'` h = morphIO' $ \w -> w m `Control.Exception.catch` \e -> w (h e) then catch' fails to actually catch anything: *Main> throwIO NonTermination `catch` \NonTermination -> return "moo" "moo" *Main> throwIO NonTermination `catch'` \NonTermination -> return "moo" *** Exception: <<loop>> Am I doing something wrong? Anders

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Continual delivery status notification failures
by Ivan Miljenovic 20 Apr '10

20 Apr '10

Can the owner of c10b66c97b5cd09384aa9f82ecd95c2b(a)orangeat.blackberry.com please fix their emails up (or the haskell-cafe admins remove that address), as I keep getting delivery status notification failure messages over a week after I send an email to the list (I would have emailed that address directly, except if they can't get -cafe they probably can't get any emails...). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Strange error with type classes + associated types
by Brent Yorgey 20 Apr '10

20 Apr '10

Hi all, Consider the following declarations. > -- from vector-space package: > (*.*) :: (HasBasis u, HasTrie (Basis u), > HasBasis v, HasTrie (Basis v), > VectorSpace w, > Scalar v ~ Scalar w) > => (v :-* w) -> (u :-* v) -> u :-* w > > -- my code: > data Affine v = Affine (v :-* v) v > > instance (HasBasis v, HasTrie (Basis v), VectorSpace v) => Monoid (Affine v) where > mempty = Affine idL zeroV > mappend (Affine a2 b2) (Affine a1 b1) = Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) When I try to compile this, I get the following error: No instance for (HasTrie (Basis u)) arising from a use of `*.*' at Diagrams.hs:107:50-58 Possible fix: add an instance declaration for (HasTrie (Basis u)) In the first argument of `Affine', namely `(a2 *.* a1)' In the expression: Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) In the definition of `mappend': mappend (Affine a2 b2) (Affine a1 b1) = Affine (a2 *.* a1) (lapply a2 b1 ^+^ b2) This seems bizarre to me; it seems like GHC ought to be able to infer that in my use of (*.*), u,v, and w are all instantiated to the v in the instance declaration, and hence all the required constraints are satisfied. I have no idea why it would be complaining about u --- there's nothing called u in my instance declaration. Can someone more well-versed in the intricacies of type checking with associated types explain this? Or is this a bug in GHC? -Brent

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