January 2013 - Haskell-Cafe

Heads up: planned removal of String instances from HTTP package
by Ganesh Sittampalam 30 Jan '13

30 Jan '13

Hi, tl;dr: I'm planning on removing the String instances from the HTTP package. This is likely to break code. Obviously it will involve a major version bump. The basic reason is that this instance is rather broken in itself. A String ought to represent Unicode data, but the HTTP wire format is bytes, and HTTP makes no attempt to handle encoding. This was discussed on the libraries@ list a while back, but I'm happy to discuss further if there's a general feeling that this is a bad thing to do: http://www.haskell.org/pipermail/libraries/2012-September/018426.html I will probably upload the new version in a week or two. Cheers, Ganesh

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Darcs Hacking Sprint #8 (February 15-17, Paris)
by Guillaume Hoffmann 30 Jan '13

30 Jan '13

Dear hackers, the Darcs project is glad to announce that the next Hacking Sprint will take place in Paris on February 15-17, at the IRILL (near Place d'Italie). Please check out the details at: http://darcs.net/Sprints/2013-02 Here are three things to know 1. Everybody is welcome to join us. We'd love to have you, whatever your Haskell or Darcs hacking experience. Also, if you've got a wacky idea for the future of version control, or a cool use for the Darcs library, you should join us too :-) 2. Please let us know if you're attending. Just add your name to http://wiki.darcs.net/Sprints/2013-02 and you're good to go. You can also send us an email. 3. We can reimburse travel costs (within reason!). Let us know if you'd like a reimbursement, and save your receipts. Many thanks to everybody who participated in our fundraising drives or who gave money on the side. Thanks also to the Software Freedom Conservancy for making fundraising and reimbursements so painless! If you can't join us in person, but you'd like to cheer us on, say hello at http://darcs.net/Donations ! Looking forward to seeing you! Guillaume

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Handling exceptions or gracefully releasing resources
by Thiago Negri 30 Jan '13

30 Jan '13

`Control.Exception.bracket` is a nice function to acquire and release a resource in a small context. But, how should I handle resources that are hold for a long time? Should I put `Control.Exception.finally` on every single line of my finalizers? What exceptions may occur on an IO operation? Every IO function has the risk of throwing an exception? Thanks, Thiago.

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Uniplate and rewriting with different types
by Chris Mears 29 Jan '13

29 Jan '13

Hi all, I have a question about the Uniplate library, regarding rewriting with transformations that have different types. With the following type, and transformation functions: data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1,t2,t3 :: Even -> Maybe Even t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 (EvenOne (OddZero x)) = Just $ EvenZero (OddOne x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing it is easy to combine the transformations into a single transformation, because they all have the same type. The result can then be passed to the Uniplate's "rewriteBi" function: allts x = t1 x `mplus` t2 x `mplus` t3 x example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBi allts example But if one of the transformations has a different type, you can't do it this way. For instance, redefine t2 to have a different type: t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing and you are stuck because the functions of different types can't be combined into a single transformation. My question is: is there a good way to combine the transformation functions if they have different types? I have come up with a possible solution (see below), but I am not sure that it is the right approach, and it is probably inefficient. Chris Mears {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Control.Monad import Control.Monad.State import Data.Generics import Data.Generics.Uniplate.Data data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing -- The transformers are wrapped in an existential type. data WrappedTransformer from = forall to. (Data to, Biplate from to) => WrappedTransformer (to -> Maybe to) -- Apply a single transformation, and return "Just x" if the -- transformation fired (where x is the result of the rewriting), and -- "Nothing" if no transformation fired. rewriteBiCheck :: Biplate from to => (to -> Maybe to) -> from -> Maybe from rewriteBiCheck f e = case runState (rewriteBiM check e) False of (e', True) -> Just e' (_, False) -> Nothing where check x = case f x of Nothing -> return Nothing Just y -> put True >> return (Just y) -- Apply a list of wrapped transformations until no more -- transformations can be applied. rewriteBiList :: forall from. [WrappedTransformer from] -> from -> from rewriteBiList transformers m = go transformers m where go :: [WrappedTransformer from] -> from -> from go [] m = m go ((WrappedTransformer t):ts) m = case rewriteBiCheck t m of Just m' -> go transformers m' Nothing -> go ts m -- Test case. example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBiList [ WrappedTransformer t1 , WrappedTransformer t2 , WrappedTransformer t3 ] example

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catamorphisms and attribute grammars
by Petr P 29 Jan '13

29 Jan '13

Dear Haskellers, I read some stuff about attribute grammars recently [1] and how UUAGC [2] can be used for code generation. I felt like this should be possible inside Haskell too so I did some experiments and I realized that indeed catamorphisms can be represented in such a way that they can be combined together and all run in a single pass over a data structure. In fact, they form an applicative functor. [1] http://www.haskell.org/haskellwiki/Attribute_grammar [2] Utrecht University Attribute Grammar Compiler To give an example, let's say we want to compute the average value of a binary tree. If we compute a sum first and then count the elements, the whole tree is retained in memory (and moreover, deforestation won't happen). So it's desirable to compute both values at once during a single pass: -- Count nodes in a tree. count' :: (Num i) => CataBase (BinTree a) i count' = ... -- Sums all nodes in a tree. sum' :: (Num n) => CataBase (BinTree n) n sum' = ... -- Computes the average value of a tree. avg' :: (Fractional b) => CataBase (BinTree b) b avg' = (/) <$> sum' <*> count' Then we can compute the average in a single pass like runHylo avg' treeAnamorphism seed My experiments together with the example are available at https://github .com/ppetr/recursion-attributes I wonder, is there an existing library that expresses this idea? Best regards, Petr Pudlak

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quotRem and divMod
by Artyom Kazak 29 Jan '13

29 Jan '13

Hi! I’ve always thought that `quotRem` is faster than `quot` + `rem`, since both `quot` and `rem` are just "wrappers" that compute both the quotient and the remainder and then just throw one out. However, today I looked into the implementation of `quotRem` for `Int32` and found out that it’s not true: quotRem x@(I32# x#) y@(I32# y#) | y == 0 = divZeroError | x == minBound && y == (-1) = overflowError | otherwise = (I32# (narrow32Int# (x# `quotInt#` y#)), I32# (narrow32Int# (x# `remInt#` y#))) Why? The `DIV` instruction computes both, doesn’t it? And yet it’s being performed twice here. Couldn’t one of the experts clarify this bit?

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Mac os x (Intel, 10.6.8) problem compiling yesod-core-1.1.7.1
by jean-christophe mincke 28 Jan '13

28 Jan '13

Hello GHC version : 7.4.2 When I do a cabal-dev instal yesod-core, I get the following error: Loading package blaze-builder-conduit-0.5.0.3 ... linking ... done. Loading package hashable-1.2.0.5 ... linking ... ghc: lookupSymbol failed in resolveImports /Users/V3/windev/Haskell/Z/cabal-dev//lib/HShashable-1.2.0.5.o: unknown symbol `_hashable_siphash24_sse2' ghc: unable to load package `hashable-1.2.0.5' Has anyone encountered the same problem? Thank you J-C

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Suggestiong for inter-thread communication
by Erik de Castro Lopo 28 Jan '13

28 Jan '13

Hi all, I am in the process of writing a Haskell program consisting of two threads, one for performing a calculation and one for an Ncurses UI (haskell-ncurses). The calculation thread needs to feed a stream of numbers back to the UI thread (about 1 value per second) and the UI needs to take input from the user and will pass parameter changes to the calculation thread using an IORef and atomicModifyIORef. However, I'm not sure how to hande the UI thread. The UI thread would normally wait for Ncurses input using getEvent, but I need advice on how to the the data from the calculation thread. Any advice or things to try? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Munich Haskell Meeting
by Heinrich Hördegen 28 Jan '13

28 Jan '13

Dear Haskellers, our monthly get-togethers in Munich this year start on Thursday, 31ts of January at 19h30. Our meeting venue will be Cafe Puck once again. If you want to join, please go to http://www.haskell-munich.de/dates and click the button. Of course, you can also invite all those interested in functional programming. Have a successful week, Heinrich

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GHC.Generics and newtypes
by Roman Cheplyaka 27 Jan '13

27 Jan '13

Hi, Is it possible to generate different instances for newtypes and datatypes using GHC.Generics? Roman

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