July 2010 - Haskell-Cafe

Weird behavior with arrow commands
by Ronald Guida 24 Jul '10

24 Jul '10

I am trying to figure out how to use GHC's arrow commands, and I found some extremely weird behavior. In GHC's manual, there is a description of arrow commands, which I don't really understand. http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html… (Primitive Constructs) I have two questions: 1. What are arrow commands supposed to do? 2. What is this code supposed to do? -- start of code -- {-# LANGUAGE Arrows #-} module Main where import Control.Arrow foo :: (b -> String) -> ((((b, Int), Float), Double) -> String) -> (b -> String) foo f g b = f b ++ " " ++ g (((b, 8), 1.0), 6.0) bar :: (t -> String) -> ((Double, Int) -> String) -> t -> String bar f g = proc x -> do (f -< x) `foo` \n m -> g -< (n) main = do putStrLn $ foo show show 17 putStrLn $ bar show show 17 putStrLn $ foo show show 42 putStrLn $ bar show show 42 -- end of code -- Output from GHCi: 17 (((17,8),1.0),6.0) 17 (6.730326920298707e-306,0) 42 (((42,8),1.0),6.0) 42 (6.730326920298707e-306,0) Output after compiling with GHC: 17 (((17,8),1.0),6.0) 17 (5.858736684536801e-270,0) 42 (((42,8),1.0),6.0) 42 (5.858736684536801e-270,0) GHC Version: The Glorious Glasgow Haskell Compilation System, version 6.12.3 Thank you -- Ron

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ANNOUNCE: Haskell Platform 2010.2.0.0
by Don Stewart 24 Jul '10

24 Jul '10

We're pleased to announce the fifth release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2010.2.0.0: http://hackage.haskell.org.nyud.net/platform/ (Caching server). The specification, along with installers (including Windows, Apple and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team

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[Haskell-cafe] a patch for base.Foreign.C.String, the current locale
by Roman Beslik 24 Jul '10

24 Jul '10

Hi. I have composed a patch that implements Foreign.C.String according to Foreign Function Interface Addendum, i.e. C strings are treated as they are encoded with the current locale. Search "Locale.hs" in the patch for further directions. Ask if you do not know how to build a patched version of the "base" library. http://www.beroal.in.ua/prg/haskell/locale.patch -- Best regards, Roman Beslik.

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Heavy lift-ing
by michael rice 23 Jul '10

23 Jul '10

Hi, I don't understand what's taking place here. >From Hoogle: ================= liftM2 :: Monad m => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Promote a function to a monad, scanning the monadic arguments from left to right. For example, liftM2 (+) [0,1] [0,2] = [0,2,1,3] liftM2 (+) (Just 1) Nothing = Nothing ================= What does it mean to "promote a function to a monad?" It would seem that the monad values must understand the function that's being promoted, like Ints understand (+). Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) Just 2 But how does one add [0,1] and [0,2] to get [0,2,1,3]? Michael

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Re: [Haskell-cafe] Heavy lift-ing
by michael rice 23 Jul '10

23 Jul '10

Thanks all, Wild, at least up to the "optional" part, which I haven't dug into yet. So the (+) for the Maybe monad and the (+) for the List monad are one in the same, the magic springs from the monads. Why is it called "lift"-ing? Michael --- On Fri, 7/23/10, Jürgen Doser <jurgen.doser(a)gmail.com> wrote: From: Jürgen Doser <jurgen.doser(a)gmail.com> Subject: Re: [Haskell-cafe] Heavy lift-ing To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Friday, July 23, 2010, 4:50 PM El vie, 23-07-2010 a las 15:05 -0400, Nick Bowler escribió: > On 11:43 Fri 23 Jul , michael rice wrote: > [...] > > But how does one add [0,1] and [0,2] to get [0,2,1,3]? > > liftM2 (+) [0,1] [0,2] gives the list > > [0+0, 0+2, 1+0, 1+2] which one could have found out by asking ghci: Prelude Control.Monad> let f a b = show a ++ " + " ++ show b Prelude Control.Monad> liftM2 f [0,1] [0,2] ["0 + 0","0 + 2","1 + 0","1 + 2"] or simpler: Prelude Control.Monad> liftM2 (,) [0,1] [2,3] [(0,2),(0,3),(1,2),(1,3)] i.e., the in the list monad, you pair each element of one list with each element of the other(s). > (recall that (>>=) in the list monad is concatMap). (>>=) = flip concatMap, to be precise. Or, concatMap = (=<<) Now let's have some fun with equational reasoning to see what liftM2 does exactly: (Only read this if you really want to!) liftM2 f a b = { definition of liftM2 } do {x <- a; y <- b; return (f x y)} = { simplified translation of do-notation } a >>= \x -> (b >>= \y -> return (f x y)) = { change (>>=) to (=<<) and flip arguments } (\x -> ((\y -> return (f x y)) =<< b)) =<< a = { specialized to the list monad } (\x -> ((\y -> [f x y])) `concatMap` b)) `concatMap` a = { change concatMap to prefix application } concatMap (\x -> concatMap (\y -> [x+y]) b) a and indeed: Prelude> concatMap (\x -> concatMap (\y -> [x+y]) [0,2]) [0,1] [0,2,1,3] with some effort, I think one can understand what happens here. It should also be clear how this is generalized to liftM3, liftM4, etc. Oh, btw, what about liftM1? Obviously, this should be the following: liftM1 f a = { definition } do { x <- a ; return f a } = { same changes as above } concatMap (\x -> [f x]) a = { definition of concatMap } concat (map (\x -> [f x]) a = { concating singletons can be simplified } map (\x -> f x) a = { eta-reduction } map f a i.e., liftM1 = map, which is indeed just fmap for lists, as already pointed out. You can use this to simplify the last line of the concatMap derivation above: concatMap (\x -> concatMap (\y -> [x+y]) b) a = { see above } concatMap (\x -> map (\y -> x+y) b) a = { use operator section } concatMap (\x -> map (x+) b) a which is about as clear as possible a definition for liftM2 (+) Jürgen

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HaTeX build failure
by Daniel Díaz 23 Jul '10

23 Jul '10

Hi, I uploaded a package, named HaTeX, to Hackage, but it gets a build failure: cabal: Couldn't read cabal file "./HaTeX/1.0.0/HaTeX.cabal It makes reference to 1.0.0 version, even when building version 1.0.1. I get the same error when I use cabal install. However, using configure>build>register I don't have any problem. I would like this package works well, but I don't know how to solve this problem. In fact, I don't know what's the problem. Here a link to the HaTeX package: http://hackage.haskell.org/package/HaTeX Any idea?

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cabal, Setup.lhs example
by Tom Hawkins 23 Jul '10

23 Jul '10

I have a script I'm using to generate some Haskell code for a library. How do I specify this flow in the cabal setup file? Would someone point me to a relevant library I can reference as an example? -Tom

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src/Text/XML/HaXml/Lex.hs:(156, 0)-(160, 22): Non-exhaustive patterns in function white
by Alexander Kotelnikov 23 Jul '10

23 Jul '10

Hello. I decided to run a couple of test XML-RPC applications using haxr. Everything worked fine while responces from my (actually, other people's) server were something relatively small (a number, a line), but with larger responces (starting around couple of kilobytes) my applications began to fail with the error I put into subject. Sounds like XML parsing problem. Though server gives a proper responce (I used tcpdump to see what is returned, looks like a valid XML with methodResponce). Any idea what is wrong and how this can be fixed? Thanks, Alexander

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Page rank and GHC docs directory organization
by Ryan Newton 23 Jul '10

23 Jul '10

GHC docs seem to have the problem that newer versions only gradually overtake older ones in page rank, resulting in the effect that if one uses Google to find library documentation, they may accidentally look at an old version. For example, if I google "Data.Data Haskell" the first link brings me to: http://www.haskell.org/ghc/docs/6.10.2/html/libraries/base/Data-Data.html Oops, version 6.10.2! I'm no web expert, but I think the problem is that the "latest" directory isn't used consistently by others and/or the fact that latest/ redirects to a concrete version number. Thus if you go to: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Data.html It redirects immediately to 6.12.2: http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base/Data-Data.html So is the 6.12.2 target accruing pagerank rather than the latest one? Even if someone links the /latest/ URL? If that's the problem, would it fix things just to make latest/ a full directory structure in its own right (a clone rather than redirect)? Cheers, -Ryan

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Hackage survey
by Sylvain Le Gall 23 Jul '10

23 Jul '10

Hello dear haskellers, I am working on OASIS-DB, a tool similar to Hackage and would like to have more information on how Haskellers use Hackage. I have setup a small poll (10 questions only) with the help of John Goerzen and Don Stewart. I will be very thankful that you answer this poll: http://polldaddy.com/s/2C49D15023CB88C6 I will share the data with the Haskell community, so that we will be able to make progress on Hackage and OASIS-DB. Cheers Sylvain Le Gall

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