September 2009 - Haskell-Cafe

[Haskell-beginners] Better way to retrieve data strucures from sql-db?
by Paolo Losi 30 Sep '09

30 Sep '09

Hi Roman, Template Haskell should be the preferred way to cope with these "boilerplate" code problems. Template Haskell: http://www.haskell.org/haskellwiki/Template_Haskell Note: I'm a beginner myself so please wait for a more informed response. Paolo 2009/9/28 Roman Naumann <roman_naumann(a)fastmail.fm>: > The following function converts the data read from an sql database to a > player data structure. > > toPlayer :: [SqlValue] -> Player > toPlayer sx = Player { > plID = fromSql (sx!!0), > plAccount = fromSql (sx!!1), > plForename = fromSql (sx!!2), > plSurename = fromSql (sx!!3), > plPos_x = fromSql (sx!!4), > plPos_y = fromSql (sx!!5) > } > > > It appears to me, that I have to write this boilerplate code every time I > introduce a new data structure that has to be read from the sql database. > Can you think of a way to automate this? > > Thanks for your replies, > Roman Naumann

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Debugging Haskell code
by Paul Moore 29 Sep '09

29 Sep '09

I'm still playing round with my random dieroll generation program. In doing so, I just hit a segmentation fault (I didn't think Haskell could *cause* a segfault!) I'm sure it's my code - I got this to compile by fiddling with types until the errors (which I didn't understand) went away. Certainly not the right way to code, I know, but never mind. The problem is that I have *no idea* how to begin debugging this. In C, Python, or any other imperative language, I'd put traces in, etc. But in Haskell, I don't even know where to start. I attach the code below. While help in the form of pointers to what I did wrong would of course be appreciated, what I'm really looking for is a pointer on how I'd find out for myself. (Hey! I just read the bit in the ghc manual which says if I am not using foreign or unsafe functions, a crash is a compiler bug. Did I find a compiler bug?) My code is below. All I did is ghc --make hist_3d6.hs, then run hist_3d6.exe. This is ghc 6.10.4 on Windows Vista Home 32-bit. Thanks for any help, Paul. import System.Random.Mersenne import qualified Data.Map as Map import Data.Map (Map) import Data.List takes :: Int -> [a] -> [[a]] takes n [] = [] takes n xs = take n xs : takes n (drop n xs) sums :: Num a => Int -> [a] -> [a] sums n xs = map sum (takes n xs) simulate :: Int -> IO [Double] simulate count = do gen <- newMTGen Nothing dice <- (randoms gen :: IO [Double]) return (take count dice) histogram :: Ord a => [a] -> [(a,Int)] histogram = Map.assocs . foldl' f Map.empty where f m k = Map.insertWith' (+) k 1 m simulation = do lst <- simulate 100000 return lst --return (histogram lst) main = do s <- simulation putStrLn (show s)

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Strong duck typing / structural subtyping / type class aliases / ??? in Haskell
by oleg＠okmij.org 29 Sep '09

29 Sep '09

Alp Mestan wrote: > Indeed, OCaml has stuctural polymorphism, it's a wonderful feature. > > *# let f myobj = myobj#foo "Hi !";; > val f : < foo : string -> 'a; .. > -> 'a = <fun>* And Haskell has that too: > -- This is how we define labels. > data Field1 deriving Typeable; field1 = proxy::Proxy Field1 > > -- This is how record selection looks like. > foo f = f # field1 The inferred type of foo is *OCamlTutorial> :t foo foo :: (HasField (Proxy Field1) r v) => r -> v It doesn't seem too different from the OCaml's type; the type variable r acts as a row type. The quoted example is the first from many others described in http://darcs.haskell.org/OOHaskell/OCamlTutorial.hs The file quotes at length OCaml's Object tutorial and then demonstrates how the OCaml code can be written in Haskell. When it comes to objects, structural subtyping, width and depth subtyping, etc., Haskell does not seem to miss match compared to OCaml. In contrast, Haskell has a few advantages when it comes to coercions (one does not have to specify the type to coerce to, as Haskell can figure that out). The other files in that directory give many more example of encoding C++, Eiffel, OCaml patterns.

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Hackage bug? Autobuild failure on literate source with ignored code blocks.
by John Millikin 29 Sep '09

29 Sep '09

According to <http://www.haskell.org/haskellwiki/Literate_programming>, the following should compile properly because the second block of code will be ignored by GHC: \begin{code} main = putStrLn "Hello world!" \end{code} \begin{code}% main = -- TODO \end{code}% However, Hackage's automatic build system seems to pre-process the literate source in such a way that the ignored block is included in the .hs source, resulting in a compilation error: http://hackage.haskell.org/packages/archive/dbus-core/0.2/logs/failure/ghc-… dist/build/tmp23596/DBus/Bus/Connection.hs:119:12: Empty 'do' construct The block in question is a TODO feature that should be included in documents generated from the LaTeX source, but does not build and shouldn't be parsed as Haskell: \begin{code}% -- TODO tcp :: A.Address -> Transport tcp a(a)(A.Address _ params) = handleTransport a connect' where connect' = do -- check host -- check port -- check family -- return handle \end{code}% Although Hackage fails to build the package, it works fine on my local system when I run "cabal configure && cabal build && cabal haddock". Is this a bug in the Hackage auto-build mechanism, or have I done something wrong when uploading the package?

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double colon equal
by xu zhang 29 Sep '09

29 Sep '09

Hi, is there anyone can tell me what does the ::= mean?And what does the code below mean? expr ::= expr addop term | term Thank you in advance!

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Exciting job opportunity
by siki 29 Sep '09

29 Sep '09

Principal investment firm based in Manhattan is looking for an outstanding software developer to maintain and develop proprietary accounting and portfolio management systems. Job duties will include coding projects as well as management of outsourced system maintenance. Candidates should have at least a bachelor’s degree in computer science from a top university and impeccable coding style. The majority of the programming is done in JAVA at present, so candidates must have extensive experience with JAVA and related technologies but we are always experimenting with the most exciting technologies (e.g. Haskell) and you will be given freedom in choosing suitable programming languages and technologies. Experience working with SQL is a must and familiarity with functional programming is a plus. We are looking for a candidate who shows strong analytical, organizational, and communication skills. Attention to detail is essential. We are a growth oriented firm that values people who take a craftsman’s pride in their work. Candidates must have a desire to constantly improve one's knowledge of programming and the financial markets. A true love of building quality software and a team spirit is strongly recommended. Background in accounting is not necessary but is definitely a plus. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. Please send your CV and cover letter to recruitment(a)karamaan.com, thank you. -- View this message in context: http://www.nabble.com/Exciting-job-opportunity-tp25667801p25667801.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Instances for Data.Text
by Pasqualino "Titto" Assini 29 Sep '09

29 Sep '09

2009/9/29 Paulo Tanimoto <tanimoto(a)arizona.edu>: > Hi Bryan and others, > > On Mon, Sep 28, 2009 at 5:29 PM, Bryan O'Sullivan <bos(a)serpentine.com> wrote: >> bytestring predates the other two libraries by several years. The underlying >> stream type for uvector and text are almost the same, so they could in >> principle be merged. There's a fair amount of duplication there, but uvector >> is in some ways more complicated and in others much less thorough than text. >> Merging them would be a lot of work! >> > > If I may free-ride on this thread: how should one go about deriving a > Data.Binary instance for text? It looks like doing it efficiently > would require using some parts of the internal module that are not > exposed, am I correct? I've been using "encodeUtf8", but that doesn't > feel right. I don't know what to do, hopefully I'm missing something > simple. This is a good point, I also need to make Data.Text an instance of a few basic classes and I am not sure that I did it correctly. So far I have: import Data.Text instance Binary Text where put = put . encodeUtf8 get = liftM decodeUtf8 get -- DOUBT: Is this correct also for Data.Text.Lazy ? instance NFData Text instance Serial Text where -- DOUBT: is this efficient? series d = [T.pack (series d :: String)] -- DOUBT: how to define this coseries rs = error "coseries" More in general: what is the right policy for instances definition? Should the library author provide them, at least for the most common and appropriate classes (at the cost of adding some additional dependencies) ? Should they go in a separate package? Should the Haskell Platform team provide some guidance on this point to library authors? titto

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ANN: atom-0.1.1
by Tom Hawkins 29 Sep '09

29 Sep '09

Atom is a Haskell DSL for designing hard real-time embedded applications. At Eaton, we use it for automotive control systems. An Atom description is composed of a set of guarded atomic actions that operate on a global program state. Atom makes it easy to manage program concurrency without the need for mutex locking and run-time task scheduling. As such, Atom can eliminate the need and overhead of an RTOS, which traditionally serves these functions. This release includes several improvements to C code generation, including a simplified rule scheduler that consumes less memory and execution time. It also ships with a new unit testing framework that allows the use of assertions and functional coverage points to assist with program verification. http://hackage.haskell.org/package/atom

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How to decide if a number is an integer?
by Magicloud Magiclouds 29 Sep '09

29 Sep '09

Hi, In other weak-type language, `round i == i` would work. But in haskell, what should I do? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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Doing people's homework?
by Michael P Mossey 29 Sep '09

29 Sep '09

I'm not really hip to the culture here so this is just an observation, but some of the recent questions posted to this list (and beginners(a)haskell.org) look a lot like someone's homework. Is anyone here concerned about avoiding giving the full answer, or maybe it's really none of our business (we aren't responsible for anyone's learning process)?

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