February 2010 - Haskell-Cafe

Streamed creation of STArray from list?
by Jason Dagit 11 Feb '10

11 Feb '10

Hello, I was wondering if there is a trick for generating a new STArray from a list in such a way that you do not have to hold both the list and array in memory? http://www.haskell.org/ghc/docs/latest/html/libraries/array-0.3.0.0/Data-Ar… As far as I can tell, that is the interface for creating ST Arrays. For example, newListArray looks almost perfect: *newListArray* :: (MArray<http://www.haskell.org/ghc/docs/latest/html/libraries/array-0.3.0.0/Data-Ar…>a e m, Ix<http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.2.0.0/Data-Ix.…>i) => (i, i) -> [e] -> m (a i e) If I know the length of the list, I might expect newListArray to have the memory behavior I want. In my case, the code calls (length xs) to calculate the length of the list. As I understand it, that will force the spine of the list into memory. Can this be avoided? The only trick that comes into mind is the one used by the C++ vector class where you dynamically resize the array and copy the values to the new array. That's potentially a lot of copying, right? Is some other array better suited for this, such as vector or uvector? Thanks, Jason

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Haskell and the Job Market, e.g. with Google
by Hans van Thiel 11 Feb '10

11 Feb '10

Hello, Somewhat in response to the original post about Haskell engineers I, II and III. This confirms the remark that Haskell experience is now being appreciated, though not (yet) used (very much). Steven Grant, recruiter from Google, asked me to bring to his attention anyone who might be suitable, so that's what I'm doing. <start quote> We are currently aggressively recruiting for a large number of engineers in EMEA. I spotted your extensive open source experience and was particularily interested to see you have worked with Haskell. I am not looking for a Haskell developer but more interested in people that have worked in exotic languages such as Haskell or Erlang or Scheme. The roles we have are heavily open sourced based and will be mainly working with Python, C, Linux, shell etc and are based in Dublin, London or Zurich. If you have any interest in discussing these further, drop me an email to stevengrant(a)google.com and we can discuss. <end quote> >From a second email: <start quote> The job specs are below. http://www.google.ie/support/jobs/bin/answer.py?answer=34884 http://www.google.ie/support/jobs/bin/answer.py?answer=34883 The roles are within a very specialist team within Google. They are a hybrid type role and are responsible for making our products reliable scalable and more efficient. <end quote> Get in touch with Steven: Steven Grant European IT Staffing Phone: +353 1 543 5083 Google Ireland Ltd., Barrow Street, Dublin 4, Ireland Registered in Dublin, Ireland Registration Number: 368047 I think this is interesting even to those who are not looking for a job right now, since it shows the current mind-set regarding Haskell, at a major and leading IT company. Best Regards, Hans van Thiel

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If monads are single/linearly threaded, doesn't that reduce parallelism?
by caseyh＠istar.ca 11 Feb '10

11 Feb '10

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HDBC convert [SqlValue] without muchos boilerplate
by Iain Barnett 11 Feb '10

11 Feb '10

Hi, I'm trying to get to grips with HDBC and have the following problem. When I run a query that returns a result set, each row comes back as a [SqlValue]. Naively, I thought the following function would convert a [SqlValue] into a string, but instead I get the error below. convrow2 :: [SqlValue] -> String convrow2 (x:xs) = foldl (\i j -> i ++ " | " ++ show j ) (show (fromSql x)) xs Prelude> :l TasksSimple.lhs [1 of 1] Compiling Main ( TasksSimple.lhs, interpreted ) TasksSimple.lhs:126:65: No instance for (convertible-1.0.5:Data.Convertible.Base.Convertible SqlValue a) arising from a use of `fromSql' at TasksSimple.lhs:126:65-73 Possible fix: add an instance declaration for (convertible-1.0.5:Data.Convertible.Base.Convertible SqlValue a) In the first argument of `show', namely `(fromSql x)' In the second argument of `foldl', namely `(show (fromSql x))' In the expression: foldl (\ i j -> i ++ " | " ++ show j) (show (fromSql x)) xs Failed, modules loaded: none. I tried looking at how to add an instance declaration for convertible, but was stumped. This code, however, works in GHCi. Would anyone know how to convert from [SqlValue] in a straightforward way without having to specify every field "by hand" ? I don't fancy doing this for each sql statement I need to run. convrow1 :: [SqlValue] -> String convrow1 [tasksid,title,added] = show ( (fromSql tasksid)::Integer ) ++ " | " ++ fromSql title ++ " | " ++ show ((fromSql added)::LocalTime) Any help is much appreciated, especially as I haven't looked at any Haskell in a while and wasn't any good with it before! Regards, Iain This is my set up: GHC is 6.10.4 HDBC is HDBC-2.1.1, HDBC-2.2.2, HDBC-postgresql-2.1.0.0,HDBC-postgresql-2.2.0.0 Convertible is convertible-1.0.5, convertible-1.0.8 OSX 10.6

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lazy'foldl
by Sebastian Fischer 10 Feb '10

10 Feb '10

Hello, I have implemented the following function: lazy'foldl :: (a -> b -> Maybe a) -> Maybe a -> [b] -> Maybe a lazy'foldl _ Nothing _ = Nothing lazy'foldl _ m [] = m lazy'foldl f (Just y) (x:xs) = lazy'foldl f (f y x) xs After hoogling its type, I found that Control.Monad.foldM :: (a -> b -> Maybe a) -> a -> [b] -> Maybe a seems like a perfect replacement because lazy'foldl f (Just x) xs == foldM f x xs holds for all finite lists xs. Here is an inductive proof: lazy'foldl f (Just x) [] == Just x == foldM f x [] lazy'foldl f (Just x) (y:ys) == lazy'foldl f (f x y) ys (if f x y == Nothing) == lazy'foldl f Nothing ys == Nothing == Nothing >>= \z -> foldM f z ys == f x y >>= \z -> foldM f z ys == foldM f x (y:ys) lazy'foldl f (Just x) (y:ys) == lazy'foldl f (f x y) ys (if f x y == Just z) == lazy'foldl f (Just z) ys (induction) == foldM f z ys == Just z >>= \z -> foldM f z ys == f x y >>= \z -> foldM f z ys == foldM f x (y:ys) I think the above equation holds even for infinite lists xs. Both functions terminate on infinite lists, if the accumulator is eventually Nothing. Do you see any differences in terms of strictness, i.e., a counter example to the above equation that involves bottom? I don't. Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Generate random UArray in constant memory space.
by Vasyl Pasternak 10 Feb '10

10 Feb '10

Hello Cafe, I tried to generate memory-efficient list of random numbers, so I've used uvector library for this task. But it doesn't work, it still allocates more than 6Gb of memory for the random list of 10 million elements. Here is the code: > import Text.Printf > import System.Random > import Control.Applicative > import System.Environment > import Data.Array.Vector > randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) > randomListU b g size = unfoldU size gen g > where gen g = let (x, g') = randomR b g > in JustS (x :*: g') > main = do > [size] <- map read <$> getArgs > let ints = randomListU (-10, 10) (mkStdGen 1) size > printf "%d\n" (sumU ints) Could someone give a hint, how to implement this function in constant memory space? Thank you in advance. Best regards, Vasyl Pasternak

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Two GET HTTP requests
by Chris Eidhof 10 Feb '10

10 Feb '10

Hi everyone, I'm trying to do a number of successive HTTP requests in one program. Here's what I tried: Approach 1: I used the 'download' package, which failed to install on OS X. It fails with "error: libio.h: No such file or directory". Approach 2: I installed the 'download-curl' package, and tried again. This seems to fail on the following example: > import Network.Curl.Download > > main = do x <- openURI "http://haskell.org" > y <- openURI "http://haskell.org/hoogle" > return () If I put a print statement around the second line of the do-statement it looks like openURI never returns. Approach 3: I used the simpleHTTP function from the HTTP package. This crashed, after I dug a little deeper into the code, it threw an error on calling the parseURI function (openFile: no such file exists). I installed the latest network package and upgraded my HTTP package, and the parseURI error went away. I felt like I was almost there, and tried the following: > simpleHTTP (getRequest "http://haskell.org") This failed with just the text "Bus error". I searched the HTTPBis git repository, but couldn't find the text "Bus error". I don't have a clue of how to fix this. I'm a bit stuck here, I would love to help fix the errors, but don't know what would be the best place to begin. If anyone can point me in the right direction, I will try to patch at least one of these packages. Thanks, -chris

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Lazy language on JVM/CLR
by Tony Morris 09 Feb '10

09 Feb '10

I have hypothesised a pure, lazy language on the JVM and perhaps the .NET CLR with FFI to .NET/Java libraries. I foresee various problems but none that are catastrophic; just often requiring a compromises, sometimes very unattractive compromises. I have authored several libraries in the same vain as pure, lazy programming to run on the JVM in Java and Scala programming languages. I expect others have forethought and perhaps even experimented with such a language. Are there any dangers to be wary of that undo the entire endeavour? Thanks for any insights. -- Tony Morris http://tmorris.net/

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Jobs
by Roderick Ford 09 Feb '10

09 Feb '10

We need two developers at the company I work for; that is, I am NOT a recruiter. My boss is asking me to search for developers for me to work with ... like minded, haskell loving, driven, c/c++ (c# if we have to) for an INITIALLY c# full-time salaried position in Arizona (or FL or UT). He is open to starting new projects using Haskell, where applicable. So, he is involving me in the interview process, and I've been less than pleased with candidate selection so far. I told him we need to find people that see the beauty of mathematics and Haskell. If you have a resume you want me to forward on, reply to me directly and I will forward on through the review process. Here is a clip from posting 1 (don't take too literally): Software Engineer with strong 'Design, Programming, Debugging' experience Required Experience 3+yrs MS C# *AND* C++ MS SQL Server MS ASP.NET MS SQL Applicants must have at least 3 years practical work experience designing and building commercial software products that have shipped on the Windows OS platform. Requires proven experience building Web GUI on Windows platform, familiarity with distributed programming and networking. Must also be able to write and debug SQL Stored Procedures. Developer will be required to take ownership of the assigned modules through design, code & unit test. Past team lead experience would be valuable. Familiarity with gcc and Linux / Unix programming would be a plus. 4 year degree in a Computer Science related field with at least 5-7 years of experience in a software development environment or similar combination of education/work experience. This position is available in the following offices: Scottsdale, Arizona Provo, Utah Melbourne, Florida Here is a clip from posting 2: Job Description As a Senior C# - Asp.net Software Engineer in this position you will operate within a group of engineers, engaged in product design and implementation of enterprise applications. This role has a very high level of technical and product knowledge spanning multiple development technologies and target platforms. The Senior Software Engineer operates unsupervised and is a self-starter. Essential Duties and Responsibilities: Performs OOD, implementation and maintenance for products composed with a variety of technologies, mostly for Windows platforms but also Linux and other platforms Provides expertise for multiple software layers and levels of abstraction: UI, business layer, database, and distributed operations. Builds a tactical consensus with the development team whilst demonstrating strong technical leadership skills Experience working in a scrum agile environment a plus Performs highly complex product design activities which may require extensive research and analysis Performs complex bug verification, release testing and beta support for assigned products Researches problems discovered by QA or product support and identifies solutions to the problems Conducts unit testing and integration testing for functionality and performance limits In absence of product architects, may serve as a product architect Researches and understands non-functional requirements beyond the marketing requirements for a product Researches new technology or development tools to remain informed of current technology Required Skills Advanced technical knowledge in software development methodologies, design and implementation in C#. Expert analytical and design skills at multi-product/multi-environment level. Understands cross-platform and/or internationalisation development. In-depth experience in research and analysis, project planning and implementation. Excellent understanding of entire development process, including specification, documentation and quality assurance. Aware of business issues as they impact overall project plans. Recognised reference and central point for technical issues across groups and products. Strong communications and interpersonal skills. Advanced PC skills. Required Experience A university degree in computer science or software engineering, preferably a masters or higher, is desireable. Requires 10 to 15 years of experience in product design Good knowledge of .Net, and ASP.NET, for C# developers. Minimum 5 Years Experience of C# language and Asp.net

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Navigating Graphs
by Matthias Görgens 09 Feb '10

09 Feb '10

Using something like zippers it is easy to navigate an acyclic graph in O(1) operations per arc you follow. Inspired by Chris Okasaki's work on queues I wondered if there is a similar approach to navigating cyclic graphs. If the graph you are navigating is static (i.e. does not have to support addition or removal of vertices in any reasonable amount of time), I guess you can get away with "Tying the know" (http://www.haskell.org/haskellwiki/Tying_the_Knot) But is there a technique that allows navigation, insertion and removal at focus in (at least amortised) O(1) operations each? As a generalisation being able to have multiple points of focus (in an acyclic graph for a start) would also be interesting. Any thoughts?

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