April 2013 - Beginners

How best to do this?
by emacstheviking 03 May '13

03 May '13

I have built a library for using the Hexwax expandIO-USB chip and I have now got some code to drive a stepper motor: doOption :: HWHandle -> Flag -> IO () doOption dev (Backward n) = do putStrLn $ "> STEP BACKWARD " ++ (show n) let x = [ stepBit b | b <- [3..0]] return () where stepBit p b = setBit p b 0 >> setBit p b 1 where setBit p b s = HW.setPortBit dev p b s >> stepDelay It feels a little smelly... if I don't assign the result of the list comprehension to "x" the return type blows at me hence the return(), to get past the door steward type checker... so what would be the "ideal" way to just execute my LC to step the bits and then return. I thought of using "liftIO" but I think that would be "backwards" here if you catch my drift. I am already in the IO monad as it is, so that has confused me because liftIO lifts something "into" the IO monad and I am already there... this project is making me work harder at my Haskell and I think that once I've completed it I will be much further advanced with it... So, if anybody cares to suggest more elegant and Haskell like ways to re-factor the above code I am more than happy to know about it! All the best, Sean.

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two System.IO.Strict modules
by Hong Yang 02 May '13

02 May '13

There are two modules called the same name: System.IO.Strict, documented respectively at http://hackage.haskell.org/packages/archive/strict-io/0.2.1/doc/html/System… http://hackage.haskell.org/package/strict-0.3.2 My questions are: 1) Will these two modules coexist in the same version of GHC? Or put it another way, both modules' .tar.gz files have System/IO/Strict.hs inside, will they overlap in physical disk? From where can I tell which locations they will be installed to? 2) If yes, which module will "import System.IO.Strict" actually use?

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Broken packages after upgrading to Ubuntu 13.04
by Johannes Engels 01 May '13

01 May '13

Hello, using the OS-specific Haskell packages of the Ubuntu distribution, I have some cabal packages installed in addition, e.g. hmatrix etc. After an upgrade to Ubuntu 13.04 most of these cabal packages are broken because some dependencies do not exist anymore or have been upgraded also, respectively. With ghc-pkg check I get messages like There are problems in package hmatrix-0.14.0.1: dependency "array-0.4.0.0-0b32f6f98c0297dbb0e5cfc0087bd1f0" doesn't exist dependency "base-4.5.0.0-c8e7184681d410015e93df85fc49e9dd" doesn't exist .... Actually now array-0.4.0.1 is installed etc. What is the recommended way to repair this? Should I do a --reinstall of the cabal packages (which should mostly also work with the upgraded dependencies)? Any hint would be welcome! Best regards Johannes Engels

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question about shared data types
by Ian Knopke 30 Apr '13

30 Apr '13

Hi everyone, I have two modules that share a common data type in their respective data structures: package Aaa where newtype Foo = Foo String data Bar = Bar { barFoo :: [Foo] } deriving (Show, Eq) ... package Bbb where newtype Foo = Foo String data Quux = Quux { quuxFoo :: [Foo] } deriving (Show, Eq) ... Both modules require the Foo newtype defined. However, in all other respects they are separate modules that can be used independently. Defining the newtype Foo twice, as above, throws a compilation error (ambigious reference). Importing one module inside another breaks independence. I've been including newtype Foo in a Common.hs module that is included in both, as I would in C or C++. This sort of thing can easily get more complicated as the program grows however. I was wondering if the Haskell community has a better solution to this that I've overlooked. Ian Knopke

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Segmentation fault
by Michael Peternell 30 Apr '13

30 Apr '13

Hi, sometimes when I start ghci, I just get a segmentation fault. Sometimes I get even 15 segmentation faults in a row, and sometimes I can open it many times and it works each time. I downloaded the Haskell Platform for Mac, 64Bit, from http://www.haskell.org/platform/mac.html I verified the SHA1-Sum, btw. I tried reinstalling the platform, and I tried deleting the ~/.ghc directory, but nothing works. I didn't do anything fancy with Haskell, I just downloaded it yesterday and I didn't tamper with any haskell-files outside of my home-directory. The only kernel extensions I have installed are the ones from VMWare Fusion and maybe flux. (i'm not sure if flux involves kernel extensions or how deeply it messes with the OS.) michael@MacMichi:~ $ rm -rf .ghc michael@MacMichi:~ $ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Segmentation fault michael@MacMichi:~ $ uname -a Darwin MacMichi.local 10.8.0 Darwin Kernel Version 10.8.0: Tue Jun 7 16:32:41 PDT 2011; root:xnu-1504.15.3~1/RELEASE_X86_64 x86_64 michael@MacMichi:~ $ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Segmentation fault michael@MacMichi:~ $ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> sum (takeWhile (<10000) (filter odd (map (^2) [1..]))) 166650 Prelude> Any insights on what is really going on behind the scenes will be appreciated. Regards, Michael

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Sqlite3 pragma statements
by Paul Higham 29 Apr '13

29 Apr '13

I am using sqlite3 as a backend database for an application that I am writing in Haskell. In the database schema there are a number of foreign key constraints but sqlite3 has the referential integrity constraint checking turned off by default. To turn on the constraint checking one needs to have issued the following command pragma foreign_keys = ON; for every connection to the database. This works if done directly in the sqlite3 shell, but I want to do in from Haskell code. The obvious thing to try would be runRaw conn "pragma foreign_keys = on" and this does in fact run with no errors. However the constraint checking does not appear to be happening since I can subsequently add rows to a table that violate the foreign key constraint. I have found no documented way to issue pragma commands in either Database.HDBC or Database.HDBC.Sqlite3, nor has a google search turned up anything that works. Does anyone have any ideas? Thanx! :: paul

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sequential code linked against nonthreaded lib vs. concurrent/parallel code linked against threaded runtime lib, both on one core
by Hong Yang 27 Apr '13

27 Apr '13

Hi, I am reading "Choosing the Right Runtime" section of the "Real World Haskell" book, and having a question. There are typically four cases as follows: case # code linking RTS -N 1 sequential non-threaded 1 2 concurrent/parallel non-threaded 1 3 concurrent/parallel threaded 1 4 concurrent/parallel threaded > 1 Case #4 may or may not be faster than #3. But #3 is definitely slower than #1 and #2, due to overhead from threaded runtime lib. Now my question is, how does #2 compare with #1? (#2 must make sense, otherwise it would not be an option, and the book authors would not have specifically talked about it in one section.) Are the authors implying #2 may be potentially much faster than #1? If there were no such potential, one would just write sequential codes without attempting concurrent/parallel style, under the circumstances where his job is limited to one core. Thanks, Hong

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Haskell training in San Francisco Bay Area and New York
by Duncan Coutts 26 Apr '13

26 Apr '13

Well-Typed are offering Haskell courses in the San Francisco Bay Area and New York in early June. They are for professional developers who want to learn Haskell or improve their skills. There is a 2-day introductory course and a 2-day advanced course. Full course and registration details: http://www.well-typed.com/services_training Well-Typed are running these courses in partnership with FP Complete and Skills Matter. Locations, dates ================ San Francisco Bay Area * Introductory Course: June 4-5th, 2013 * Advanced Course: June 6-7th, 2013 New York * Introductory Course: June 10-11th, 2013 * Advanced Course: June 12-13th, 2013 * Early bird discount before April 29th -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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How to avoid repeating a type restriction from a data constructor
by gs 25 Apr '13

25 Apr '13

In http://code.accursoft.com/binding/src/c13ccbbec0ba8e326369ff2252863f20a891e… there's a data constructor data Source v a = Variable v => Source {bindings :: v [Binding a], var :: v a} Source is used many times in this file and in http://code.accursoft.com/binding/src/c13ccbbec0ba8e326369ff2252863f20a891e…, and each time, I have to repeat the context Variable v. Is there any way to avoid this redundancy?

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What is the functional way of implementing a function that takes a long time to execute?
by Costello, Roger L. 25 Apr '13

25 Apr '13

Hi Folks, Suppose a function takes a long time to do its work. Perhaps it takes minutes or even hours to complete. While it is crunching along, it would be nice to have some insight into its status such as (1) how close is it to completing? (2) what part of the task is it currently working on? It might even be nice to be notified when it is finished. What is the functional way of implementing the function? /Roger

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