January 2015 - Beginners

abstracting parametrized record types
by Alia 30 Apr '15

30 Apr '15

Hi Folks, Here's a problem that's frustrating me: I have defined a parametrized record type which looks like this: data Job a = Job { jobID :: String , jobName :: String , jobTable :: String , jobSource :: IO [a] , jobProcessors :: [a -> a] } where a is a data model type that maps on to a database table, for example: data Person = Person { name :: String , age :: Int } deriving (Show) data Car = Car { brand :: String , value :: Double , year :: Int } deriving (Show) I would like to define a higher-level record type which contains, for configuration purposes, a certain set of jobs for execution. Let's say we call it a JobSet and which could possibly look like this: data JobSet = JobSet { jobsetID :: String , jobsetName :: String , jobs :: [Job] <-- yes I know this is not legal } Is there a legal haskell way to achieve the above objective without having to do something like this which hardcodes the model type into the jobset schema? data JobSet = JobSet { jobsetID :: String , jobsetName :: String , personJobs :: [Job Person] , carJobs :: [Job Car] } Many thanks for any enlightenment on this front. Alia

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Avoiding stack space overflow
by Hans Georg Schaathun 11 Mar '15

11 Mar '15

Hi, can someone give some hints on how to get around a stack space overflow? My problem is with the training function for a neural network: trainNetwork :: Double -> Samples -> Int -> Network -> Network trainNetwork _ _ 0 n = n trainNetwork eta samples c n = trainNetwork eta samples (c-1) $! epoch eta n samples epoch :: Double -> Network -> Samples -> Network So trainNetwork runs epoch c times, each time taking a Network in and modifying the Network as output. Clearly, space complexity can be made constant in c, but I get stack overflow if and only if c is too large. As you can see, I have tried to make the epoch evaluation strict ($!). I have also tried bang patterns on the input parameter n, and I have tried rewriting with foldr/foldl/foldl', and I have tried switchin the inner and outer calls (epoch vs. trainNetwork), all to no avail. I reckon this loop like pattern should be fairly common ... does it have a common solution too? TIA -- :-- Hans Georg

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Cannot find cause of space leak
by YCH 01 Feb '15

01 Feb '15

Hello Haskellers, http://lpaste.net/119693 ~~~ {-# LANGUAGE TemplateHaskell #-} import Test.Tasty import Test.Tasty.TH import Test.Tasty.QuickCheck main :: IO () main = defaultMain tests tests = $(testGroupGenerator) -- Problem 15 -- -- (**) Replicate the elements of a list a given number of times. -- -- Example: -- -- * (repli '(a b c) 3) -- (A A A B B B C C C) -- -- Example in Haskell: -- -- >>> repli "abc" 3 -- "aaabbbccc" prop_repli xs n = a && b where types = ((xs::[Int]), (n::Int)) ys = repli xs n ys' = groupByN ys n where groupByN [] _ = [] groupByN zs m = take m zs : groupByN (drop m zs) m a = and $ fmap ((==n).length) ys' b = and $ fmap f ys' where f (x:xs) = all (==x) xs repli :: [a] -> Int -> [a] repli [] _ = [] repli (x:xs) n | n <= 1 = (x:xs) | otherwise = repli' x n ++ repli xs n where repli' x' 0 = [] repli' x' n' = x':repli' x' (n'-1) ~~~ > prop_repli xs n = a && b If I set 'n' as ' n>=1', it's ok. But with just n, does not terminate. Any advice would be appreciated. Thanks. -- YCH

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Cabal problems - as usual..
by Gregory Guthrie 31 Jan '15

31 Jan '15

I have a very basic usage pattern and installation on a few machines, and yet continually run into the often discussed cabal-maze of problems on all of them. I have read all of the many explanations of why it is a hard problem, but so far the only solution I have heard for repair is to erase the installed library files, and start over. This can be hard if one has any external libraries installed which were themselves a lot of work to get functioning, an example are graphics libraries (like WxWin). Has anyone created any scripts which would note everything which is installed, clear out all libraries, and then try to re-install everything? (This is on Windows....) Or would this typically just run into the same problems? Doing a cabal update, it notes a new version of cabal -install, so trying that fails, and now it looks like cabal itself is broken (no installs work). Any short-term \remedies, and/or long term better approaches? >cabal update Downloading the latest package list from hackage.haskell.org Skipping download: Local and remote files match. Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install >cabal install cabal-install Resolving dependencies... cabal: Could not resolve dependencies: trying: Cabal-1.20.0.2/installed-a16... (user goal) trying: ghc-syb-utils-0.2.3/installed-f83... (user goal) next goal: ghc (dependency of ghc-syb-utils-0.2.3/installed-f83...) rejecting: ghc-7.8.3/installed-a4e... (conflict: Cabal==1.20.0.2/installed-a16..., ghc => Cabal==1.18.1.3/installed-421...) rejecting: ghc-7.4.2/installed-4d6... (conflict: ghc-syb-utils => ghc==7.8.3/installed-a4e...) Backjump limit reached (change with --max-backjumps). Note: when using a sandbox, all packages are required to have consistent dependencies. Try reinstalling/unregistering the offending packages or recreating the sandbox.

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install trouble: diagrams
by Gregory Guthrie 31 Jan '15

31 Jan '15

Trying to install Diagrams, but am stuck in a typical Cabal maze of install errors; any hints or help? >cabal install Diagrams Resolving dependencies... Configuring arithmoi-0.4.1.1... Building arithmoi-0.4.1.1... Preprocessing library arithmoi-0.4.1.1... ... <no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM [ 1 of 34] Compiling Math.NumberTheory.Primes.Sieve.Indexing ( Math\NumberTheory \Primes\Sieve\Indexing.hs, dist\dist-sandbox-b31b5bcf\build\Math\NumberTheory\Primes\Sieve\Indexing.o ) <no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM Failed to install arithmoi-0.4.1.1 cabal: Error: some packages failed to install: arithmoi-0.4.1.1 failed during the building phase. The exception was: ExitFailure 1 diagrams-1.2 depends on arithmoi-0.4.1.1 which failed to install. diagrams-contrib-1.1.2.5 depends on arithmoi-0.4.1.1 which failed to install. > cabal install llvm ... Failed to install type-level-0.2.4 cabal: Error: some packages failed to install: llvm-3.2.0.2 depends on type-level-0.2.4 which failed to install. llvm-base-3.2.0.2 failed during the configure step. The exception was: ExitFailure 1 type-level-0.2.4 failed during the building phase. The exception was: ExitFailure 1 >Cabal install type-level ... src\Data\TypeLevel\Num\Ops.hs:336:10: Illegal instance declaration for `Div10 x q' The liberal coverage condition fails in class `Div10' for functional dependency: `q -> x' Reason: lhs type `q' does not determine rhs type `x' In the instance declaration for `Div10 x q' Failed to install type-level-0.2.4 cabal: Error: some packages failed to install: type-level-0.2.4 failed during the building phase. The exception was: ExitFailure 1 ?? -------------------------------------------

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Project Euler #01 on HackerRank, Performance issue‏
by Jean Lopes 29 Jan '15

29 Jan '15

Hello everyone! I'm learning Haskell by solving simple exercises on www.hackerrank.com... The problem to be solved: https://www.hackerrank.com/contests/projecteuler/challenges/euler001 as stated in this section: https://www.hackerrank.com/environment constraints version: haskell-plataform 2013.2.0.0 time limit: 5 seconds memory: 512mb I keep getting "timed out" on some test cases (#2 and #3). Trying to process 10^5 numbers between 1 to 10^9 *seems* impossible to me (Please, prove me wrong!) here is my code: import Data.Maybe import qualified Data.ByteString.Char8 as B nearestMultipleOf :: Integral a => a -> a -> a nearestMultipleOf n k = if mod n k == 0 then n else nearestMultipleOf (n-1) k sumMultiplesOf :: Integral a => a -> a -> a sumMultiplesOf n k = foldl (+) 0 [k,k*2..nearest] where nearest = nearestMultipleOf n k solution :: Integral a => a -> a solution n = s03 + s05 - s15 where s03 = sumMultiplesOf (n-1) 3 s05 = sumMultiplesOf (n-1) 5 s15 = sumMultiplesOf (n-1) 15 main = do c <- B.getContents let ns = tail $ B.lines c putStr $ unlines $ map show $ map (solution . fst . fromJust . B.readInt) ns as always, any input is really welcome!

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GPIO/I2C/PWM and Haskell
by Michal Kawalec 29 Jan '15

29 Jan '15

Hey list, I got myself a Beaglebone and will use it to stabilize and fly a drone with Haskell. The ghc support seems good enough (text doesn't compile, everything else does), but I have a question about inputs/outputs. They are all set through sysfs, and I think a wrapper over it would be easiest and most straightforward to have. The operation of most pins goes along the lines of 'activate a pin -> do some stuff -> turn it off'. Is there some particular mechanism/library you would suggest me to look into to handle that? Any thoughts or own experience with doing similar things with Haskell? Any tips will be appreciated:) Michal

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Fixed Point function
by Animesh Saxena 27 Jan '15

27 Jan '15

I stumbled across this example as a shiny way of explaining why Lazy eval matters.... fact = fix $ \f n -> if n == 0 then 1 else n * f (n-1) fix f = f (fix f) With lazy eval, I get fact 4 = fix $ \f 4 (......) since haskell goes outside to inside I have, fac 4 = $ \f 4 (......) (fix $ \f 4 (......)) Now the first chunk can get evaluated, but what about the rest of the expression. The outer "f" from "fix f" function has the parameter (fix f). So when writing "f (fix f)" apparently f got evaluated without using this parameter. I understand that the anonymous lambda does have 4 as the param, but somehow "f (fix f)" doesn't feel complete. -Animesh

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Defining ExtensionClass (Maybe a) instance in xmonad.
by Dmitriy Matrosov 26 Jan '15

26 Jan '15

Hi. I've tried to define (Maybe a) instance for ExtensionClass from XMonad/Core.hs in such way, that extensionType value would use the same data constructor as was used for the type a itself. But the code below typechecks only, if i add (Show a) and (Read a) constraints to (Maybe a) instance definition, what makes such definition useless for types, which do not have these instances and do not want to use PersistentExtension . How can i define (Maybe a) instance without (Show a) and (Read a) constraints? > {-# LANGUAGE ExistentialQuantification #-} > import Data.Typeable > -- This one does not typecheck > --instance ExtensionClass a => ExtensionClass (Maybe a) where > instance (Show a, Read a, ExtensionClass a) => ExtensionClass (Maybe a) where > initialValue = Nothing > extensionType x = let Just i = (Just initialValue) `asTypeOf` x > in case extensionType i of > PersistentExtension _ -> PersistentExtension x > StateExtension _ -> StateExtension x Here is class definition from XMonad/Core.hs: > class Typeable a => ExtensionClass a where > initialValue :: a > extensionType :: a -> StateExtension > extensionType = StateExtension > data StateExtension = > forall a. ExtensionClass a => StateExtension a > | forall a. (Read a, Show a, ExtensionClass a) => PersistentExtension a -- Dmitriy Matrosov

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ghc failed to compile trifecta while installing idris
by Robert Dodier 25 Jan '15

25 Jan '15

Hi, I'm trying to install Idris. I am working on Ubuntu 14.04. ghc --version reports 7.6.3. After installing Haskell via apt-get, I then executed cabal update, which succeeded. I then tried cabal install idris and it eventually failed while trying to install trifecta. I then tried cabal install -v3 idris and got the following info about the trifecta problem, which I've appended to this message as a PS. Maybe I can patch Highlight.hs? How would I go about that? Unpack the tar.gz, patch the file, and repack it? For the record, I tried to follow the instructions at: https://github.com/idris-lang/Idris-dev/wiki/Idris-on-Ubuntu but ran into various errors, e.g., cabal install cabal-install failed with ExitFailure 139. Thanks for any advice. best, Robert Dodier PS. The tail end of the output of cabal install -v3 idris: [ 8 of 13] Compiling Text.Trifecta.Highlight ( src/Text/Trifecta/Highlight.hs, dist/build/Text/Trifecta/Highlight.o ) *** Parser: *** Renamer/typechecker: src/Text/Trifecta/Highlight.hs:46:15: Ambiguous occurrence `Comment' It could refer to either `Text.Blaze.Internal.Comment', imported from `Text.Blaze.Internal' at src/Text/Trifecta/Highlight.hs:35:1-26 or `Text.Parser.Token.Highlight.Comment', imported from `Text.Parser.Token.Highlight' at src/Text/Trifecta/Highlight.hs:36:1-34 Upsweep partially successful. *** Deleting temp files: Deleting: /tmp/ghc13030_0/ghc13030_0.s Warning: deleting non-existent /tmp/ghc13030_0/ghc13030_0.s link(batch): upsweep (partially) failed OR Main.main not exported; not linking. *** Deleting temp files: Deleting: /tmp/ghc13030_0/ghc13030_0.hscpp *** Deleting temp dirs: Deleting: /tmp/ghc13030_0 /usr/bin/ghc returned ExitFailure 1 Failed to install trifecta-1.5 World file is already up to date. cabal: Error: some packages failed to install: idris-0.9.16 depends on trifecta-1.5 which failed to install. trifecta-1.5 failed during the building phase. The exception was: ExitFailure 1

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