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ghc --make Vs runhaskell
by Madhu Babu 03 Feb '15

03 Feb '15

Hi !, I have the following program from “Learn You a Haskell” ( http://learnyouahaskell.com/input-and-output#randomness ). When i run this using runhaskell, code works perfectly fine. However, when i compile and run the binary file, code behaves differently. Basically, in the following code, we print a line first and the read the line from stdin. This behavior works perfectly fine when using “runhaskell guess.hs”. But when i say “./guess”, i had to type in a number directly ( i.e. first prompt is skipped ). btw.. i am using mac yosomite command terminal. Any help greatly appreciated.. Thanks a lot in advance.. import System.Random import Control.Monad(when) main = do ranGen <- getStdGen let (rand,_) = randomR (1,10) ranGen :: (Int,StdGen) putStr "Guess a number between 1 and 10 : " numStr <- getLine when(not $ null numStr) $ do let num = read numStr if rand == num then putStrLn "Yuhaa!! you are right baby!" else putStrLn $ "Sorry dude!! it was " ++ show rand newStdGen main

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Vim + Syntastic + ghc-mod: over-verbose error messages
by Matthew Moppett 03 Feb '15

03 Feb '15

I'm not sure if this is the right forum to raise this issue, but: Trying out the Syntastic plugin in Vim, I've found that it displays confusingly over-verbose error messages that contain unnecessary information about the module location of identifiers -- e.g.: Config.hs|13 col 7 error| Couldn't match type `(Config.ConfigElem, GHC.Base.String)' with `[GHC.Types.Char]'Expected type: [GHC.Base.String] -> containers-0.5.0.0:Data.Map.Base.Map Config.ConfigElem GHC.Base.String Actual type: [(Config.ConfigElem, GHC.Base.String)] -> containers-0.5.0.0:Data.Map.Base.Map Config.ConfigElem GHC.Base.StringIn the expression: Data.Map.Strict.fromList :: [GHC.Base.String] -> containers-0.5.0.0:Data.Map.Base.Map Config.ConfigElem GHC.Base.StringIn an equation for `Config.foo': Config.foo = Data.Map.Strict.fromList :: [GHC.Base.String] -> containers-0.5.0.0:Data.Map.Base.Map Config.ConfigElem GHC.Base.String Syntastic uses the output from "ghc-mod check" to generate this, and in turn I think ghc-mod relies on ghc, since options can be passed to ghc from ghc-mod using the -g option flag. Does anyone know of a way to modify the behaviour of Syntastic/ghc-mod/ghc to produce output that's more readable?

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Build woes (cabal configure can't find sandboxed deps)
by Alex Hammel 02 Feb '15

02 Feb '15

Hi list, I've got a number of modules in a project set up like this: project |-- foo |-- bar |-- baz `-- sandbox where 'foo', 'bar' and 'baz' are modules which depend on one another, and 'sandbox' is a sandbox. Before building from scratch, I do a $(cabal sandbox init --sandbox=../sandbox) in each of the module folders. If I run $(cabal install ./foo ./bar ./baz) from the projects/ directory, everything is happy. However, if I reconfigure one of the sub-modules with different flags, it fails with missing dependencies: # in foo/ $ cabal build Package has never been configured. Configuring with default flags. If this fails, please run configure manually. # module builds correctly $ cabal configure --enable-tests Resolving dependencies... Configuring foo-0.1... $ cabal build dist/setup/setup.hs:13:8: Could not find module ‘System.FilePath.Glob’ Perhaps you meant System.FilePath (from filepath-1.3.0.2) System.FilePath.Posix (from filepath-1.3.0.2) Use -v to see a list of the files searched for. Glob is installed in the sandbox. I've tried pointing to the sandbox config file manually with both $(cabal --sandbox-config-file=project/foo/cabal.sandbox.config) and $(cabal --sandbox-config-file=project/cabal.sandbox.config) with the same result. Cabal insists that Glob isn't installed until I nuke the foo/dist directory and start over. I figure there's some flag I need to pass to `cabal configure` that I'm missing, but I can't figure out what it is. Anybody have any hints? Cheers, Alex

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Cabal reset/restart
by Gregory Guthrie 31 Jan '15

31 Jan '15

I did a complete reset and restart of Haskell & Cabal, and trying to re-install my former packages on a new clean system, most went easily but there are already a number which would not install. (Deleted .ghc, .cabal, and .sandbox; reinstalled Platform). 1) "ghc-pkg check" indicates a lot of issues (although most seemed to be warnings about missing .hi or haddock html files). Many seem to be related to looking for older versions, e.g. it complains that: There are problems in package Win32-2.2.2.0: Warning: library-dirs: E:\Plang\Haskell Platform\lib\Win32-2.2.2.0 doesn't exist or isn't a directory Warning: include-dirs: E:\Plang\Haskell Platform\lib\Win32-2.2.2.0\include doesn't exist or isn't a directory import-dirs: E:\Plang\Haskell Platform\lib\Win32-2.2.2.0 doesn't exist or isn't a directory cannot find any of ["libHSWin32-2.2.2.0.a","libHSWin32-2.2.2.0.p_a","libHSWin32-2.2.2.0-ghc7.8.3.so","libHSWin32-2.2.2.0- ghc7.8.3.dylib","HSWin32-2.2.2.0-ghc7.8.3.dll"] on library path And indeed it is not there, but Win32-2.3.0.2 is present, and similarly for most all others that I see. So why are these being reported as problems, and what to do? 2) cabal install of several of my previous packages fails; is there just some intrinsic incompatibility of these packages, or some other error? An example: C:\Users\myUser\.cabal-sandbox>cabal install cgi Resolving dependencies... In order, the following would be installed: exceptions-0.6.1 (reinstall) changes: mtl-2.1.3.1 -> 2.2.1, transformers-0.3.0.0 -> 0.4.2.0 network-2.6.0.2 (new version) cgi-3001.2.2.0 (new version) cabal: The following packages are likely to be broken by the reinstalls: temporary-1.2.0.3 ghc-mod-5.2.1.2 either-4.3.3 monad-journal-0.7 Use --force-reinstalls if you want to install anyway. 3) ghc-pkg check indicates a lot of broken packages for a new installation, Although all those that I checked of these are from old versioning as above, and there really is a good and more current version installed: The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. Win32-2.2.2.0 time-1.4 template-haskell-2.7.0.0 regex-base-0.93.2 process-1.1.0.1 pretty-1.1.1.0 parsec-3.1.3 old-time-1.1.0.0 old-locale-1.0.0.4 integer-gmp-0.4.0.0 HTTP-4000.2.5 hpc-0.5.1.1 hoopl-3.8.7.3 haskell98-2.0.0.1 haskell2010-1.1.0.1 haskell-platform-2012.4.0.0 ghc-prim-0.2.0.0 ghc-7.4.2 filepath-1.3.0.0 fgl-5.4.2.4 extensible-exceptions-0.1.1.4 directory-1.1.0.2 deepseq-1.3.0.0 containers-0.4.2.1 cgi-3001.1.7.4 Cabal-1.14.0 bytestring-0.9.2.1 binary-0.5.1.0 base-4.5.1.0 array-0.4.0.0 zlib-0.5.4.0 xhtml-3000.2.1 vector-0.10.0.1 transformers-0.3.0.0 text-0.11.2.3 syb-0.3.7 stm-2.4 split-0.2.1.1 regex-posix-0.95.2 regex-posix-0.95.2 regex-compat-0.95.1 regex-compat-0.95.1 random-1.0.1.1 QuickCheck-2.5.1.1 primitive-0.5.0.1 parallel-3.2.0.3 OpenGL-2.2.3.1 network-2.3.1.0 HUnit-1.2.5.1 -------------------------------------------

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Cannot find cause of space leak
by YCH 31 Jan '15

31 Jan '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 30 Jan '15

30 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 30 Jan '15

30 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 28 Jan '15

28 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 28 Jan '15

28 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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