February 2015 - Beginners

File I/O: reading from, writing to, same file in one function
by Geoffrey Bays 05 Feb '15

05 Feb '15

Question: As I work my way through Learn You as Haskell, I have been writing small File IO programs. Unlike the example in that book, where the author reads from one file, writes altered data to a temp file and then deletes the original file and renames the temp file, I have been trying to read from a given file, alter the data and then overwrite the same file with the new data. Is this possible? In a main do block I have tried: contents <- readFile fileName -- do stuff to contents-- writeFile fileName result After using appendFile to add items to the file, when I call the function to read and write I get this error: openFile: resource busy, (file is locked) I have also tried using the file handles like so: handle <- openFile fileName ReadWriteMode contents <- hGetContents handle -- do stuff to contents -- hPutStr handle results hClose handle I have also tried opening with one handle to read, closing it, then using another handle for writing, but the error message is that the handle to the file is closed. I am thinking that the the author of LYAH maybe has good reasons to use a temp file, or is there another way that is not too difficult to grasp?? Many thanks, Geoffrey

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Re: [Haskell-beginners] [Haskell-cafe] ghc --make Vs runhaskell
by Brandon Allbery 04 Feb '15

04 Feb '15

On Wed, Feb 4, 2015 at 10:10 PM, Richard A. O'Keefe <ok(a)cs.otago.ac.nz> wrote: > Given things like sockets, STREAMS, and ptys, I am not > sure that it is _possible_ for a Unix system to determine whether two > streams > are or are not "related" reliably. > It's not. I was not implying that it was, or that it was even a good idea to try to do so. On the other hand, flushing *every* line-buffered FILE is rather overkill but less heinous than highly dubious and untestable assumptions about relationships. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] [Haskell-cafe] ghc --make Vs runhaskell
by Madhu Babu 03 Feb '15

03 Feb '15

Thanks a lot. .this really helps.. On Feb 3, 2015 10:35 PM, "Tristan Seligmann" <mithrandi(a)mithrandi.net> wrote: > On 4 February 2015 at 05:23, Madhu Babu <madhub.bits(a)gmail.com> wrote: > > 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 ). > > This is most likely due to stdio buffering; usually stdin/stdout are > line buffered, so data you write with putStr may not actually be > written until a newline is written. This post / thread details some > ways to handle this problem: > > https://www.haskell.org/pipermail/beginners/2010-March/003692.html > -- > mithrandi, i Ainil en-Balandor, a faer Ambar >

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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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