March 2013 - Beginners

can't make distribution, requires rerunning ./configure
by Josh Stratton 04 Apr '13

04 Apr '13

I recently downloaded the ghc distribution for Linux (x86_64) and had difficulty installing to my machine. The configure step seems to have completed successfully, but when I run the followup "make install", it fails suggesting that I haven't run the configure command yet. atlas => ./configure --prefix=/opt/ghc checking for path to top of build tree... /tmp/ghc-7.6.2 Build platform inferred as: x86_64-unknown-linux Host platform inferred as: x86_64-unknown-linux Target platform inferred as: x86_64-unknown-linux GHC build : x86_64-unknown-linux GHC host : x86_64-unknown-linux GHC target : x86_64-unknown-linux checking for perl... /rel/map/generic-2012.18.last/bin/perl checking if your perl works in shell scripts... yes checking for a BSD-compatible install... /usr/bin/install -c checking whether ln -s works... yes checking for gsed... sed checking for gcc... /usr/bin/gcc checking for gcc... /usr/bin/gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... no checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether /usr/bin/gcc accepts -g... yes checking for /usr/bin/gcc option to accept ISO C89... none needed checking version of gcc... 4.4.6 checking how to run the C preprocessor... /usr/bin/gcc -E checking whether ld understands --hash-size=31... checking whether ld understands --reduce-memory-overheads... checking for extra options to pass gcc when compiling via C... -fwrapv checking Setting up CFLAGS, LDFLAGS, IGNORE_LINKER_LD_FLAGS and CPPFLAGS... done checking Setting up CONF_CC_OPTS_STAGE0, CONF_GCC_LINKER_OPTS_STAGE0, CONF_LD_LINKER_OPTS_STAGE0 and CONF_CPP_OPTS_STAGE0... done checking Setting up CONF_CC_OPTS_STAGE1, CONF_GCC_LINKER_OPTS_STAGE1, CONF_LD_LINKER_OPTS_STAGE1 and CONF_CPP_OPTS_STAGE1... done checking Setting up CONF_CC_OPTS_STAGE2, CONF_GCC_LINKER_OPTS_STAGE2, CONF_LD_LINKER_OPTS_STAGE2 and CONF_CPP_OPTS_STAGE2... done checking for .subsections_via_symbols... no checking whether your assembler supports .ident directive... yes checking for GNU non-executable stack support... yes checking for grep that handles long lines and -e... /bin/grep checking for egrep... /bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking size of void *... 8 checking for ar... /usr/bin/ar checking whether /usr/bin/ar is GNU ar... yes checking for ar arguments... q checking whether ranlib is needed... no configure: creating ./config.status config.status: creating settings config.status: creating mk/config.mk config.status: creating mk/install.mk **************************************************** Configuration done, ready to 'make install' (see README and INSTALL files for more info.) **************************************************** atlas => make install make -r --no-print-directory -f ghc.mk install BINDIST=YES NO_INCLUDE_DEPS=YES ghc.mk:122: *** Please run ./configure first. Stop. make: *** [install] Error 2

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lazy mapM
by Ovidiu D 03 Apr '13

03 Apr '13

Hi again, Given the following code: g :: IO String -> IO String f :: [IO String] -> IO [ String ] f = mapM g The implementation of f is wrong because I would like to: 1. Make f behave lazy Its input list is made of lines read from stdin and I want it to process lines one by one as they are entered by the user. 2. Implement f such that it stops consuming items from the input list when the input item meets some condition. For example: isExit item = ("exit" == item) I tried to implement my own custom iteration by recursion but I got stuck in the combination of IO and list monads. Any help is appreciated. Thanks!

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[IO String] to IO [String]
by Ovidiu D 01 Apr '13

01 Apr '13

I have the function f which reads lines form the stdin and looks like this: f :: [IO String] f = getLine : f What I don't like is the fact that the line processing I'm doing will have to be in the IO Monad I would like to make this function to have the signature f : IO [String] ...such that I can get rid of the IO monad and pass the pure string list to the processing function. Can I do this? Thanks

6 15

finding the cause of an error (here head: empty list)
by Nathan Hüsken 31 Mar '13

31 Mar '13

Hey, I have written a program which, when executed produces: Main: Prelude.head: empty list Now, I can go through my program and replace all "head" with (\a -> trace ("<line-nr>: length=" ++ (show (length a))) a) Actually, I already did that, and by this I found the error. But I wonder if there would have been an easier way? Has anyone any debug advice how I could find out which call to "head" causes this without so much typing work? Thanks! Nathan

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cabal install local troubles (osx)
by Miro Karpis 31 Mar '13

31 Mar '13

please, can you help me with following. I have installed haskell on mac from the package. When I check cabal -V I can see 1.14.0 That is because the cabal is loading from /Library/Haskell/bin. After that I run 'cabal install cabal-install', I get following output: Installing executable(s) in /Users/miro/Library/Haskell/ghc-7.4.2/lib/cabal-install-1.16.0.2/bin Updating documentation index /Users/miro/Library/Haskell/doc/index.html So the new version of cabal is installed in another place. When I install new package via cabal, it uses the 1.14.0 version. I have tried to copy the new 1.16.0.2 version to bin, but that just didn't help because the new packages have been installed somewhere else. Please what is the proper way to fix this? many thanks, miro

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Quickcheck2 - writing a modifier for test data
by Nathan Hüsken 31 Mar '13

31 Mar '13

Hey, Using quickcheck2 I want to generate test data, that is a tuple of integers where both elements are >=0 and < 8. So I am trying to write a modifier (in the sense of the modifiers found in Modifiers.hs): newtype ValidPos = ValidPos Pos deriving Show instance Arbitrary ValidPos where arbitrary = do x <- elements [0..7] y <- elements [0..7] return (ValidPos (x,y)) prop_dummy (ValidPos pos) = fst pos > 0 tests = [ testGroup "Tests" [ testProperty "dummy" prop_dummy ] ] main = defaultMain tests doing so gives me this error: No instance for (QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary ValidPos) arising from a use of `testProperty' Possible fix: add an instance declaration for (QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary ValidPos) In the expression: testProperty "dummy" prop_dummy In the second argument of `testGroup', namely `[testProperty "dummy" prop_dummy]' In the expression: testGroup "Tests" [testProperty "dummy" prop_dummy] I declared the instance, why canit not be found? Thanks! Nathan

2 2

point-free + IO Monad
by Ovidiu D 31 Mar '13

31 Mar '13

I have the following code: f1 :: String -> String f2 :: IO String -> IO () f2 a = do b <- a putStr $ f1 b How can I write the function f2 in a point-free style? I have tried this: f2 = return.f1 >>= putStr ...but it doesn't work. Thanks!

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"downcasting"
by Emmanuel Touzery 29 Mar '13

29 Mar '13

Hello, using the Text.Json library (and I'm reading I should be using Aeson instead... I'll migrate to it later), I'm a bit frustrated. At one point I'm getting a JSValue object, which could be anything, including a JSObject. But in this case I know I'm looking at a JSObject. I don't mind if Haskell crashes if that's not the case (for now), either it's a JSObject or the JSON I'm parsing is invalid. I hoped to do that conversion using pattern matching by giving non-exhaustive pattern match, considering only cases where the JSValue is in fact a JSONObject but somehow I can't make it... getObject :: JSValue -> JSObject JSValue getObject x@(JSObject _) = x --> doesn't compile the function returns a JSValue, I somehow wanted to point out to the compiler that if it gets there it's in fact a JSObject. getObject :: JSValue -> JSObject JSValue getObject (JSObject x) = JSObject x -> same as with the other attempt.. the compiler is saying the function is returning a JSValue: Couldn't match expected type `JSObject JSValue' with actual type `JSValue' In the return type of a call of `JSObject' In the expression: JSObject x In an equation for `getObject': getObject (JSObject x) = JSObject x I think I'm missing an obvious pattern in Haskell here, probably something I've already used elsewhere too, but somehow I don't make it. Otherwise answers saying that if I need to make such a "cast" then I'm using that library the wrong way are welcome (together to maybe a link to a tutorial explaining it the right way), but by curiosity I would still be interested in how to actually do that "cast" in this case. Thank you! Emmanuel

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stack overflow summing numbers read from a big file
by Axel Wegen 25 Mar '13

25 Mar '13

When I try to run the following code on a 50M file consisting of one number per line I receive a `Stack space overflow' error and the program seems to consume a lot of memory. Why does that happen? And how can I fix the problem without increasing the Stack with -Ksize hoping that it will be big enough? Any general advice on processing big files with Haskell? -- sumFile.hs -- adapted from Real World Haskell's SumFile.hs at the beginning of -- Chapter 8 main = interact sumFile where sumFile = show . sum . map read . words ghc -o sumFile sumFile.hs ./sumFile < ./numbers

4 7

How to design functions so they can be part of other larger systems?
by Costello, Roger L. 24 Mar '13

24 Mar '13

Hi Folks, I read this statement by Tim Berners-Lee [1]: It is not only necessary to make sure your own system is designed to be made of modular parts. It is also necessary to realize that your own system, no matter how big and wonderful it seems now, should always be designed to be a part of another larger system. Recently I have been working hard to learn how to better modularize. But now TBL says that I must do more - I must not only modularize well, but I must also build the modules so that they can be part of other larger systems. How do I design modules so that they may be part of other larger systems? Are there any articles that give guidelines on how to do this? What are your thoughts on how to do this? /Roger [1] http://www.w3.org/DesignIssues/Principles.html

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