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Problem "grouping" a list
by João Paulo Pizani Flor 24 Nov '10

24 Nov '10

Hello dear Haskellers! I've been a user and big fan of Haskell for a while, but only now I'm writing my first "big" project in Haskell (some thousands of lines of code perhaps). It's an interpreter for a programming language, the source code is music! Yes, sheet music! :D OK, so my specific problem goes like this: I have a list of tuples :t myList [ (Float, Integer) ] And I want to "group" this list into a nested list groupAtoms :: [ (Float,Integer) ] -> [ [(Float,Integer)] ] Of course, I want that the concatenation of the groups yield me the original list, i.e, (foldl (++) [] . groupAtoms == id), and the criterion that defines a group is that: "The sum of the first elements in the tuples comprising the list must be greater than or equal to 1.0". That is, given a list of tuples, the boolean predicate deciding whether this list is a PROPER group (True) or TOO SMALL (False) is: \g -> sum (map fst g) >= 1.0 Although the criterion is very clear, I've tried hard until now and couldn't come up with a function for producing the nested list based on this grouping criterion. I am sure that the Haskell Hierarchical Libraries have something to help me, but only now I see I'm still a big noob :P Could someone please help me writing this function? My best regards from Brazil, João Paulo Pizani Flor joaopizani(a)gmail.com Federal University of Santa Catarina

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Portability of a Haskell program
by Ondřej Garncarz 23 Nov '10

23 Nov '10

I want to run my Haskell program which I've compiled on other computer, so I try: $ ./eternitypar ./eternitypar: error while loading shared libraries: libgmp.so.10: cannot open shared object file: No such file or directory $ LD_PRELOAD=./libgmp.so.10 ./eternitypar ./eternitypar: /lib/libc.so.6: version `GLIBC_2.7' not found (required by ./libgmp.so.10) $ LD_PRELOAD=./libc.so:./libgmp.so.10 ./eternitypar ERROR: ld.so: object './libc.so' from LD_PRELOAD cannot be preloaded: ignored. ./eternitypar: /lib/libc.so.6: version `GLIBC_2.7' not found (required by ./libgmp.so.10) $ LD_PRELOAD=./ld-linux.so.2:./libc.so:./libgmp.so.10 ./eternitypar ERROR: ld.so: object './ld-linux.so.2' from LD_PRELOAD cannot be preloaded: ignored. ERROR: ld.so: object './libc.so' from LD_PRELOAD cannot be preloaded: ignored. ./eternitypar: /lib/libc.so.6: version `GLIBC_2.7' not found (required by ./libgmp.so.10) So I download a package from http://new-www.haskell.org/ghc/download_ghc_7_0_1#x86_64linux and I try: $ ./configure checking for path to top of build tree... utils/ghc-pwd/ghc-pwd: error while loading shared libraries: libgmp.so.3: cannot open shared object file: No such file or directory configure: error: cannot determine current directory Why is it looking for such an old version, when 7.0.1 should be up-to-date? But fine, I can replace ghc-pwd with symlinked /bin/pwd, but I have a problem with some pgcc installed on the machine, so I run ./configure --with-gcc=/usr/bin/gcc: (/usr/bin/gcc exists and works) checking for path to top of build tree... /u/gar061/tmp/ghc-7.0.1 checking for perl... /usr/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 ar... /usr/bin/ar checking whether /usr/bin/ar is GNU ar... yes checking for ar arguments... q checking for gcc... /tools/pgi725/linux86-64/7.2-5/bin/pgcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... no checking whether /tools/pgi725/linux86-64/7.2-5/bin/pgcc accepts -g... yes checking for /tools/pgi725/linux86-64/7.2-5/bin/pgcc option to accept ISO C89... none needed checking whether ranlib is needed... no checking for sed... /bin/sed configure: error: gcc is required It's ignoring my choice of GCC. Installation of 6.12.3 runs the same way. Then I move to compilation from source codes, but here - http://new-www.haskell.org/ghc/download_ghc_7_0_1#sources - I read: "The source distribution needs an installed GHC (version 6.10 at least)." Oh, so I need the application to build the application? What's the reason? Is there any other way to make it work? Thanks, Ondřej Garncarz

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Parsing an integer
by Tim Baumgartner 22 Nov '10

22 Nov '10

Hi folks, since I always get great answers here, I have another question. I'm currently parsing a (positive) integer using data Token = Number Int -- ... number :: Parser Token number = many1 digit ↠ (read >>> Number >>> return) -- = many1 digit ↠ return∘Number∘read But I wonder what an advanced haskeller would code instead. Particularly, I'd like to get rid of the 'return'. Thanks in advance Tim

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Ignoring the result of a monadic computation
by Tim Baumgartner 22 Nov '10

22 Nov '10

Hi, while learning about monads, I had something like do line <- getLine something putStrLn line and I wondered if I could write it in one line, without naming of parameters. I finally came up with getLine >>= ignore something >>= putStrLn using ignore :: Monad m => m a -> b -> m b ignore m a = m >> return a I'm satisfied with this solution but searching hoogle I didn't find a standard function for my ignore. Am I missing something? Cheers, Tim -- GRATIS! Movie-FLAT mit über 300 Videos. Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome

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Chain computations
by Tim Baumgartner 21 Nov '10

21 Nov '10

Hi, I really like fooling around with operators. In elementary school, we always had to make chain computations, e.g. teacher: 2 teacher: +1 teacher: *2 teacher: divided by 3 Again, I didn't find the following operator on hoogle (very similar to ($)): (->>) :: a -> (a -> b) -> b a ->> f = f a Using it, we can nicely write down the above computation: 2 ->> (+1) ->> (*2) ->> (`div` 3) Cheers Tim

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Rekommendation for select i GUI
by Anders Persson 20 Nov '10

20 Nov '10

Hi! i have installed Haskell Platform om a Windows Vista, I will also run Haskell on a OS/X machine. I have looked at a good GUI tool, tested wxHaskell, and hs2GTK but no one seems easy to be install, lot off errors, etc... Have i missed something and could i get some comments om a GUI toolkit to select. Best regards Anders -- med vänliga hälsningar Anders ______________________________________________ Anders Persson anders.u.persson(a)gmail.com

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Re: [Haskell-cafe] About "Fun with type functions" example
by Bastian Erdnüß 19 Nov '10

19 Nov '10

On Nov 19, 2010, at 1:07, Daniel Peebles wrote: > The best you can do with fromInt is something like Int -> (forall n. (Nat n) > => n -> r) -> r, since the type isn't known at compile time. Or you put it in the type class as fromInt :: Int -> n and do in Zero: fromInt _ = Zero and in the other: fromInt _ = Succ $ fromInt (undefined :: n). Shouldn't that work, too? > On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly <arnaud.oqube(a)gmail.com>wrote: > >> Thanks a lot, that works perfectly fine! >> Did not know this one... >> BTW, I would be interested in the fromInt too. >> >> Arnaud >> >> On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink <hesselink(a)gmail.com> >> wrote: >>> On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly <arnaud.oqube(a)gmail.com> >> wrote: >>>> Hello, >>>> I am trying to understand and use the Nat n type defined in the >>>> aforementioned article. Unfortunately, the given code does not compile >>>> properly: >>> >>> [snip] >>> >>>> instance (Nat n) => Nat (Succ n) where >>>> toInt _ = 1 + toInt (undefined :: n) >>> >>> [snip] >>> >>>> And here is the error: >>>> >>>> Naturals.hs:16:18: >>>> Ambiguous type variable `n' in the constraint: >>>> `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 >>>> Probable fix: add a type signature that fixes these type variable(s) >>> >>> You need to turn on the ScopedTypeVariables extension (using {-# >>> LANGUAGE ScopedTypeVariables #-} at the top of your file, or >>> -XScopedTypeVariables at the command line). Otherwise, the 'n' in the >>> class declaration and in the function definition are different, and >>> you want them to be the same 'n'. >>> >>> Erik >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Package Documentation (Haddock)
by Bastian Erdnüß 19 Nov '10

19 Nov '10

Hi, I found the Haddock documentation to the packages that came with the Haskell Platform locally on my machine. 1) It's a little bit uncomfortable to always pick the right one by hand. Is there a more comfortable way like doing e.g. 'haddock Data.List' (that's not working) to direct to the documentation of a certain module? 2) I cannot find the documentation to the packages I post installed with cabal. I tried to build them using e.g. 'cabal haddock Yampa' but that's not working. How do I do it right? Thanks for your help in advance, Bastian

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arrow type signature question
by MH 19 Nov '10

19 Nov '10

I am looking at signatures for Arrow and Composable classes and I cannot understand some of them. Could you please explain me the following: Let's take for example the following: class FunAble h => FunDble h where resultFun :: (h b -> h b') -> (h (a->b) -> h (a->b')) class FunAble h where secondFun :: (h b -> h b') -> (h (a,b) -> h (a,b')) -- for 'second' in the signatures: resultFun :: (h b -> h b') -> (h (a->b) -> h (a->b')) secondFun :: (h b -> h b') -> (h (a,b) -> h (a,b')) if (h b -> h b') is the input of these functions where does 'a' comes from in the output? Thanks,

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Cost center annotations - where/let difference
by Daniel Seidel 18 Nov '10

18 Nov '10

Hi, I was doing some heap profiles and ran into the following issue. Consider the program fib.hs: module Main where fib i | i == 0 || i == 1 = 1 | i > 1 = fib (i-1) + fib (i-2) | otherwise = 0 testWhere = {-# SCC "Where" #-} res where res = fib 35 testLet = {-# SCC "Let" #-} let res = fib 35 in res If I choose main = print testLet compile with: ghc --make -prof -auto-all -caf-all -O2 fib.hs and run: ./fib +RTS -hc -hCLet -L60 the fib.hp file will contain entries as expected. If I choose main = print testWhere compile with: ghc --make -prof -auto-all -caf-all -O2 fib.hs and run: ./fib +RTS -hc -hCWhere -L60 the fib.hp file will contain the timestamps, but entries for measured heap consumption. Can anyone please tell me, if this is the expected behavior? I'm not very experienced in benchmarking and was a bit irritated by that difference. Cheers, Daniel.

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