- Beginners - Haskell.org

Data type
by Shishir Srivastava 05 May '15

05 May '15

Hi, Could anyone please have a look and let me know how you can create the instance of this data type ? --- data Person = Person { firstName ::String -> Int } --- I've seen this type of syntax in a lot of places in haskell code where a new data type is defined in terms of functions rather than concrete data types. I am not trying to achieve anything out of this but purely as an exercise in understanding the record syntax. As far as I understand the data type is function based and takes a 'function' instead of a value of a concrete type so how does one create an instance of this type. Thanks, Shishir

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Data type
by Shishir Srivastava 05 May '15

05 May '15

Thanks Matthew - that was crisp ! Cheers, Shishir

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Variable re-use
by Shishir Srivastava 03 May '15

03 May '15

Hi, Please can anyone explain how does 'a' get re-used in the code below. My understanding so far of haskell is that variables are not allowed to mutate or re-assigned. --- do a <- [1,2,3] a <- [a+1] return a [2,3,4] --- Thanks, Shishir

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definition of >>= for lists
by Stanislaw Findeisen 03 May '15

03 May '15

Hi Where is the definition of (>>=) for lists? I was trying to locate it using Hoogle but no luck. :( -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 9EC2 5620 2355 B1DC 4A8F 8C79 0EC7 C214 E5AE 3B4E

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Re: [Haskell-beginners] : GCJ 2015 - Round 1A Problem - Haircut
by Gregory Guthrie 03 May '15

03 May '15

Seems to me that an estimate won't help, one has to know exactly how much to skip over. I did this: Find the lowest common multiple (lcm), and then from that the shortest cycle of services, And then skip any multiple of those from the customer queue. sim :: Problem -> [Result] sim (n, ts) = serve tSkip (servers ts) -- optimize; skip ahead over server cycles... where tsLcm = (foldl lcm $ head ts) $ tail ts tCycle = sum $ map (div tsLcm) ts tSkip = n - (div n tCycle)*tCycle (Some fixup is needed for tSkip=0, one easy one is to force an extra cycle.) > ---------------------------------------------------------------------- > Message: 1 > Date: Sat, 18 Apr 2015 10:29:11 -0400 > From: Doug McIlroy <doug(a)cs.dartmouth.edu> > To: sourabh.s.joshi(a)gmail.com > Cc: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] GCJ 2015 - Round 1A Problem - Haircut > Message-ID: <201504181429.t3IETBfo025201(a)coolidge.cs.dartmouth.edu> > Content-Type: text/plain; charset=us-ascii > > > Can someone tell me how I could have avoided or fixed this? > > The trouble manifested as stack overflow on solving > > > https://code.google.com/codejam/contest/4224486/dashboard#s=p1 > > For efficiency, you'll probably need more cleverness in math than in Haskell. You can predict > an approximate start time for the Nth customer's service from the average per-customer > service time M, where 1/M = sum 1/M_k. > Starting from that estimate, one can skip over almost the entire service simulation. > > Doug McIlroy > >

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Haskell ghc, linking error when trying to compile a program using ghc and Euterpea
by Florian Gillard 02 May '15

02 May '15

I am checking out the computing music development library Euterpea, and I am trying to compile the following basic program: import Euterpea t251 :: Music Pitch t251 = let dMinor = d 4 wn :=: f 4 wn :=: a 4 wn gMajor = g 4 wn :=: b 4 wn :=: d 5 wn cMajor = c 4 bn :=: e 4 bn :=: g 4 bn in dMinor :+: gMajor :+: cMajor main = play t251 the program works fine inside ghci, but when I try to compile it using ghc test.hs I get the following error message: Linking test ... /usr/bin/ld: /home/fayong/.cabal/lib/PortMidi-0.1.3/ghc-7.6.3/libHSPortMidi-0.1.3.a(ptlinux.o): undefined reference to symbol 'pthread_create@@GLIBC_2.2.5' //lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line collect2: error: ld returned 1 exit status It seems to be a linking error but I don't know how to fix it. (I tried to pass -lpthread as an option but it didn't work) I installed Euterpea via cabal, on linux mint 17

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Re: [Haskell-beginners] Loop
by Shishir Srivastava 01 May '15

01 May '15

Hi Marcin, Thanks for your response. Yes my intention to use 'do' was to implement it in so called 'imperative' style coding. So if I understand it correctly there is no other way to achieve it via 'do' w/o using the monad transformers? Thanks again Shishir

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Loop
by Shishir Srivastava 01 May '15

01 May '15

Hi, Please can anyone point out the correct way of doing this - I am simply trying to print "Test" in a loop based on the counter derived from the list - ---- do _ <- [1..4] b <- putStrLn "Test" return b --- The intended output is to print 'Test' 4 times. Clearly there is a mismatch between the Monad types but I can't see how to achieve this without causing the conflict. Thanks, Shishir

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abstracting parametrized record types
by Alia 29 Apr '15

29 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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Data and Typeable for an opaque type
by Elise Huard 28 Apr '15

28 Apr '15

Hi, I'm working on an open source patch, but stumbled upon the following problem: I'm introducing a type that is basically a pointer in a FFI library, pointing to an opaque type (which I'm told is common practice). -- | An opaque type encapsulating a font in C. data Font_Opaque type Font = Ptr Font_Opaque The problem is that insertion in the current code would require this to be an instance of Data and Typeable. In this case (gloss library) I've not 100% understood why that's a requirement, I've been grepping (case-insensitive) for typeOf, constr, gfold, and I've not found any obvious use. Anyway, the rough-and-ready approach for this would be to manually define instances of Data and Typeable for the opaque type - my question is: what would sensible implementations look like for such a type? Thank you, Elise

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