April 2015 - Beginners

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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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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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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Re: [Haskell-beginners] This year's Google Summer of Code
by Shishir Srivastava 28 Apr '15

28 Apr '15

Brilliant work ! Congrats to ever so helpful Sumit Sahrawat ! Cheers, Shishir

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This year's Google Summer of Code
by Kim-Ee Yeoh 28 Apr '15

28 Apr '15

will see one of the actively helpful members of this list, Sumit Sahrawat, help contribute to making a full-featured Computer Algebra System an open-source reality. He will take the IHaskell package (inspired by IPython) one step closer to feature parity with Mathematica: https://www.google-melange.com/gsoc/project/details/google/gsoc2015/sumit_s… Sumit: Congrats for bagging GSOC ! -- Kim-Ee

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Partition a number or optimize
by Animesh Saxena 27 Apr '15

27 Apr '15

I have the following function to optimize myfunction = max(p `div` c + p `mod` c) Here p is an array of numbers and c is also an array of numbers I need to find c’s which minimize the value of my function. for example if p = [1,10000]. c = 2,8 or c = 1,9 or c=5,5 number of c is length of p which is 2 (in this case). Sum of c is given as a constraint, so for example if sum is 10, then c’s can be 2,8 or 1,9 or 5,5 or 4,6 or…..etc etc. Length of c will be equal to length of p array. Hope the problem statement is clear In haskell terms it will be, let my_min p c = foldl1 max $ zipWith (\p c -> (p `div` c) + (p `mod` c)) p c One way is to use various combinations of c using integer partitioning. Now i wrote a crude parts function to do this nparts 0 = [] nparts n = [n] : [x:xs | x <- [1..n`div`2], xs <- nparts(n - x)] which is a bad bad example of recursion coz it’s slow! *Main> nparts 5 [[5],[1,4],[1,1,3],[1,1,1,2],[1,1,1,1,1],[1,2,2],[1,2,1,1],[2,3],[2,1,2],[2,1,1,1]] I can then filter this with length of p and get the required options of c I can then call the function “ my_min” with this list (simple map application) Problem is if i do *Main> filter (\x->length x == 2) $ nparts 100 [[1,99] and it hangs…for a lot of time….so time constraint? Any other approaches to this problem to make the partitions speedier. I know it’s an optimization problem…but all non linear optizers will use floats…and fail to converge…this is an integer optimizing problem. Any ideas??

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How to "add column" to a Table?
by martin 27 Apr '15

27 Apr '15

Hello all, I suppose the natural representation for a Table is a List of Tuples. Or more generally I'll probably end up with a type "Table a" where a stands for the Columns. I cannot figure out how to add a new Column to a Table. What would be the type of such an operation? If I just want to add an empty Column (which what an RDBMS would do), would I do something like this? addColumn :: ColumnDef -> Table a -> Table b But what is this ColumnDef? It would need to contain the name and type of the new column. To implement this operation I would have to somewehre add a column to something. But what is the type of this something. I don't see how I can add a component to a Tuple in a generic way, or can I? I browsed some code on Hackage and found code which uses Data.Proxy. It this the right direction? Or do I need to look at GADTs, or template haskell? Or am I overcomplicating things, and there is a true haskellish solution?

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fmap Maybe
by Shishir Srivastava 27 Apr '15

27 Apr '15

Hi, Please can someone explain why these two expression behave differently - ---- fmap (\x -> x) Just 2 Just 2 ----- fmap (\x -> x+2) Just 2 *No instance for (Num (Maybe a0)) arising from a use of `it'* *In a stmt of an interactive GHCi command: print it* ---- The first expression evaluates fine whereas the second one fails. However if I do - ---- fmap (\x -> x+2) $ Just 2 Just 4 ---- Then the second expression also returns the Maybe value. Why is $ needed in second expression but not in the first one ? Thanks, Shishir

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IO and purity
by Shishir Srivastava 26 Apr '15

26 Apr '15

Hi, Can someone please explain how IO operations do not fit in the pure category of mathematical function in that they have to be implemented via Monads. For e.g. the getLine function has the type IOString and it reads the input from the user. Now as I see it the output of getLine will always be same if the input remain same (i.e. for input "X" getLine will always return "X" ) which is the constraint on mathematical functions. Therefore I don't see why monads are necessary for implementing IO in pure languages. I can understand why Date and Random functions have be implemented via monads because their output will always change. Thanks, Shishir

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