- Beginners - Haskell.org

Re: [Haskell-beginners] Better Code
by Carlo Matteo Scalzo 14 Jan '17

14 Jan '17

Hi Saqib, perhaps something like this? divide :: [Int] -> [[Int]] divide [] = [] divide (x:xs) = divideAux xs [] [x] x divideAux :: [Int] -> [[Int]] -> [Int] -> Int -> [[Int]] divideAux [] result current max = result ++ [current] divideAux (x:xs) result current max = if x - max > 1 then divideAux xs (result ++ [current]) [x] x else divideAux xs result (current ++ [x]) x This can of course be generalised to other types (as per Joel's requirement) just by replacing the check on line 7, i.e., 'x - max > 1'. Happy to explain/talk about the code if you'd like me to :-) Cheers, Carlo On Fri, Jan 13, 2017 at 7:14 PM, Joel Neely <joel.neely(a)gmail.com> wrote: > Had a chance to chat with ghci, so earlier conjecture not confirmed: > > Prelude Data.List> groupBy (\x y -> x == y-1) [1,2,3,7,8,10,11,12] > > [[1,2],[3],[7,8],[10,11],[12]] > > So close but no cigar. > > On Fri, Jan 13, 2017 at 10:05 AM, Saqib Shamsi <shamsi.saqib(a)gmail.com> > wrote: > >> Hi, >> >> The problem that I wish to solve is to divide a (sored) list of integers >> into sublists such that each sublist contains numbers in consecutive >> sequence. >> >> For example, >> *Input:* [1,2,3,7,8,10,11,12] >> *Output:* [[1,2,3],[7,8],[10,11,12]] >> >> I have written the following code and it does the trick. >> >> -- Take a list and divide it at first point of non-consecutive number >> encounter >> divide :: [Int] -> [Int] -> ([Int], [Int]) >> divide first [] = (first, []) >> divide first second = if (last first) /= firstSecond - 1 then (first, >> second) >> else divide (first ++ [firstSecond]) (tail second) >> where firstSecond = head second >> >> >> -- Helper for breaking a list of numbers into consecutive sublists >> breakIntoConsecsHelper :: [Int] -> [[Int]] -> [[Int]] >> breakIntoConsecsHelper [] [[]] = [[]] >> breakIntoConsecsHelper lst ans = if two == [] then ans ++ [one] >> else ans ++ [one] ++ >> breakIntoConsecsHelper two [] >> where >> firstElem = head lst >> remaining = tail lst >> (one, two) = divide [firstElem] >> remaining >> >> >> -- Break the list into sublists of consective numbers >> breakIntoConsecs :: [Int] -> [[Int]] >> breakIntoConsecs lst = breakIntoConsecsHelper lst [[]] >> >> -- Take the tail of the result given by the function above to get the >> required list of lists. >> >> However, I was wondering if there was a better way of doing this. Any >> help would be highly appreciated. >> >> Thank you. >> Best Regards, >> Saqib Shamsi >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >> > > > -- > Beauty of style and harmony and grace and good rhythm depend on > simplicity. - Plato > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -- Carlo Matteo Scalzo Mobile: +44 (0) 7934 583582 E-mail: cmscalzo(a)gmail.com

1 0

Runtime error while feeding a binary to stdin
by Manuel Vázquez Acosta 11 Jan '17

11 Jan '17

Hi all, I'm quite new to Haskell. While following the "Real World Haskell" and doing some experimentation I came up with a anoying situation: Trying to read data from stdin it seems that binary data is not allowed. A simple "copy" program: -- file: copy.hs import System.IO main = do input <- hGetContents stdin hPutStr input Fails when I run it like: $ ghc copy.hs $ ./copy < input > output copy: <stdin>: hGetContents: invalid argument (invalid byte sequence) input contains binary data. In fact of all the following programs only the first works with binary data: copy:: IO () copy = do bracket (openBinaryFile "input" ReadMode) hClose $ \hi -> do bracket (openBinaryFile "ouput" WriteMode) hClose $ \ho -> do input <- hGetContents hi hPutStr ho input copy2:: IO () copy2 = do -- Doesn't work with binary files source <- readFile "input" writeFile "output" source copy3:: IO () copy3 = do -- Doesn't work with binary files either interact (map $ \x -> x) copy4:: IO () copy4 = do input <- hGetContents stdin hPutStr stdout input But I lost any chance of piping and/or using '<', '>' in the shell. Best regards, Manuel.

3 2

dynamic code compilation and loading
by Dennis Raddle 10 Jan '17

10 Jan '17

Can I get a pointer to how to do dynamic code compilation and loading into a running program? D

1 0

Autogenerating tuple constructor of length n
by Manikanda Krishnan 10 Jan '17

10 Jan '17

Hi, I am exploring haskell. Is there a way to generate a tuple constructor of n elements at runtime? Basically I want to convert a list of lists into a list of n tuples. Thanks in advance Mani

1 0

Rose tree
by mike h 10 Jan '17

10 Jan '17

Hi, I’m rapidly descending into a spiral of confusion! :) How do I add something to a rose tree? I have data Tree a = Node a [Tree a] deriving (Show) add :: Eq a => a -> a -> Tree a -> Tree a and the first ‘a’ is the thing to add and the second is its parent node. i.e. add :: Eq a => a -> a -> Tree a -> Tree a add x px (Node x' []) = Node x' [Node x []] ….. I’ve tried various ways for the more general case but always seem to lose part of the tree. Any advice would be very welcome. Thanks Mike

1 1

Functor instance
by sasa bogicevic 10 Jan '17

10 Jan '17

Hi all, Can someone explain to me why exactly when defining the Functor instance for type that has polimorphic parameter constraint I am not allowed to put that parameter in definition. So basically: data Four a b c d = Four a b c d instance Functor (Four a b c) where <-- why can't I specify also param d here ?? fmap f (Four a b c d) = Four a b c (f d) I asked the same question on IRC and got the answer that it is because of Functor has kind * -> *. I thought I understand it but it is not clear to me completely. I understand type kinds on examples I looked at before but when looking at this Functor instance why exactly I don't specify all params for the type and while we are at it why don't I also specify f param for the Functor like this "instance Functor f (Four a b c d) where ...". Thanks, Sasa { name: Bogicevic Sasa phone: +381606006200 }

4 5

fnc explanation
by sasa bogicevic 03 Jan '17

03 Jan '17

Hi, I have some trouble understanding the following function: http://lpaste.net/350771 If someone can take the time and explain the part after WHERE clause that would be great since I kind of know what the function does but have problems with the param order for the go anonymous function. Thanks, Sasa { name: Bogicevic Sasa phone: +381606006200 }

3 3

Alternative
by Imants Cekusins 29 Dec '16

29 Dec '16

discovered this handy snippet today (duh :): >>> foldl (<|>) Nothing [Nothing, Just 1, Nothing, Just 2] Just 1 basically, pick first Just from a list of Maybes http://hackage.haskell.org/package/base-4.9.0.0/docs/Control-Applicative.ht…

4 10

Mutable grid
by mike h 27 Dec '16

27 Dec '16

Hi, I’m looking a problem where I have an NxN grid of ints. I need a function like setValue x y newVal I have tried using [[Int]] but it does become messy when splitting , dropping and then ++ back together. What other options are available to represent a mutable grid? Many thanks Mike

3 6

[Netwire] Problems constructing a simple Event Wire
by Francesco Ariis 27 Dec '16

27 Dec '16

Hello list, I am learning netwire and decided to write a simple toy program to get to know the library better. The structure is extremely simple: 1. a MVar is created and an endless loop fills it with Char 2. a Wire is created (using mkGen_) to provide Events (from the source described in (1)) 3. the final Wire performs some basic filtering/merging on (2) and outputs a behaviour which is then printed on screen. [[[ I attach a commented .hs, `main` to test, 'x' and C-c to exit ]]] The final wire `testWire2 = hold . (keyQ &> keyA)` doesn't work as expected, i.e. it responds to Q keypresses but not to A ones. I suspect the problem lies in how I used `mkGen_` to create the 'source' wire, but I am not sure to fix it. Any help appreciated! -F

1 0