- Beginners - Haskell.org

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 09 Jan '17

09 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 09 Jan '17

09 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 09 Jan '17

09 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 09 Jan '17

09 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 02 Jan '17

02 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 28 Dec '16

28 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

ANNOUNCE: Haskell Communities and Activities Report (31st ed., November 2016)
by Mihai Maruseac 24 Dec '16

24 Dec '16

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (31st edition, November 2016) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2016/report.pdf http://haskell.org/communities/11-2016/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of February 2016: target deadline for contributions to the May 2017 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

1 0