May 2015 - Beginners - Haskell.org

best way to code this
by briand＠aracnet.com 03 Jun '15

03 Jun '15

Hi, A simple example of something I was trying to do but it had some questions along the lines of "the best way to do this". Given a list l = [a] and an a-list alist = [ (a,b) ] the idea is to find all the items in l which are in alist and then create a list [b] so the overall function should be [a] -> [ (a,b) ] -> [b] the solution is straightforward: l1 = filter (\x -> isJust (lookup x alist)) l l2 = map (\x -> fromJust (lookup x alist)) l1 `fromJust` used in the construction of l2 won't fail, because only the elements for which the lookup succeeded are in l1. This would be something called `filterMap` but I couldn't find such a function in the list library, but it seems like there just has to be one defined in a library somewhere. the above seems clumsy, i'm wondering how to make it "more pretty". generally i was also wondering if the above construction is as inefficient as it looks because of the double-lookup, or would the compiler actually be able to optimize that code into something more efficient ? this code is not being used on large sets of data so efficiency doesn't matter, I'm just curious. Thanks, Brian

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ANNOUNCE: Haskell Communities and Activities Report (28th ed., May 2015)
by Mihai Maruseac 29 May '15

29 May '15

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (28th edition, May 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2015/report.pdf http://haskell.org/communities/05-2015/html/report.html 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 September 2015: target deadline for contributions to the November 2015 edition of the HC&A 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 October (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 and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Re: [Haskell-beginners] Continuations
by Shishir Srivastava 29 May '15

29 May '15

Hi Amy Thanks for explanation. Yes I can now see that equivalence relation between your example and mine. It does sometime feel alien to see variables representing partially applied functions (e.g k in this case) when you are coming from the imperative land. Cheers, Shishir Srivastava

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Continuations
by Shishir Srivastava 29 May '15

29 May '15

Hi, Reading on continuation I've came across this new style of creating the functions which I guess is not very clear in how it works --------------- Prelude> let add_cps x y = \k -> k (x+y) Prelude> add_cps 3 4 $ print 7 --------------- I have some questions as to 1) what is the role of variable 'k' and what eventually happens to it. 2) How does print work after the $ because there is clearly no parameter being passed to it. Thanks, Shishir Srivastava

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Data.Vector in Haskell
by Dananji Liyanage 28 May '15

28 May '15

Hi All, I'm trying to extract elements from a Vector using 'head' and 'tail' function in Data.Vector package, but it gives me the following error now. <interactive>:41:18: Couldn't match expected type ‘Data.Vector.Vector a’ with actual type ‘vector-0.10.9.1:Data.Vector.Vector Int’ NB: ‘Data.Vector.Vector’ is defined in ‘Data.Vector’ in package ‘vector-0.10.12.3’ ‘vector-0.10.9.1:Data.Vector.Vector’ is defined in ‘Data.Vector’ in package ‘vector-0.10.9.1’ Relevant bindings include it :: a (bound at <interactive>:41:1) In the first argument of ‘Data.Vector.head’, namely ‘it’ In the expression: Data.Vector.head it It was working fine before. Any help would be appreciated. Thanks in advance! -- Regards, Dananji Liyanage

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Can't figure out my error here
by Barry DeZonia 28 May '15

28 May '15

Hello, I have a small piece of code that does not compile and I'm having trouble figuring out why. Here is the relevant snippet: readChunks :: Handle -> [String] -> IO [String] readChunks handle accum = do chunk <- readHeaderChunk handle if isLast chunk then return (accum ++ chunk) else return (readChunks handle (accum ++ chunk)) isLast :: [String] -> Bool readHeaderChunk :: Handle -> IO [String] And here is the single compiler error: hacks.hs:48:18: Couldn't match expected type `[String]' with actual type `IO [String]' In the return type of a call of `readChunks' In the first argument of `return', namely `(readChunks handle (accum ++ chunk))' In the expression: return (readChunks handle (accum ++ chunk)) What I'm most confused about is that chunk is passed to isLast as a [String] with no compiler error but cannot be passed to accum ++ chunk that way. Or so it seems. Can someone she some light on this? Thanks.

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Working with Haskell matrices
by Dananji Liyanage 27 May '15

27 May '15

Hi All, I'm implementing a puzzle where the input is given as a matrix, using Data.Matrix package in Haskell. How do I implement similar functions to `union`, which are available for lists on a matrix? -- Regards, Dananji Liyanage

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Using IO values for computations
by Dananji Liyanage 25 May '15

25 May '15

Hi All, I'm writing a code, where the input is read from a text file using: readValues = readFile "Input.txt" Since the type of this is 'IO String', I can't use this in the consequent functions. For an example: I want to split this as follows within another function extractInput url method template | isURI url == True = getList values components | otherwise = [] where components = splitTemplate readValues values = getURL (splitURL url) method This gives the following error: Couldn't match type ‘IO String’ with ‘[Char]’ Expected type: String Actual type: IO String How can I solve this? Thanks in advance! -- Regards, Dananji Liyanage

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Crawling page and making json out of it
by René Klačan 24 May '15

24 May '15

Hi all, I have a small example that opens page and creates a json out of it https://gist.github.com/reneklacan/262eef04a3bc67607dd8 It works correctly and it returns desired results. But I am sure there is a better way (or more Haskell way) how to write it. Especially I don't really like how *getInfo* function looks like. Get you please give me an advice how to make it more ellegant or tell me what I'm doing wrong. Thanks Rene

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ClassyPrelude vs. Haskell.Language.Interpreter
by Mike Meyer 23 May '15

23 May '15

I'm trying to run interpreted code via ClassyPrelude, and getting some results that make me suspect a bug in the Prelude's type system. Or maybe the interpreter. Anyway, here's a bit of code that works as expected: {-# LANGUAGE NoImplicitPrelude #-} import ClassyPrelude import Language.Haskell.Interpreter main :: IO () main = do fun <- runInterpreter $ makeFun "reverse" case fun of Left e -> print e Right f -> readFile "/etc/motd" >>= hPut stdout . f makeFun expr = do set [languageExtensions := [NoImplicitPrelude]] setImportsQ [("ClassyPrelude", Nothing)] interpret expr (as :: Text -> Text) I don't think I can simplify this any further. It works as expected, and also works as expected, and prints out the contents of /etc/motd reversed. However, if you change the type signature in the last line from Text -> Text to LText -> Ltext (to get lazy text), you get no output. But if you change the function in the first line after main from "reverse" to "id", it works. So far, it might be an issue with lazy IO. However, change the type signature in the last line to LText -> Text. In this case, there is no output for either value of the expression. I expect an error in this case, as neither id nor reverse should be able to have the type LText -> Text! So, is there something I missed in either ClassyPrelude or the Interpreter? Or is this a subtle interaction, in which case can someone suggest a workaround? Or have I found a bug in one of the two?

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