July 2013 - Beginners

14 Aug '13

Every OO language which supports generics allows a declaration such as List<Show> alist, where Show is an interface. Any type implementing Show can be put in alist, and any Show operation can be performed on the alist's members. No casts, wrappers, or other special types and plumbing are needed. Why isn't it possible to do this directly in Haskell?

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Shorten this code
by Nadav Chernin 01 Aug '13

01 Aug '13

Hi, all Below is my solution to SPOJ->Polybius square<http://www.spoj.com/problems/POLYBIUS> Please try to shorten it: *import Data.Maybe* *d=[1..5]* *f s=unwords$map(\c->fromJust$lookup c((' ',""):('J',"24"):zip(['A'..'I']++['K'..'Z'])[show(x+10*y)|y<-d,x<-d]))s* *main=getLine>>(interact$unlines.map f.lines)* Thanks, Nadav

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Viewing Template Haskell output?
by Lee Short 30 Jul '13

30 Jul '13

I'm working with some Yesod code, and it's complaining about some missing function definitions. I'd really like to see the code that Template Haskell is outputting -- this is hard to debug otherwise. The --ddump-splices output is managable at this early stage, but I can't imagine using it once I add more functionality and the code grows. Is there way to get the template haskell output code?

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listenOn specific IP
by vold 30 Jul '13

30 Jul '13

I'm trying to write a simple server with network. listenOnSource takes a socket number, but tries to bind to every available IP, which isn't what I need. How do I tell it to bind to a specific address?

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Question about Regular expression pattern consuming.
by S. H. Aegis 29 Jul '13

29 Jul '13

Hi. I'm newbie to Haskell. I want to get date, it's type is [[String]]. The problem is that "the string that is used in regular expression pattern" is consumed. ie, disappear. Here is my code. ------------------------------------------------------------ import Text.Regex import Control.Applicative main :: IO() main = do myIn <- readFile "Data.dat" print $ lines <$> intoEachPt myIn intoEachPt :: String -> [String] intoEachPt = splitRegex (mkRegex "20[0-9]{13}AH021") ----------------------------------------------------------- How can I fix this? Data: ....there is many DIGIT.....201306000300001AH02112361640 9....... Output: [[....there is many DIGIT..."],["12361640 9......]....] I hope: [[....there is many DIGIT..."],["201306000300001AH02112361640 9......]....]

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Cannot Show Table
by Guenter Fiebig 26 Jul '13

26 Jul '13

Hello, me, I am a very beginner. I build a GUI with Glade containing an input- and an output table as well as two buttons. My target is to take the user-input from the input table, process it and put the result to the output table. My first problem is to display the content from the input table for control. With " onClicked inButton (putStr "Hello, World!") " haskell works fine without errors. With " onClicked inButton (??? inTable) " I do not find the right command to display within Prelude. Thanks for help, Guenter inButton <- builderGetObject builder castToButton "button1"; outButton <- builderGetObject builder castToButton "button2"; inTable <- builderGetObject builder castToTable "table1"; --onClicked inButton (putStr "Hello, World!"); --0nly for testing onClicked inButton (??? inTable);

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Hamlet hyperlinks in Yesod
by Kimberley Hoff 25 Jul '13

25 Jul '13

Hi, new member here! I've really no prior experience with web development but trying to learn as fast as possible. I'm trying to get a hyperlink in a Hamlet file to connect to the right text. It goes to the right place, but the words I want it to link from don't even appear in the browser view of the web page. I've tried copying and pasting from other Hamlet files in the project but it doesn't seem to work. This is the code that's been giving me difficulties: <p> Welcome! Please read the <a href=@{WikiR}>How to Help # / wiki page to find out more about our particular needs at this time. I am trying to get "How to Help" as the link but in the browser view "How to Help wiki page" doesn't even appear and the rest of the sentence is the link. Can anyone tell me what I'm doing wrong? Thanks, Kim Hoff

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Module name from a function name?
by Jong-won Choi 25 Jul '13

25 Jul '13

Hi, I am self-teaching Haskell and also try to fix a slightly broken Emacs mode for Haskell. I'd like to browse Haskell documentation from Emacs - C-c C-d on a function name, say. Current Emacs Lisp Haskell mode implementation uses return value of ':info', for example, GHC.List for length (the function name I want to browse). Unfortunately, this does not work because Glasgow Haskell's HTML doc uses module name Data.List(i.e. Data-List.html) instead of GHC.List. I'd like to get 'Data.List' from 'length'. How can I do this? For example, if it is Common Lisp, this can be achieved by: (symbol-package 'length) One more thing. I think :info command is only for GHC. Is there any portable ways which will work on other implementations as well? Thanks! - Jong-won Choi

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Pipe and typeclasses
by Emmanuel Surleau 24 Jul '13

24 Jul '13

Hello, I have seen from time to time the pipe symbol in type signatures, like so: class Monad m => MonadReader r m | m -> r where How should I interpret that ? Or more to the point, how does the compiler interpret it? Thanks, Emm

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Why does this list comprehension return an empty list?
by Costello, Roger L. 23 Jul '13

23 Jul '13

Hi Folks, I have a list of singletons: xs = [("a")] f is a function that, given an argument x, it returns the argument: f x = x g is a function that, given an argument x, it returns the empty list: g x = [] I have a list comprehension that extracts the singletons from xs using f and g, and creates a pair from their output: [(a,b) | a <- f xs, b <- g xs] I executed this and the result is the empty list: [] That is odd. Why is the empty list the result? Consider this list comprehension that extracts the singletons from xs using f only, and creates a pair by matching up the singleton from xs with the empty list: [(a,[]) | a <- f xs] The result is this: [("a",[])] That is the result that I expected to get with the first list comprehension. Would you explain why this: [(a,b) | a <- f xs, b <- g xs] yields [], whereas this: [(a,[]) | a <- f xs] yields [("a",[])]? Also, how would I modify this: [(a,b) | a <- f xs, b <- g xs] so that it produces this: [("a",[])] /Roger

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