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Can't figure out my error here
by Barry DeZonia 29 May '15

29 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 26 May '15

26 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 25 May '15

25 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 24 May '15

24 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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Couldn't match expected type `(a0 -> [a0]) -> [a0]', with actual type `[a0]'
by Roelof Wobben 23 May '15

23 May '15

Hello, For some reasons my file's are corrupted. I had repair them with success except this one. Here is the code : toDigits number | number <= 0 = [] | otherwise = toDigits' [] number where toDigits' acc 0 = acc toDigits' acc number = ((number `mod` 10 ): acc) (toDigits' (number `mod` 10)) main = print $ toDigits 123 and here is the error: Couldn't match expected type `(a0 -> [a0]) -> [a0]' with actual type `[a0]' The function `(number `mod` 10) : acc' is applied to one argument, but its type `[a0]' has none In the expression: ((number `mod` 10) : acc) (toDigits' (number `mod` 10)) In an equation for toDigits': toDigits' acc number = ((number `mod` 10) : acc) (toDigits' (number `mod` 10)) Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Question regarding CIS 194 Homework Assignment 1
by Vaibhav Goel 20 May '15

20 May '15

My apologies, this is only tangentially related to Haskell and I am not looking for a solution. I am confused with the assignment itself and hence asking here. The assignment (https://www.seas.upenn.edu/~cis194/spring13/hw/01-intro.pdf) says: ====== BEGIN Double the value of every second digit beginning from the right. That is, the last digit is unchanged; the second-to-last digit is dou- bled; the third-to-last digit is unchanged; and so on. For example, [1,3,8,6] becomes [2,3,16,6] Add the digits of the doubled values and the undoubled dig- its from the original number. For example, [2,3,16,6] becomes 2+3+1+6+6 = 18 ======== END Firstly, I am confused as to how the doubled values are being added to the undoubled number in the above example. It looks like only the individual numbers of the doubled values are being added Secondly, later on in the assignment: ======== BEGIN Example : validate 4012888888881881 = True Example : validate 4012888888881882 = False ========= END If we are to follow the algorithm described (double the value of "every second digit" beginning from the right, last digit unchanged", then the above numbers are identical EXCEPT For the last digits (1 and 2) Any help is appreciated in decoding these instructions. Again please do not provide Haskell code, since I want to attempt to write it myself. I am just looking for help with the algorithm. Regards,

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Iterating through lists
by Dananji Liyanage 19 May '15

19 May '15

Hi All, I'm building a 9x9 grid like list, and I want to extract each column of that grid. My input is a list of integers as follows; input = [0, 0, 0, 0, 0, 0, 4, 0, 9, 0, 0, 0, 0, 8, 0, 0, 5, 0, 7, 0, 2, 4, 5, 3, 6, 0, 0, 6, 7, 0, 0, 0, 1, 5, 0, 2, 2, 0, 8, 7, 0, 4, 3, 0, 1, 9, 0, 4, 5, 0, 0, 0, 8, 6, 0, 0, 6, 3, 1, 9, 8, 0, 7, 0, 2, 0, 0, 7, 0, 0, 0, 0, 1, 0, 7, 0, 0, 0, 0, 0, 0] I tried list comprehension with 'take' function in lists as follows; columns xs = [x | x <- (take 1 xs)] : columns (drop 9 xs) --> columns input This extracts only the first column. How I can go for the other columns? -- Regards, Dananji Liyanage

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which ones to use
by Roelof Wobben 18 May '15

18 May '15

Hello, My linux distro has haskell-platform not avaible. But things like ghc and cabal are avaible. Which ones does I need so I can programm in Haskell with SublimeText 3 as editor ? Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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profiling :: memory allocation
by Konstantin Saveljev 18 May '15

18 May '15

Hello, I'm trying to figure out how to reduce the allocation rate in my program. But I'm stuck with the profile report which has the top allocator to be shown like this: COST CENTRE MODULE %time %alloc >>= Internal 20.5 22.6 I do not understand what it actually means? Can anyone explain to me or is there anything I can do so it shows some more information (instead of showing the bind operator >>=) ? Thanks, Konstantin

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