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can I have a case statement with a pattern matching
by Roelof Wobben 14 May '15

14 May '15

Hello, I try to make make the assignment of CIS194 where I must multiply every second item from the right by 2 So I thought this could be working : -- | Convert a digit to a reversed list for example 123 becomes [3,2,1] revtoDigits :: Integer -> [Integer] revtoDigits number | number <= 0 = [] | number > 0 = number`mod` 10 : revtoDigits (number `div` 10) -- | Convert a digit to a list for example 123 becomes [1,2,3] toDigits :: Integer -> [Integer] toDigits number | number <= 0 = [] | number > 0 = toDigitsAcc [] number toDigitsAcc:: [Integer] -> Integer -> [Integer] toDigitsAcc acc number | number <= 0 = acc | number > 0 = toDigitsAcc ((number `mod` 10) : acc) (number `div` 10) doubleEveryOther :: [Integer] -> [Integer] doubleEveryOther list = case (length list) `mod` 2 of 0 -> doubleEveryOther [] = [] doubleEveryOther (x:y:xs) = x*2 : y : xs (length list) `mod` 2 of 0 -> doubleEveryOther [] = [] doubleEveryOther (x:y:xs) = x : y*2 : xs -- | The main entry point. main = print $ doubleEveryOther [1,2,3] but apperantly you cannot have pattern matching after a case statement because I get a error on the = of a empty list Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Date Time in Haskell
by Dananji Liyanage 14 May '15

14 May '15

Hi All, I'm trying out the date time conversions in Haskell. How do I convert a keyboard input (IO String) to UTCTime? This is how I'm doing (probably there's a better way), convDay x = read (x ++ " 00:00:00") :: UTCTime Another thing, how do I convert getCurrentTime (IO UTCTime) to UTCTime in order to get the time difference between the keyboard input date and today? -- Regards, Dananji Liyanage

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Function Composition
by Shishir Srivastava 13 May '15

13 May '15

Hi, Could someone please point out what is the difference between using $ and parenthesis in the function composition below. Why does the first expression work whereas the second fails. --- $$ head.head$[[1,2],[3,4]] 1 $$ head.head([[1,2],[3,4]]) <interactive>:122:12: Couldn't match expected type ‘a -> [c]’ with actual type ‘[t0]’ Relevant bindings include --- Thanks, Shishir

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accumenlator problem
by Roelof Wobben 13 May '15

13 May '15

Hello, I have this : -- | Convert a digit to a list for example 123 becomes [1,2,3] toDigits :: Integer -> [Integer] toDigits number | number <= 0 = [] | number > 0 = toDigitsAcc [] number toDigitsAcc:: [Integer] -> [Integer] -> [Integer] toDigitsAcc acc number | number <= 0 = acc | number > 0 = (number `mod` 10) : acc toDigitsAcc (number `div` 10) bur there are error on both last lines of he functions. Any tips how to solve this ? Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Are these solution the Haskell way ?
by Roelof Wobben 13 May '15

13 May '15

Hello, For practising pattern matching and recursion I did recreate some commands of Data,list. My re-implementation of ++ : plusplus :: [a] -> [a] -> [a] plusplus [] [] = [] ; plusplus [] (xs) = xs plusplus (xs) [] = xs plusplus (xs) yx = plusplus' (reverse xs) yx plusplus' :: [a] -> [a] -> [a] plusplus' [] (xs) = xs plusplus' (x:xs) yx = plusplus' xs (x:yx) main = print $ plusplus ["a","b"] ["c","d"] my re-implementation of init : import Data.Maybe -- | The main entry point. init' :: [a] -> Maybe [a] init' [] = Nothing init' [x] = Just [] init' (x:xs) = Just (x:fromMaybe xs (init' xs)) main = print . init' $ [1,3] my re-implementation of last : -- | The main entry point. last' :: [a] -> Maybe a last' [] = Nothing last' [x] = Just x last' (_:xs) = last' xs main = print . last' $ [] Now I wonder if these solutions are the haskell way ? if not so, how can I improve them , Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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another type problem
by Roelof Wobben 13 May '15

13 May '15

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how can I make this work
by Roelof Wobben 12 May '15

12 May '15

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AMQP and nested exception handlers
by Alex 12 May '15

12 May '15

Hi: I am writing a small application which receives HTTP requests, translates them to JSON, and queues the requests using RabbitMQ. I am using exceptions to handle extreme cases, such as when a client's HTTP request lacks a critical header: lookupHeader :: RequestHeaders -> HeaderName -> Text lookupHeader hs h = decodeUtf8 $ fromMaybe notFound (lookup h hs) where notFound = throw $ MissingHeader $ decodeUtf8 $ original h The problem I am running in to is that the header isn't actually looked up until I call the AMQP library's publishMsg function. If I purposely do not supply a critical header, I get the following error printed to the screen: ConnectionClosedException "ConnectionClosedException \"UNEXPECTED_FRAME - expected content header for class 60, got non content header frame instead\"" If I add the following line just above the publishMsg function (to force evaluation), my exception handler is called successfully: print $ encode req As a result, I suspect that this is due to the fact that the "throw MissingHeader" is getting captured by the AMQP library. What's the best way to deal with this situation? -- Alex

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why is this type wrong
by Roelof Wobben 12 May '15

12 May '15

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Reading, Processing, and Writing I/O
by Dananji Liyanage 12 May '15

12 May '15

Hi All, As a newcomer to Haskell, I'm following a tutorial to learn I/O in Haskell. As an exercise I'm writing a small code to read from an input text file and process it and then write it back to an output file. The problem is that, it doesn't write to the output file after the input is being processed. Here's my code: module Main where import System.IO main = do input <- readFile "input.txt" let reversedInput = reverseInput (convert input) writeFile "reversed.txt" (reversedInput) -- These two lines are not printed on the shell putStrLn (show reversedInput) putStrLn "Writing to file...." -- Reversing the input reverseInput input = (last input) : reverseInput (init input) -- Convert from String IO -> String convert x = show (x) -- Regards, Dananji Liyanage

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