May 2015 - Beginners - Haskell.org

Error while using type and data constructor
by Nishant 15 May '15

15 May '15

Hi, I was writing some code for Tree. 1 : data Tree a = NULL | Node a (Tree a) (Tree a) works fine. But, 2 : data Tree a = NULL | Tree a (Node a) Tree a does not work. GHCI says Not in scope " type constructor or class 'Node'. Why is 2 declaration not valid though second definition looks much more closer to what a binary tree actually means ? Regards Nishant Verma

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CANCEL ME PLEASE
by Letizia Galli 15 May '15

15 May '15

-- Letizia Galli 0039 328 6917393 www.letiziagalli.it

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cabal problem
by Roelof Wobben 15 May '15

15 May '15

Hello,, I want to use the module Digits which according to Hoogle is in Data.Digits But when I do cabal install Data.Digits I see this error message : cabal install Data.Digits cabal: The file does not exist 'Data.Digits' What do I do wrong ? Roelof --- Dit e-mailbericht is gecontroleerd op virussen met Avast antivirussoftware. http://www.avast.com

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Questions about optparse-applicative
by Mike Meyer 15 May '15

15 May '15

In trying to convert eddie (http://hackage.haskell.org/package/eddie) from CmdArgs optparse-applicative, I've run into a couple of problems. First question: I have options whose legality depends on other options: two that can't be specified together, and a couple that require one of the two, among other things. Is there some way to specify such constraints when building the parser? Maybe a way to print just the usage message if I check these myself? Maybe a better approach? It looks like if I replace main in test/Example/Hello.hs with: main = handleParseResult . Failure $ parserFailure (prefs idm) (info sample briefDesc) ShowHelpText mempty I should just get the Usage message output, but instead I get the full help text. Second, and possibly related, it's not at all clear what fullDesc does in that example program. In particular, removing it doesn't seem to change the program behavior. Is there some behavior I've missed, or is this an error?

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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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