Max Voit schreef op 5-2-2015 om 13:01:

Hi Roelof,

On Thu, 05 Feb 2015 12:37:12 +0100 Roelof Wobben wrote:

...

main :: IO () main = do toDigits 123 src/Main.hs@14:5-14:17 Couldn't match expected type ‘IO ()’ with actual type [Integer] … In a stmt of a 'do' block: toDigits 123 In the expression: do { toDigits 123 } since your do block consists only of one expression, actually your main looks like:

main :: IO () main = toDigits 123

Perhaps this explains where the problem lies.

Considering, however, do-notation: The last statement of a do-block needs to be an expression of the type of the do-block. Translating do-notation to bind operator sequences[1] may be helpful in understanding this.

best, Max

[1] http://en.wikibooks.org/wiki/Haskell/do_notation _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Thanks, Another question : how can I display the array ? putStrln works only for strings, Roelof

Max Voit schreef op 5-2-2015 om 13:35:

On Thu, 05 Feb 2015 13:25:16 +0100 Roelof Wobben wrote:

...

Another question : how can I display the array ?

putStrln works only for strings, Yep, so you need to convert it to a string and putStrLn that. Kindly enough typing ":i []" into ghci tells you, that lists have an instance for "show".

Anonther option would be to convert every single list element to a string and putStrLn that. "map" comes in handy here (or mapM_ if you directly want to use print instead of putStrLn . show).

best, Max _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Thanks that part worked fine. One part I cannot figure out is how to make this : -- | Main entry point to the application. module Main where -- | convert a number to a array in pieces where a negative number will be a empty array. toDigits :: Integer -> [Integer] toDigits n | n < 0 = [] | otherwise = n `mod` 10 : [] -- | The main entry point. main :: IO () main = do print $ toDigits 123 Now it showing only [1] or [23] I have to find out how I can do something like this toDigits 23 [1] But I cannot do this that way because it has to be Integer -> [Integer] Who has a tip for me ? Roelof

Am 05.02.2015 17:29 schrieb "Marcin Mrotek" :

Well, toDigits is always finishing its work after dividing the input

number once, and always returns lists of at most one element. You need it to call itself recursively until it's done, and append the calculated digit to the result:

toDigits :: Integer -> [Integer] toDigits n | n <= 0 = [] | otherwise = n `mod` 10 : toDigits (n `div` 10)

This doesn't feel right: toDigits 10 = 0 : toDigits 1 = 0 : 1 : toDigits 0 = 0 : 1 : [] = [0,1] I'd expect the result to be [1,0]...

The <= is necessary, otherwise the function would loop infinitely on 0.

This version will print the digits in reverse, so you might move the code to a local function in toDigits, and make toDigit call it and then reverse the result.

I'd strongly recommend reading a Haskell tutorial (for example

http://learnyouahaskell.com ) to learn the basics.

Kind regards, Marcin Mrotek

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

@Norbert Melzer - Yeah, I just wanted to provide a solution without shuffling Roelof's code too much, so he'd recognize how is it different from his version. I did write that this is incomplete and requires the list to be reversed later. @Roelof Wobben - Maybe try a different one? There are several popular choices other than LYAH: * FP Complete Shool of Haskellhttps://www.fpcomplete.com/school * Wikibooks: http://en.wikibooks.org/wiki/Haskell * Real World Haskell: http://book.realworldhaskell.org (Warning: while I remember learning from this book some time ago and it was fine, every now and then I see people saying that it's outdated and some examples don't compile) Kind regards, Marcin Mrotek

Hello, Roelof. Your function is done. But it should be named 'toDigitsReversed' or so. So if you could write another 'reverse' function then you can compose like this. toDigits = reverse . toDigitsReversed -- YCH