[Haskell-beginners] Questions About Rose Trees Analysis

[Lorenzo Isella]

On Thu, Dec 17, 2015 at 12:10:01PM +0100, Henk-Jan van Tuyl wrote:
>On Thu, 17 Dec 2015 11:37:24 +0100, Lorenzo Isella 
><lorenzo.isella at gmail.com> wrote:
>:
>>
>>data Rose a = a :> [Rose a]
>>deriving (Eq, Show)
>>
>>and the root can be detected simply as
>>
>>root (a :> rs) = a
>>
>>I would like to have the expression (with the ":>" notation for the
>>Node) of the function to find the children of the root. Example of
>>expected behavior of this function children
>>
>>children (1 :> [2 :> [], 3 :> []]) = [2 :> [], 3 :> []]
>>
>>On top of that, I am trying to get the functions to have the functions
>>
>> size :: Rose a -> Int
>> leaves :: Rose a -> Int
>>
>>that count the number of nodes in a rose tree, respectively the number
>>of leaves (nodes without any children).
>>For the children function, I have tried stuff like
>>
>>children (a :> rs) = rs
>>
>>quite unsuccessfully.
>:
>
>Your definition of children is correct, what is the message you get 
>from the compiler/interpreter?
>
>Regards,
>Henk-Jan van Tuyl


Hello,
This is the situation: my script rose.hs is given by


data Rose a = a :> [Rose a]

root (a :> rs) = a

children (a :> rs) = rs


and this is what happens when I load it and apply the children
function on a rose tree


$ ghci
GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :load rose.hs
[1 of 1] Compiling Main             ( rose.hs, interpreted )
Ok, modules loaded: Main.
*Main> children (1 :> [2 :> [], 3 :> []])

<interactive>:3:1:
    No instance for (Show (Rose t0)) arising from a use of ‘print’
    In a stmt of an interactive GHCi command: print it
	

I do not really understand what goes wrong and any suggestion is
appreciated.
Cheers

Lorenzo