Re: [Haskell-beginners] Questions About Rose Trees Analysis

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 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