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

It seems to me like there might be benefit to special casing the error message when (as in this case) the only thing wrong with an expression is that the resulting type is missing the Show instance implicitly demanded by ghci. On Sun, Dec 20, 2015 at 4:06 AM, Lorenzo Isella wrote:

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

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