[Haskell-cafe] about the concatenation on a tree

[Thomas Davie]

On 31 Dec 2008, at 21:18, Henk-Jan van Tuyl wrote:

> On Wed, 31 Dec 2008 17:19:09 +0100, Max cs <max.cs. 
> 2009 at googlemail.com> wrote:
>
>> Hi Henk-Jan van Tuyl,
>>
>> Thank you very much for your reply!
>>
>> I think the concatenation should be different to thhe
>>
>> treeConcat :: Tree a -> Tree a -> Tree a
>>
>> the above is a combination of two trees instead of a concatenation,  
>> so
>> I think the type of treeConcat should be:
>>
>> treeConcat :: Tree (Tree a) -> Tree a
>>
>> instead. How do you think? : ) I tried to implement it .. but it  
>> seems
>> confusing.. to me
>>
>> Thanks
>>
>> Max
>
> Hello Max,
>
> The function
>  treeConcat :: Tree (Tree a) -> Tree a
> cannot be created, as it has an infinite type;

It does?  How did he type it then?  And yes, it can be created
concatT :: Tree (Tree a) -> Tree a
concatT (Leaf t) = t
concatT (Branch l r) = Branch (concatT l) (concatT r)

It's also known as join on trees (as I explained a bit more in my  
response on haskell-beginners).

Bob