[Haskell-cafe] lists of arbitrary depth

[Shlomi Vaknin]

Thank you Bob,
your example clarified how actually using such data type would appear in
haskell. I naively thought it would be as simple as defining a regular list,
but i see it is slightly more strict than that. I appreciate your help!

Vadali

On Tue, Jul 13, 2010 at 11:18 AM, Thomas Davie <tom.davie at gmail.com> wrote:

>
> On 13 Jul 2010, at 10:11, Shlomi Vaknin wrote:
>
> > Thank you all for replying!
> >
> > I am really beginning my baby steps in this fascinating language, and was
> just wondering if it was possible to naturally scan lists with arbitrary
> lists (aka trees :) ).
>
> Trees aren't lists, Trees are trees...  Here's how you create one*
>
> data Tree a = Leaf
>             | Branch a [Tree a]
>
> This roughly says, a tree can be made up of two possible things... First,
> it could be a leaf, and secondly, it could be a branch with an element at
> it, and a list of subtrees.
>
> Some example trees using this data type:
>
> 1) Leaf
> 2) Branch 5 []
> 3) Branch 5 [Leaf]
> 4) Branch 5 [Branch 10 [], Branch 20 [], Leaf, Branch 50 [Branch 10 [],
> Leaf]]
>
> When you have really strong typing, you also have very well specified
> types.  You don't just use a list as if it were a tree, you declare what a
> tree is.
>
> Bob
>
> * In this case an arbitrarily branching tree, we could ofc declare
> different forms of tree here.
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