[Haskell-cafe] I just don't get it (data structures and OO)

Phlex Phlex at telenet.be
Sun Jun 3 03:41:29 EDT 2007

Donald Bruce Stewart wrote:
> Phlex:
>> On the other side, using the functional paradigm, it seems to me that 
>> the function i use in order to create a _new_ inhabitant with a 
>> different age will need to have knowledge of the country over it, the 
>> planet ..and so on up to the universe...as i need to update all these 
>> structures to reflect the change. This is pretty bad and most probably 
>> doesn't need to be like this.
> Nope, its not done like that. You share as much of the original
> structure as you can, as a general principle.
> Imagine updating a node in a tree by just detaching and reattaching a
> pointer.
>     [1]                         [1]
>     / \                         / \
>   [2] [3]     update node 5   [2] [3] 
>        / \     with value 7       / \
>      [4] [5]                     [4] *
> and share the rest of the structure. Since the rest isn't mutable
> anyway, you can share all over.
> -- Don

That's precisely the thing i don't understand.
In order to update node 3 with a new pointer, i need to mutate it, so i 
need to recreate it, and so on up to node 1.

Now in this exemple, it's ok since that's a regular tree and the process 
can be automated, but when each node has a different type, it can become 
quite hairy.


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