Chris Angus:
>
> Would it not be better to tag a start point then we can manipulate this
> easier
> and move it back to a singly linked list etc.
>
> data Db a = Dd (Db a) a (Db a)
> | DStart (Db a) a (Db a)
>
> ...
Well, I am sufficiently old to confess that one of my favourite OO
languages, and the one where I found doubly-linked lists for the first
time was ...
Yes, Simula-67.
Actually *they did* that. A "node" had two sub-classes, the link and the
head, and the link chain was doubly attached to the head. This structure
has been havily used for the maintenance of the co-routine bedlam
exploited in simulation programs.
The idea of double lists was to permit a fast two-directional
navigation,
and the ease of insertion/deletion.
But in Haskell, where the beasts are not mutable:
... Actually, has anybody really used them for practical purposes?
Jerzy Karczmarczuk
Caen, France
