[Haskell] implementing pointers-based data structure

[Duncan Coutts]

On Wed, 2006-03-15 at 17:21 +0100, Sebastian Sylvan wrote:
> On 3/15/06, minh thu <noteed at gmail.com> wrote:
> > hi everybody,
> >
> > i have to implement some data structure which is usually implemented
> > with pointers in imperative languages (can think of it as a double
> > linked list).
> >
> > i'd like to know how i have to do that.
> >
> > is there any standard way of converting pointer-based data structure
> > into an inductively-defined data type (like standard haskell list) ?
> > is-it possible ?
> >
> > or would it be good to define the data structure in c and write a
> > haskell wrapper around the c code ?
> >
> > thank you a lot, bye,
> > vo minh thu
> 
> You can use references, IO, ST or STM.
> 
> You can also use laziness (untested!):
> 
> data DLink a = (DLink a) a (DLink a) | Nil
> 
> test = d1
>   where d1 = Nil 5 d2
>             d2 = d1 6 Nil
> 
> test here is a linked list containing 5 and the next node containing
> 6. Both nodes have references to the next and previous links (wich is
> Nil at the ends). The magic here is laziness. You reference d2 in the
> definition for d1 and d1 in the definition for d2. It gets sort of
> clumsy to work with, though. You're probably better off using STRefs
> (for example) if you really need that type of data structures...

I wrote a talk once on this topic:
http://web.comlab.ox.ac.uk/oucl/work/duncan.coutts/papers/recursive_data_structures_in_haskell.pdf

Duncan