> >From: Benoit de Boursetty <debourse@email.enst.fr>
> >
> ># let translate_tree =
> >    let rec aux father (Classical_node (data, sons)) =
> >      let rec this_node = Node (father, data,
> >                                List.map
> >                                  (aux (Some this_node))
> >                                  sons)
> >      in this_node
> >    in aux None
> 
> >So, is there an applicative workaround? I know how to do it with mutable
> >values / references, but...

As usual: hide the side effect into the lazy side of the language that
is usually considered as pure and applicative, add a bit of
eta-expansion if your language does not know lazy evaluation properly,
and you get a simple workaround:

let translate_tree t =
    let rec aux father (Classical_node (data, sons)) =
      let rec this_node = lazy (Node (father, data,
                                List.map
                                  (aux (Some (Lazy.force this_node)))
                                  sons))
      in Lazy.force this_node
    in aux None t;;
val translate_tree : 'a classical_tree -> 'a tree = <fun>

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/