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Date: Wed, 8 Dec 1999 06:07:15 +0100 (MET)

From: Benoit de Boursetty <debourse@email.enst.fr>

To: caml-list@inria.fr

Subject: Ask for explanation -- possibly repeated

Hi

My question must already have been asked, but the error I get from the

compiler does not seem to be listed in the documentation

("This kind of expression is not allowed as right-hand side of `let rec'")

So here's the situation. I have a basic, standard structure for trees.

# type 'a classical_tree = Classical_node of 'a * 'a classical_tree list

And now I'd like to move to another tree structure (cyclic by default)

where each node or leaf points to its parent node, if there is one.

So I want to use the following type:

# type 'a tree = Node of 'a tree option * 'a * 'a tree list

which means: Node (parent node, data, sons).

I can easily create such typed values by hand:

# let rec root = Node (None, 0, [son1; son2])

and son1 = Node (Some root, 1, [])

and son2 = Node (Some root, 2, []);;

But I can't manage to write the translation function from the first type

to the second one in a purely applicative fashion. It is intuitive to

write:

# 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

but of course this doesn't work ("This kind of expression is not allowed

as right-hand side of `let rec'", as they say) and I understand why. It's

because the contents of "this_node", not yet defined, could be looked at

in function "aux". In fact this is a correct algorithm only because

argument "father" is not accessed to in function "aux".

So, is there an applicative workaround? I know how to do it with mutable

values / references, but...

Benoit de Boursetty.

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