[Haskell-cafe] How to give unique name/id to nodes outside any monad ?

[minh thu]

Hi,

I'd like to process some kind of graph data structure,
say something like

data DS = A [DS] | B DS DS | C.

but I want to be able to discover any sharing.
Thus, in

b = B a a where a = A [C],

if I want to malloc a similar data structure,
I have to handle to the node representing B
two times the same pointer (the one returned
after allocating A [C]).

To discover sharing, I thought it would be
necessary to give unique name to node and
then compare them while traversing the graph.
I could give the name by hand but it would be
cumbersome. But at least it would not require
any monad for the bookkeeping of ungiven
names. Is it possible to give those names
automatically but outside any monad ?

Thanks,
Thu