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

minh thu noteed at gmail.com
Thu Jan 8 03:53:46 EST 2009

Well, the processing of the data structure has to be done in the IO monad.
What is the library you talk about ? Could it give the "stable names"
(in IO) for
each node of the mentioned graph (I mean, after the graph has been constructed
purely) ?


2009/1/8 Lennart Augustsson <lennart at augustsson.net>:
> Of course you don't need a monad, but you need to do the same
> operations as you would with a state monad to number the nodes.  This
> is the only way in (pure) Haskell.  There is no object identity in
> Haskell, so if you want the nodes to have identity you need to provide
> it.
> GHC does have a library for stable names which (in the IO monad)
> allows you to get something akin to the address of a value in memory.
> But that's not the functional way of doing this.
>  -- Lennart
> On Thu, Jan 8, 2009 at 9:28 AM, minh thu <noteed at gmail.com> wrote:
>> 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
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