[mercury-users] Self-referential data structures

[Peter Schachte]

On Mon, 1 Dec 1997, Fergus Henderson wrote:

> I have just thought of another solution.  We could provide a new
> procedure in the `store' module to create a circular element. 
> This procedure would take as an argument a function which when
> given a reference (mutvar) to type T produces a value of type T.

Could you give us a quick example of how this could be used?

...
> This avoids the need for null references (and hence the need to
> document for every procedure taking a reference what happens if
> the reference is null).  It avoids the need to use a maybe(T) type.

How?  In many uses of circular structures, there's still the need for
empty structures.  How can you handle these without some use of a
maybe type, or making everything semidet/nondet?


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