12.1 Encapsulating Failure

For returning the result of the unification process, independently of failure or not, you can use the Oz search facilities as follows:

{Browse {Search.base.all proc{$ X} {Constrain X _} end}}

This works fine for returning failure in terms of an empty list rather than a programming error. But there is still a problem with returning the result of unification when coreferences occur. How can you return the result in the following example?

declare proc{CoRef X} Y in X=f(Y Y) end 
{Browse {Search.base.all proc{$ X} {CoRef X} end}}

Here you have lost the coreference between the first and second subtree of X. This problem can be solved in Oz as described in the next section.