10.5 Feature Constraints

A feature tree is a ground record whose sub-values are also records. Oz provides feature constraints, i.e partial (underspecified) descriptions of feature trees. Feature unification is the process of solving feature constraints.

For instance, you can tell the following feature constraint to the Oz constraint store:

declare X AVM1 in 
AVM1^dtrs^2^1 = unit(cat:vp nb:sg)
{Browse AVM1}

The feature selector ``hat'' ^ allows to describe a fields of a feature tree without specifying the set of all features of this tree a priori. The hat selector provides for open feature (tree) descriptions. Apart from this, the two feature selectors ``hat'' ^ and ``dot'' . do not differ.

Next you can add the following additional constraint for fixing the arity of AVM1:

AVM1 = unit(cat:s
            nb:X
            dtrs:[unit(cat:np
                       nb:X)
                  unit(cat:vp
                       nb:X)])

Note also, how the coreferences due to the variable X are treated correctly by unification.