Record Constraints

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  • General

    Oz supports fine-grained constraints for the description of records as they are typically used in natural language processing with constraint-based grammars.

    Record Construction

    In Oz 1, one could only describe records whose label and arity was fully known, i.e., determined ones, using an equation like the following.

    X = s(np:N vp:V) 
    

    Feature Constraints

    The most important additional record constraint in Oz 2 is the feature constraint X^Y = Z. Such a constraint blocks until Y is determined and then constrains X to denote a record whose arity includes feature Y and whose subtree at Y is Z. These constraints are immediately available for further processing. For instance, telling the constraints

    X^np = N1
    X^np = N2
    

    results in equating N1 and N2.

    Path Equations

    Nesting feature constraints yields what computational linguists call path equations:

    VP^head^subject = NP^head
    

    Label Constraint

    It is possible to fix the label of a record without giving any of its features. The constraint:

    {TellRecord F X}
    

    blocks until F is a determined literal and then a tells the store that X is a record with label F.

    Notation

    There is a convenient notation for a labelling and a number of feature constraints which resembles the construction of a determined record. The statement:

    X = f(a:Y b:Z ...) 
    

    is equivalent to the constraints:

    {TellRecord f X}
    X^a = Y
    X^b = Z
    

    Type Test

    As soon as the store contains a labelling or a feature constraint for X, this variable is known to denote a record. While the type test {IsRecord X} blocks until X is determined, the test:

    {IsRecordC X}
    

    reduces as soon as possible.

    Record Inspection

    The procedures for record inspection:

    {Label X}
    {HasFeature X f}
    

    need no longer block until X is determined. They reduce as soon as the label of X is known, respectively as soon as X is constrained to have feature f.

    Record Propagator

    There is a record propagator which detects when new features have been added to a record.

    {Record.monitorArity X P Ls}
    

    connects the record X to the stream Ls (i.e., a non-terminated list). As soon as X becomes known to have a feature F, this feature appears on the stream. Record.monitorArity is a primitive from which user-defined propagators can be derived: By observing Ls, one can react to new arity information. P is a nullary procedure that kills the primitive propagator and disconnects R from L.


    Martin Müller