Keyed Properties

A Keyed Property is similar to a Numbered Property except that it may only be accessed by indexing (so-called square-bracket indexing) and indices are not restricted to integers but may be arbitrary arrays.

To implement a Keyed Property, only a get and/or a set function are required. APL does not attempt to validate or resolve the specified indices in any way, so does not require the presence of a shape function for the Property.

However, APL does check that the rank and lengths of the indices correspond to the rank and lengths of the array to the right of the assignment (for an indexed assignment) and the array returned by the get function (for an indexed reference). If the rank or shape of these arrays fails to conform to the rank or shape of the indices, APL will issue a RANK ERROR or LENGTH ERROR.

Note too that indices may be elided. If KProp is a Keyed Property of Instance I1, the following expressions are all valid.

      I1.KProp
      I1.KProp[]←10
      I1.KProp[;]←10
      I1.KProp['One' 'Two';]←10
      I1.KProp[;'One' 'Two']←10

When APL calls a monadic get or a set function, it supplies an argument of type PropertyArguments, which identifies which dimensions and indices were specified. See PropertyArguments Class.