Empty Arrays of Instances: How?

To cater for the need to handle empty arrays of Instances as easily as non-empty arrays, a reference to an empty array of Class Instances is handled in a special way.

Whenever a reference or an assignment is made to the content of an empty array of Instances, the following steps are performed:

  1. APL creates a new Instance of the same Class of which the empty Instance belongs
  2. the default (niladic) Constructor is run in the new Instance
  3. the appropriate value is obtained or assigned:
    • if it is a reference is to a Field, the value of the Field is obtained
    • if it is a reference is to a Property, the PropertyGet function is run
    • if it is a reference is to a Method, the method is executed
    • if it is an assignment, the assignment is performed or the PropertySet function is run
  4. if it is a reference, the result of step 3 is used to generate an empty result array with a suitable prototype by the application of the function {0⍴⊂⍵} to it
  5. the Class Destructor (if any) is run in the new Instance
  6. the New Instance is deleted

Example

:Class Bird
    :Field Public Species
    
    ∇ egg spec
      :Access Public Instance
      :Implements Constructor
      ⎕DF Species←spec
    ∇
    ∇ default
      :Access Public Instance
      :Implements Constructor
      ⎕DF Species←'Default Bird'
      #.DISPLAY Species
    ∇
    ∇ R←Speak
      :Access Public
      #.DISPLAY R←'Tweet, Tweet, Tweet'
    ∇
    
:EndClass ⍝ Bird

First, we can create an empty array of Instances of Bird using 0⍴.

      Empty←0⍴⎕NEW Bird 'Robin'

A reference to Empty.Species causes APL to create a new Instance and invoke the niladic Constructor default. This function sets Species to 'Default Bird'and calls #.DISPLAY which displays output to the Session.

      DISPLAY Empty.Species
.→-----------.
|Default Bird|
'------------'

APL then retrieves the value of Species ('Default Bird'), applies the function {0⍴⊂⍵} to it and returns this as the result of the expression.

.⊖---------------.
| .→-----------. |
| |            | |
| '------------' |
'∊---------------'

A reference to Empty.Speak causes APL to create a new Instance and invoke the niladic Constructor default. This function sets Species to 'Default Bird'and calls #.DISPLAY which displays output to the Session.

      DISPLAY Empty.Speak
.→-----------.
|Default Bird|
'------------'

APL then invokes function Speak which displays 'Tweet, Tweet, Tweet' and returns this as the result of the function.

.→------------------.
|Tweet, Tweet, Tweet|
'-------------------'

APL then applies the function {0⍴⊂⍵} to it and returns this as the result of the expression.

.⊖----------------------.
| .→------------------. |
| |                   | |
| '-------------------' |
'∊----------------------'