rosetta-constraint-genericity.html

Rosetta Example: Constrained genericity

package req nx

Define the two classes Eatable and Fish. Eatable is a class for all eatable things, a Fish is a subclass ant therefore eatable.

nx::Class create Eatable
nx::Class create Fish -superclass Eatable {
  :property name
}

A FoodBax may only contain eatable items. Therefore with we define items as a property of type Eatable" which has a multiplicity of +0..n (might contain 0 to n eatable items). Furthermore, we define items as incremental, such we can add / remove items with item add or item remove.

nx::Class create FoodBox {
  :property -incremental item:object,type=::Eatable
  :public method print {} {
    set string "Foodbox contains:\n"
    foreach i ${:item} {append string "   [$i cget -name]\n"}
    return $string
  }
}

Demonstrating the behavior in a shell:

Create two fishes, Wanda and Nemo:

% set f1 [Fish new -name "Wanda"]
% set f2 [Fish new -name "Nemo"]

Create a Foodbox and add the two fishes:

% set fb [FoodBox new]
% $fb item add $f1
% $fb item add $f2

Return the print string of the contents:

% $fb print
Foodbox contains:
   Nemo
   Wanda