# Listing of doc/example-scripts/rosetta-constraint-genericity.tcl

## Rosetta Example: Constrained genericity

For details see http://rosettacode.org/wiki/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