Perhaps Ferret's most unique feature is its ISA inheritance system. The name ISA is borrowed from Perl, where package variables @ISA consist of parent package names. However, Ferret's inheritance system is quite different.

Ferret objects do not inherit from classes; they inherit from other objects. As silly as it may sound, objects can belong to any number of classes. But they don't inherit from those classes; they inherit from their prototype objects.

If an object is an instance of the String class, for instance, its *ISA (list of parent objects) would include the String.proto object.

Example class

Below is an example of a Ferret class from the standard library.

package Math
class Point
init {
    need @x: Num, @y: Num
method distanceTo {
    need $pt2: Point
    $dx = @x - $pt2.x
    $dy = @y - $pt2.y
    return sqrt($dx ^ 2 + $dy ^ 2)
prop distanceFromOrigin {
    return @distanceTo(*class(0, 0))
prop pretty {
    return "(@x, @y)"
method toString {
    return @pretty
method description {
    return "Point" + @pretty
func midpoint {
    need $pt1: Point, $pt2: Point
    return *class(
        x: ($pt1.x + $pt2.x) / 2,
        y: ($pt1.y + $pt2.y) / 2
func distanceBetween {
    need $pt1: Point, $pt2: Point
    return $pt1.distanceTo($pt2)

Initializing an object as a certain type is as simple as adding an object to its *ISA list. However, this is not typically done directly. Consider this example.

# Create an empty object. This is like {} in JavaScript.
$obj = (:)
# Make the object an instance of Point.
Math::Point.init($obj)(0, 0)
# The object is now a Point representing the origin.
# Therefore, $obj.*ISA includes Math::Point.proto.


[ Point ](
    x = 0
    y = 0

This is equivalent to below, as calling a class creates an empty object and initializes it in one step.

$obj = Math::Point(0, 0)

Example of basic inheritance

Because Ferret objects can inherit from any objects, a class is not required for inheritance. Below is an example of basic inheritance without a class.

# create a basic object representing a male being.
$male = (gender: "male")
# create a basic object representing a specific person.
$person = (name: "Jake", age: 22)
# add $male to $person's *ISA list


[ Object ](
    age = 22
    name = "Jake"
    (gender) = "male"