Internals: inheritance (of M2 types)

Creating new subtypes

The method new Type of Foo creates a new type that inherits from Foo. Consider the snippet below

X0 = new Type of BasicList
X1 = new Type of X0

Any method that accepts a BasicList will also accept an instance of X0 and X1, and any method that will accept an instance of X0 will also accept an instance of X1. The list of ancestors of a type can be viewed with ancestors.

Looking up methods

Let's create two new types, and install a method that prints a message

Y0 = new Type of BasicList
Y1 = new Type of Y0
X1 ** Y0 := (x, y) -> "X1 ** Y0"

Note that (new X1) ** (new Y0) and (new X1) ** (new Y1) will both work, since Y1 inherits from Y0. Furthermore, (new X0) ** (new Y1) will not work, as X0 is an ancestor of X1. The function that gets called with given types can be recovered using lookup, e.g. lookup(symbol **, X1, Y1) will return the function above that returns the string "X1 ** Y0".

Hashcodes

For garbage collection purposes, the method is installed in the youngest type with respect to the hashcode via the function hash. The function youngest returns the youngest of two things. In this case, the method is stored under Y0, see e.g. peek Y0. Note that the hashcode reflects the age of mutable objects only.

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