= Reachability

Reachability is a notion dealing with the graph of heap objects
maintained at runtime.  Nodes in the graph are heap objects and edges
correspond to the pointers between heap objects.  As the program runs,
it allocates new objects (adds nodes to the graph), and those new
objects can contain pointers to other objects (new edges in the
graph).  If the program uses mutable objects (refs or arrays), it can
also change edges in the graph.

At any time, the program has access to some finite set of _root_
nodes, and can only ever access nodes that are reachable by following
edges from these root nodes.  Nodes that are _unreachable_ can be
garbage collected.

== Also see

 * <<MLtonFinalizable#>>
 * <<MLtonWeak#>>
