orchid/notes/expr_refs.md

Reference hierarchy of the host

Reference loops are resource leaks. There are two primary ways to avoid reference loops; a strict hierarchy between types is the easiest. The expression tree uses a less obvious hierarchy where expressions are only able to reference expressions that are older than them.

• Trees reference Constants
• Constants reference their constituent Expressions
• Expressions reference Atoms
• During evaluation, Constants replace their unbound names with Constants
  • There is a reference cycle here, but it always goes through a Constant.

    todo A potential fix may be to update all Constants to point to a dummy value before freeing Trees

• Atoms reference the Systems that implement them
• Atoms may reference Expressions that are not younger than them
  • This link is managed by the System but tied to Atom and not System lifecycle
  • Atoms can technically be applied to themselves, but it's a copying apply so it probably isn't a risk factor
• Systems reference the Extension that contains them
• Extensions reference the Port that connects them
  • The Extension signals the remote peer to disconnect on drop
  • The port is also referenced in a loose receiver thread, which always eventually tries to find the Extension or polls for ingress so it always eventually exits after the Extension's drop handler is called