19 lines
1.3 KiB
Markdown
19 lines
1.3 KiB
Markdown
# Reference hierarchy of the host
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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.
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- Trees reference Constants
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- Constants reference their constituent Expressions
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- Expressions reference Atoms
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- During evaluation, Constants replace their unbound names with Constants
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- There is a reference cycle here, but it always goes through a Constant.
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> **todo** A potential fix may be to update all Constants to point to a dummy value before freeing Trees
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- Atoms reference the Systems that implement them
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- Atoms may reference Expressions that are not younger than them
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- This link is managed by the System but tied to Atom and not System lifecycle
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- Atoms can technically be applied to themselves, but it's a copying apply so it probably isn't a risk factor
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- Systems reference the Extension that contains them
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- Extensions reference the Port that connects them
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- The Extension signals the remote peer to disconnect on drop
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- 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
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