Eidolon
An experimental runtime in Rust where small programs run inside a world that records every event with its causes, so you can rewind it, ask why a value changed, and run what would have happened instead.
- Year
- 2026
- Kind
- Systems · Rust
- Status
- Shipped
A program in Eidolon is a small world: cells, processes, nodes and links, rules that must always hold, inputs, and faults injected on purpose. Every state change is an event in a hash-chained ledger that records the values it read, the branch it ran under and the message it consumed. From that ledger alone you can rebuild any past tick and trace why a value is what it is.
A what-if is a second run with one edit, compared at the exact event where the two histories part. The same engine searches interleavings for one that breaks a rule: on a two-teller bank it finds the schedule that loses a deposit after 17 states. It also fuzzes and shrinks failures, crashes each node in turn, and runs in the browser as WebAssembly.
The network is a simulation inside one process; machines can be linked over TCP, but the nodes and links within a world are modelled. A recorded real program can be asked why but not what if, because Eidolon cannot re-run it. The symbolic solver handles one unknown at a time; many unknowns is listed as next.
