Composition is a pure fold.

The composed view does not exist until it is read. compose/2 takes a chain of bundles and a map of overrides, folds the chain with Map.merge — base first, each descendant after — then merges the overrides last and drops the :extends key. Right-most wins: a descendant overrides its parent, and the entity’s overrides override them all.

Resolution stays pure by taking its boundary as an argument. resolve/3 walks the :extends chain root-first through a fetch function the caller supplies, then hands the chain to compose/2; whether fetch reads a property store or a snapshot, the composing logic is the same pure fold. The side effect is injected, so the merge itself touches nothing outside its arguments.

Codemojex composes a tile’s effective properties at the moment it needs them. The base tile, the power-up that extends it, and the placed tile’s overrides fold into one map on read — not stored pre-merged, not stale when the base changes. Change the base bundle and every entity that extends it shows the change the next time it is composed.