Public reducer descriptors for graph state fields, plus the pure reduction engine the runtime applies at every commit.

A reducer folds the field's prior committed value with the step's writes (deterministically sorted by writer node ID):

new_value = reduce(reducer, current_committed_value, sorted_writes)

Built-in reducers

For :append and :union, a list write concatenates its elements and any other write appends as a single element; to append one element that is itself a list, wrap it in an outer list.

Options

• :append — unique: true drops duplicate elements (first occurrence wins); max_length: n keeps only the last n elements (sliding window), applied after unique.
• :union — by: key dedupes elements by the value at key (a string or a list of strings forming a path); elements missing the key dedupe by their whole value.
• :merge — deep: true merges nested maps recursively instead of replacing them.

Option keys are stored as strings (constructors normalize atom keys), matching the wire format. Unknown option keys are stored but ignored.

Initial values

Accumulating reducers have a natural zero used as the field's effective default when it declares none: [] for :append/:union, %{} for :merge, 0 for :sum. A field's explicit default acts as the base the first commit folds into — including :first_value, where an explicit default counts as the value already being set.

Write validation

Node writes to a field validate against a reducer-dependent shape rather than the committed field schema (which stays the truth for the committed value): :append/:union writes validate against the list schema's item (a list write validates each element), :merge writes validate as a map fragment of the field schema with top-level required fields relaxed, and :sum writes must be a number of the schema's type.