only

any →
only(
    predicate: predicate,
    accumulator: accumulator
) → any

Forwards only items satisfying the predicate to the wrapped accumulator.

Parameters

Name	Type	Required	Description
predicate	predicate	Yes	Specifies the predicate deciding which items participate.
accumulator	accumulator	Yes	Specifies the accumulator receiving matching items.

Argument evaluation

Visits each item of the collection supplied as pipeline input to this only call in source order.

• predicate: Evaluated once per visited item, with that item as its context. Direct field selectors such as .active read that item; arguments within the predicate follow their normal evaluation rules.
• accumulator: Created once before accumulation, retaining the surrounding evaluation context. Receives only matching items; its arguments follow the wrapped accumulator evaluation rules.

Behavior

The input and final result types are inherited from the wrapped accumulator. Initialization, empty-input and no-match results, completion, and failures are unchanged. Null items are evaluated by the predicate like any other item. Predicate results must be Boolean. Matching items retain source order without materializing a filtered collection. Direct pipeline calls implicitly fold the wrapper.

Examples

{1, 2, 3, 4} | only(is-even, count) → 2
{10, #null, 30} | fold(only(is-not-null, count)) → 2

Kind: Accumulator
Scope: array
Aliases: None