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filter — thin by predicate

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Category: filter (body-layer core) / Signature: filter(on: P) / filter(x => condition)

Stream -> Stream / name settled (spec §5.4)

Meaning

Keeps only the points that satisfy the predicate. Both forms are taken (where was folded into filter; ADR-25):

• Premise predicate filter(on: P) — keeps only the points that belong to the axis P (a name that resolves to a valid-point stream under the in-scope premise, or a derived stream). Calendar dependence starts here (I8).
• Value-expression predicate filter(x => condition) — decides with a lambda binding each point x. Inside the lambda, the value functions of cycle labels (weekday(x)) and window-to-value projections (ordinalIn and the like) are available.

Examples

Keep business days only (2026-01-01 is a holiday; 1/3 and 1/4 are a weekend):

# eval: 2026-01-01..2026-01-08
@JP
everyDay |> filter(on: bizDay)
#=> 2026-01-02 2026-01-05 2026-01-06 2026-01-07

Keep Mondays only (label predicate):

# eval: 2026-01-01..2026-01-20
@JP
everyDay |> filter(d => weekday(d) == Mon)
#=> 2026-01-05 2026-01-12 2026-01-19

Combined with a projection, “select by window coordinates” — the 11th of every month:

# eval: 2026-01-01..2026-04-01
@JP
everyDay |> filter(d => ordinalIn(day, month, d) == 11)
#=> 2026-01-11 2026-02-11 2026-03-11

Pitfalls

• filter is not a generator — it is a transform (a stage), so it cannot stand at the head of an expression (F45; everyDay |> filter(…)).
• on: and the lambda are two mouths of the same word. Never pass both at once.
• Generator purity (I8): dependence on business days always goes at or after the filter. This keeps the front of the expression (the calendar-system part) stable under calendar swaps.
• filter(on: P) is membership by point equality — it requires the input and the axis to agree in alignment (ADR-36 · spec §4.5; a mismatch is a static error, never a silent no-match). on: also accepts direct entity names (on: TSE) (ADR-35).
• To thin by window membership, use coincides in a value-expression predicate — excluding ad-hoc closure “days” from timed notifications is filter(t => not coincides(closures, day, t)) (not expressible as an equality difference; F68 = ADR-38).

Related

roll (nudges instead of thinning) · ordinalIn · cycle (the source of label predicates) · I8 · ADR-25/26.

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