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last — select the last in each window

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Category: selector (body-layer core) · Signature: last : Stream(windowed) -> Stream · name settled (spec §5.4)

Meaning

Selects the last element within each window. Window-relative (I4). The default target is the innermost window; when nesting makes it ambiguous, name the target window explicitly with of: w (§4.3; the example in first). “Month-end”, “fiscal-year-end”, “the last day of the week” — all of them are really this.

monthEnd, which looks like a generator, is in fact a public boundary word of a primitive definition, monthEnd = month |> last — a reuse of last, not a separate mechanism (spec §3.6).

Examples

The last day of each month (calendar-day based; business-day adjustment is the job of a downstream roll — the I8 separation):

# eval: 2026-01-01..2026-04-01
@JP
everyDay |> within(month) |> last
#=> 2026-01-31 2026-02-28 2026-03-31

The last day of the week (Sunday under wkst: Mon):

# eval: 2026-01-05..2026-01-26
@JP
everyDay |> within(week) |> last
#=> 2026-01-11 2026-01-18 2026-01-25

Pitfalls

• Leap years are handled by the calendar system’s value expression (isLeap) — February’s last automatically returns the 28th in a common year and the 29th in a leap year (“leap is a value, not a window”; spec §3.6).
• “3 business days before month-end” cannot be written with last alone — nudge onto a business day with roll(Preceding, on: bizDay), then shift(-3, unit: bizDay) (representative example §7.1).

Related

first · nth · roll · shift · public boundary words (spec §3.6).

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