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Section 21 of 26

Terminals and concurrency

Normal close and abnormal terminal projections

For closing flow:

items >> consume
items| >> finish
items! >> explain
items; >> resting

Law:

items     ordinary values
items|    meaningful normal close; legal only for T...|
items!    abnormal terminal truth
items;    observed quiescence, a non-terminal track

Silence/quiescence is neither | nor !. The explicit ; projection reports quiescence; it does not close a flow or terminate a live plot. Normal-close and quiescence tracks carry Empty. An abnormal track carries the exact fault type declared by its endpoint; an undeclared abnormal contract refuses.

endpoint! is not exception throwing. It exposes typed semantic terminal truth as an ordinary graph track.

Successful internal fallback/replan does not manufacture a semantic ! if the endpoint continues satisfying its contract.

No try, catch, throw, hidden unwinding, or global error bus.

The following complete source is a checking/expansion fixture from the canonical expansion tests:

plot observe-terminals {
    source: test/closing-source
    finish: test/unit-sink
    resting: test/unit-sink
    explain: test/fault-sink

    source| >> finish
    source! >> explain
    source; >> resting
}

The fixture catalog supplies the named test/ fores and proves three distinct typed tracks. These names are not installed application kinds. Wiring ! into a Empty sink, projecting | from a standing flow, or projecting an undeclared fault refuses. Successful parsing alone is not terminal execution evidence.

Provenance: #3970, #3999, #4001.

Terminal transduction and default abnormal propagation

gears/plots transform values plus terminal truth.

Reusable terminal laws must distinguish at least:

A closing-flow consumer whose fore declares T...| receives normal-close semantics as part of that port contract. Authors should not need to wire items| merely so a collector knows its input ended.

Normal close may trigger finite flushing before downstream close when the exact contract says so. Abnormal terminal must not masquerade as normal-close flushing unless a reviewed fault-finalization law explicitly permits it.

Do not universally copy the last payload into close. Retain last-value context explicitly (for example through keep/T?) only where needed.

Default abnormal composition rule:

An unhandled abnormal terminal in a child gear/plot propagates as abnormal terminal truth of the containing plot.

Catching/routing x! is not by itself successful recovery. The containing plot must still satisfy its checked obligations. If recovery faults, the unrecovered terminal propagates.

Keep three layers distinct:

mechanism/Back trouble
recovery/fallback/replan activity
semantic endpoint abnormal termination

Successful internal recovery remains evidence and must not manufacture semantic !.

Provenance: #3999, #4000, #4047.

Semantic cancellation: ~

Preferred/canonical control projection:

deadline >> work~
work! >> explain

work~ is an explicit semantic cancellation request only when that gear's fore declares such a cancellable control surface.

It is distinct from:

Sending cancellation does not prove it succeeded. Current expansion checks the semantic cancellation law and the exact control port, rather than guessing from a port name. This fixture is accepted only with the reviewed cancellable contract from the same canonical expansion tests:

plot cancel-work {
    deadline: test/deadline
    work: test/cancellable-work

    deadline >> work~
}

A similar-looking gear with a port named cancel, but without the declared cancellation law, refuses.

Provenance: #4049.

Multi-input temporal relationships must be explicit

A pure expression never secretly synchronizes two independent runtime inputs.

Distinct operations remain distinct:

These are ordinary semantic gears/plots with exact finite pending-state, pressure, and terminal laws. With compatible exact fores in scope, the glyph prelude makes the relationship visible:

left &> right >> paired
current-left <> current-right >> latest-pair
note @ save-request >> snapshot
first ?> second >> winner
a >< b >> merged

The glyphs respectively name flow/zip, state/combine-latest, current/sample, flow/race, and flow/merge. The examples are scoped cord fragments; their exact result shapes and terminal behavior come from the selected fores. A glyph binding or parsed cord does not establish universal execution support for every combination. See the glyph checking tests and glyph composition.

No hidden combineLatest, timestamp-proximity join, or source-order synchronization.

Fan-in

Independent producers may not silently write the same ordinary single-producer input.

Merge/race/join must be explicit.

The comma-list merge sketch is rejected. Canonical concise merge uses the standard >< gear glyph from #4335, e.g. a >< b >> merged; checked expansion still contains one ordinary flow/merge gear.

Arbitration

Ambiguous races refuse at checking unless exclusivity is proven by routing or an explicit arbitration kind owns the competition.

No “first thread wins,” source-order priority, or hidden last-writer-wins.

Provenance: #4046, #4050, #4064.

cord delivery, fan-out and pressure

Ordinary fan-out remains boring source:

temperature >> display
temperature >> history
temperature >> safety

Its behavior belongs to checked cord/port contracts, not source order or a hidden dispatcher.

Relevant contracts must define:

Default direction: ordinary fan-out preserves delivery promises. A slow branch must not silently make another branch lossy.

Authors should not spell queue byte arithmetic when a reviewed finite profile/default can supply it safely.

Fan-out is legal only when the info/resource ownership contract permits duplication/shared reference. Capability/resource-valued values must not become forgeably cloneable.

Independent fan-in remains explicit under section 16: merge, race, zip, combine-latest, sample and joins are distinct meanings.

Provenance: #4045, #4046.