Overview

The h8io.stages.std package holds the stages that stand on their own: they answer to their input, and none of them wraps another stage whose behaviour they alter. Most are built from the immutability hierarchy, so they carry no state between runs unless their whole purpose is to carry some.

Stage	What it does
Const	ignores its input, always yields the same value
Identity	passes its input through unchanged
Swap	swaps the two elements of a pair
Coalesce	collapses an Either[T, T] to the value either side holds
Complete	passes the input through and signals the end of the unit of work
DeadEnd	yields nothing, with Status.Complete — the end-of-stream marker
Countdown	passes n values through, then completes and resets
GlobalSoftDeadline	passes values through until a deadline fixed at construction
Scan	folds its own inputs into a running accumulator across runs

The first four never fail and never change: they are Fn stages. Four of them — Identity, Swap, Coalesce and Complete — hold no value at all, so each is a single instance defined over Any and cast to what the use site needs by a typed apply[T], rather than allocated afresh — see Polymorphic Singletons.

The last two carry something between runs. Countdown counts, and Scan accumulates — both by evolving into the next generation of themselves rather than by mutating, and both resetting on any status that is not Success. GlobalSoftDeadline sits between: it never evolves, but the clock it reads moves on its own.