Pimm / ruply / 35594786767

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Pull #10

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fix(types): type callbacks which always throw as rejections in asynchronous chains

A callback typed `never` (one which always throws) ended a chain as
`never`, even when the chain was already asynchronous at that point,
where the promise actually rejects; an `async` one (typed
`Promise<never>`) vanished from a runIf chain altogether.

The state machine gets a fourth state: a step which never produces a
value ends a synchronous chain as `never`, as before, and an
asynchronous one as a rejection, as the callback then throws inside a
`then`; a step which never resolves is a rejection from the start. The
result matches the equivalent `.then` chain. In a runIf chain a step
which is always null-ish ends the chain the same way, and once the
chain is asynchronous that would put a `Promise<never>` next to the
promise of the null-ish value; as any promise may reject, that adds
nothing, and it is dropped.

apply gets its own helper rather than passing its value through the
fold as a result: the value is what apply returns, so a promise value
with a throwing callback becomes a rejection, while TypeScript keeps
inferring the value from the call's context and passing generic
values through unchanged.

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