Too many math bullshit notations.

// Composition g ∘ double: number → string

Mixing the names here.
It's either g ∘ f, or toString ∘ double.

ppa: Promise<Promise<A>>

It is impossible to create a value that's a promise that resolves with a promise. You can trick TypeScript into believing a value to be that type, but it's erroneous.

G: Async → Sync would be…

There's no "Async → Sync" unless you invent time travel.