I had a muck around with go2 generics with my toy-ish futures package
https://github.com/anacrolix/futures. The go1 implementation is in master,
and a working go2 implementation in the go2 branch (using channels of
different types instead of the attempt that follows). The package provides
one function AsCompletedDelayed, that allows to favour futures over others
with timeouts. The timeouts are implemented using the future type *F[int],
where as the futures the user provides as arguments are *F[T]. In the
implementation for AsCompletedDelayed I need to pass both types of futures
to another function AsCompleted[T](fs ...*F[T]) <-chan *F[T], then
differentiate them when they're returned: I could get back a "timeout"
future, or a user/argument future. To do this I created another type
timeoutOrResult[T] struct { timeout bool; timeoutIndex int; result T },
however now I ran into the situation where I need to "map" the futures of
type *F[int], and *F[T] to *F[timeoutOrResult[T]]. This seems non-trivial:
I believe in another language I would make F a Functor, and map the
timeouts to something like `Either int T`. It is possible to write an Fmap
on my *F type, but now I need to create new types, and break out an
interface, and the implementation quickly increases in complexity.
This seems like a situation where the go1 style of doing this was easier
albeit without enforcing the result types of the futures in the return chan
for AsCompleted and AsCompletedDelayed: I could pass arbitrary *Fs to
AsCompleted, and then compare the returning *F against a map[*F]struct{}
that tracked which ones were timeouts.
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