Construct an async generator

An async generator constructs iterable functions that are also awaitables. They support both the yield() and await() syntax. An async iterator can be looped within async functions and iterators using await_each() on the input of a for loop.

The iteration protocol is derived from the one described in iterator. An async iterator always returns a promise. When the iterator is exhausted, it returns a resolved promise to the exhaustion sentinel.

async_generator(fn)

await_each(x)

Arguments

fn

An anonymous function describing an async generator within which await() calls are allowed.

x

An awaitable value, i.e. a promise.

Value

A generator factory. Generators constructed with this factory always return promises::promise().

See also

async() for creating awaitable functions; async_collect() for collecting the values of an async iterator; coro_debug() for step-debugging.

Examples

# Creates awaitable functions that transform their inputs into a stream
generate_stream <- async_generator(function(x) for (elt in x) yield(elt))

# Maps a function to a stream
async_map <- async_generator(function(.i, .fn, ...) {
  for (elt in await_each(.i)) {
    yield(.fn(elt, ...))
  }
})

# Example usage:
if (interactive()) {
  library(magrittr)
  generate_stream(1:3) %>% async_map(`*`, 2) %>% async_collect()
}