Function flatMap

flatMap<V, R>(mapper: (item: V) => Awaitable<Data<R>>): Task<V, R>
Creates a task expanding each item into a data source.

Each item is converted into a Data value and replaced in the feed by the items that value contributes, according to its shape; conversion is lazy, one item at a time, and expansions are emitted in source order. An item expanding to nothing simply drops out of the feed.

The mapper returns a batch of items rather than a single value, so an item replaced by exactly one value expands to an array wrapping it.
Type Parameters
- V
  The type of input items
- R
  The type of output items
Parameters
- mapper: (item: V) => Awaitable<Data<R>>
  The function converting each item into the data source replacing it; an empty result, as well as an undefined value within the expansion, contributes nothing, so expansion doubles as filtering
Returns Task<V, R>
A task yielding the items contributed by the expansion of each source item
Example
```
await pipe(
  (items(1, 2, 3))
  (flatMap(n => [n, n*2]))
  (toArray())
);  // [1, 2, 2, 4, 3, 6]
```
See
map to replace each item with a single value
- Defined in tasks/flatMap.ts:52