Issue #17165 has been reported by sawa (Tsuyoshi Sawada).

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Feature #17165: Add `filter` and `flatten` keywords to `Enumerable#map`
https://bugs.ruby-lang.org/issues/17165

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
I had a use case to do `map` on an enumerable, with 1-level flattening, while skipping `nil` values.

There are convenient `Enumerable#flat_map` and `Enumerable#filter_map` methods, but the problem is that they cannot be used at the same time. I had to chose to do either of the following:

```ruby
array
.filter_map do |foo|
  bar = baz(foo)
  next unless bar
  bar.map{...}
end
.flatten(1)
```

```ruby
array
.flat_map do |foo|
  bar = baz(foo)
  next unless bar
  bar.map{...}
end
.compact
```

```ruby
array
.flat_map do |foo|
  bar = baz(foo)
  next [] unless bar
  bar.map{...}
end
```

The last one of the above may not look so bad, but it requires an extra consideration, and is a bit hacky. When you are in a hurry, it just might not come to your mind.

This led me to realize that `flat_map` and `filter_map` should not be independent operations, but are rather some different modes of the operation `map`. There is no reason for the modes to be mutually exclusive of one another, and a use case that I mentioned above may arise.

I propose to add `filter` and `flatten` as optional keyword arguments to `Enumerable#map`.


```ruby
array
.map(filter: true, flatten: 1) do |foo|
  bar = baz(foo)
  next unless bar
  bar.map{...}
end
```

In fact, even when the two parameters are not used together, I believe it would be easier to the brain and I would feel much more comfortable to pass `filter: true` or `flatten: 1` to `map` when necessary rather than having to deicide whether to use `map` or `flat_map` or use `map` or `filter_map`.

Furthermore, this would make it possible to do flattening of an arbitrary depth (as specified by the parameter) during map.



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