Issue #17330 has been updated by nobu (Nobuyoshi Nakada).


It reminded me https://bugs.ruby-lang.org/issues/12075#change-57152.

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Feature #17330: Object#non
https://bugs.ruby-lang.org/issues/17330#change-88548

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
(As always "with core" method proposals, I don't expect quick success, but hope for a fruitful discussion)

**Reasons:**

Ruby always tried to be very chainability-friendly. Recently, with introduction of `.then` and `=>`, even more so. But one pattern that frequently emerges and doesn't have good idiomatic expression: calculate something, and if it is not a "good" value, return `nil` (or provide default value with `||`). There are currently two partial solutions:

1. `nonzero?` in Ruby core (frequently mocked for "inadequate" behavior, as it is looking like predicate method, but instead of `true`/`false` returns an original value or `nil`)
2. ActiveSupport `Object#presence`, which also returns an original value or `nil` if it is not "present" (e.g. `nil` or `empty?` in AS-speak)

Both of them prove themselves quite useful in some domains, but they are targeting only those particular domains, look unlike each other, and can be confusing.

**Proposal:**

Method `Object#non` (or `Kernel#non`), which receives a block, calls it with receiver and returns `nil` (if block matched) or receiver otherwise.

Prototype implementation:

```ruby
class Object
  def non
    self unless yield(self)
  end
end
```

Usage examples:

``` 
# 1. With number:

limit = calculate.some.limit
limit.zero? ? DEFAULT_LIMIT : limit
# or, with nonzero?
calculate.some.limit.nonzero? || DEFAULT_LIMIT
# with non:
calculate.some.limit.non(&:zero?) || DEFAULT_LIMIT
# ^ Note here, how, unlike `nonzero?`, we see predicate-y ?, but it is INSIDE the `non()` and less confusing

# 2. With string:

name = params[:name] if params[:name] && !params[:name].empty?
# or, with ActiveSupport:
name = params[:name].presence
# with non:
name = params[:name]&.non(&:empty?)

# 3. More complicated example

action = payload.dig('action', 'type')
return if PROHIBITED_ACTIONS.include?(action)
send("do_#{action}")
# with non & then:
payload.dig('action', 'type')
  .non { |action| PROHIBITED_ACTIONS.include?(action) }
  &.then { |action| send("do_#{action}") }

```

**Possible extensions of the idea**

It is quite tempting to define the symmetric method named -- as we already have `Object#then` -- `Object#when`:
```ruby
some.long.calculation.when { |val| val < 10 } # returns nil if value >= 10
# or even... with support for ===
some.long.calculation.when(..10)&.then { continue to do something }
```
...but I am afraid I've overstayed my welcome :)



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