Issue #15557 has been updated by zverok (Victor Shepelev).


I played a bit with idea similar to @nobu's, with this syntax:

```ruby
then_if(condition) { action }
then_if(-> { condition }) { action }
```
Explanations:
* boolean vs callable: sometimes, static conditions are useful, sligtly realistic example:

  ```ruby
  def fetch(url, parse: false)
    get(url).body
      .then_if(parse) { |res| JSON.parse(res) }
      # or, simpler:
      .then_if(parse, &JSON.:parse)
  end
  ```
* name: "it is like `then`, but conditional"

I experimented a bit with syntax like @sawa's initial proposal, and believe that it _could_ work too (but method's name should probably be `if` ;)), with the same adjustment that it can receive pre-calculated condition, too. The problem with it, though, is the introduction of the new abstraction/"mode" for the sake of nice syntax, e.g. `something.if(&:cond?)` on itself doesn't produce an easily explainable/reusable object.

The problem with @nobu's one, though, that it is pretty rarely seen in core Ruby to pass callable object instead of just a block (though, it exists, like @nobu pointed in another ticket, for example in `Enumerator::new`)

----------------------------------------
Feature #15557: A new class that stores a condition and the previous receiver
https://bugs.ruby-lang.org/issues/15557#change-76469

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I often see code like this:

 ```ruby
foo = default_definition
foo = some_method(foo) if some_condition(foo)
foo = another_method(foo) if another_condition(foo)
...
```

It would be nice if we can write this as a method chain. Since we now have the method `then`, I thought it would be a nice fit to introduce a method called `when`, such that putting it right in front of `then` would execute the `then` method as ordinarily only when the condition is satisfied, and returns the previous receiver otherwise so that the code above can be rewritten as:

```ruby
foo =
default_definition
.when{|foo| some_condition(foo)}
.then{|foo| some_method(foo)}
.when{|foo| another_condition(foo)}
.then{|foo| another_method(foo)}
```

This proposal is also a generalization of what I intended to cover by https://bugs.ruby-lang.org/issues/13807. That is,

```ruby
a.some_condition ? a : b
```

would rewritten as:

```ruby
a.when(&:some_condition).then{b}
```

The proposal can be implemented by introducing a class called `Condition`, which stores a condition and the previous receiver, and works with `then` in a particular way.

```ruby
class Object
  def when
    Condition.new(self, yield(self))
  end
end

class Condition
  def initialize default, condition
    @default, @condition = default, condition
  end

  def then
    @condition ? yield(@default) : @default
  end
end
```

And additionally, if we introduce a negated method `unless` (or `else`) as follows:

```ruby
class Object
  def unless
    Condition.new(self, !yield(self))
  end
end
```

then we can use that for purposes such as validation of a variable as follows:

```ruby
bar =
gets
.unless{|bar| some_validation(bar)}
.then{raise "The input is bad."}
.unless{|bar| another_validation(bar)}
.then{raise "The input is bad in another way."}
```




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