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Feature #15557

Updated by sawa (Tsuyoshi Sawada) almost 6 years ago

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 how 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. 

 ```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 
 ``` 

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