Feature #15557
Updated by sawa (Tsuyoshi Sawada) about 5 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 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."}
```