#### Functional objects on Ruby programming language

* Start class names with a verb;
* Public contract is a `#call` method;
  * Enables SRP;
  * Enables composition;
  * Enables polymorphism;
  * Enables to receive Proc/callables objects;
* Receive stateful/impure functions through the object initializer;
  * This permits easy mocking/substitution.
* Receive pure function inputs through the `#call` method;
* This resembles curry-like functions;
* The `#call` calling always returns a value, preferably 'result'-like objects;
  * This avoids 'primitive obssesion' anti-pattern;
  * Consider monadics operations;
* Define stateful/impure functions as default implementation, in case nothing is received;
* Stateful/impure functions are always returned as closures/lambda;
* Object state is uded only for atateful/impure collaborators.
```ruby
    extract_utm_campaign_value_from(url) # is pure
    get_page_title.call(url)             # is impure
```
* Message passing is natural;