As I mentioned in Ruby 3 Problems, Sup is filled with singleton classes. When I first encountered this programming pattern, my initial reaction was disgust, and I vowed never to use such a pattern in my own code. In fact, singletons do seem to have a bit of controversy around them, judging from the discussions I see on Stackoverflow.

But trying to port Sup without singletons looked like an overwhelming task: they're everywhere, and trying to rewrite them as normal classes was a daunting task.

So the question became: how to implement singletons in Crystal? In Sup, they are implemented with two things:

• a single instance variable for the one instantiation of the class
• a method_missing that redirects every class method to an instance method

The method_missing feature makes clever use of Ruby's dynamic features: it defines a method with the missing name, and binds that method to the single instance of the class. This can't be done in Crystal, in which all methods must be known at compile time.

My solution was to implement a hack using some macros:

• Each singleton class must call the singleton_class macro somewhere near the top of the class. This macro sets up the instance variable and defines some methods (instance, instantiated?, and deinstantiate) that Sup expected.

• The first line in the class constructor (initialize) must be a call to the macro singleton_pre_init, which ensures that there is only one instance of the class. The last line in the constructor must be a call to the macro singleton_post_init, which sets the instance variable.

• Each method in the class that is supposed to be treated as a method for the single instance must be followed by a call to singleton_method macro, which lists the name of the method, along with names of all of its arguments. This macro defines a new class method that invokes the instance method of the same name.

For more details, see the src/singleton.cr file in the source repository.