Classes and Objects

A dictionary holds data. A class holds data and the behaviour that goes with it, under a name you can check for and inherit from.

class Temperature {

  @new(celsius) {
    self.celsius = celsius
  }

  fahrenheit() {
    return self.celsius * 9 / 5 + 32
  }

  to_string() {
    return '${self.celsius}C'
  }
}

var t = Temperature(100)

echo t.fahrenheit()
echo t.to_string()

212
100C

Three facts about Zuri classes shape everything in this chapter, and it is worth having them up front.

The constructor is @new. Methods whose names start with @ are decorated methods, and each one connects the class to a piece of the language’s own syntax: construction, arithmetic, comparison, iteration, JSON encoding. There is a fixed set of them, and Decorated Methods covers all of it.

Fields are reached through self. Inside a method, self is the instance. There is no bare balance that means self.balance, and no self parameter in the declaration either.

A class is sealed. The set of fields and methods is fixed when the class is declared. Nothing at runtime can add a new field to an instance or a new method to a class, and an attempt raises an error rather than quietly creating one. That rules out a family of bugs — a typo in a field name is an error, not a new field — and it rules out monkey-patching, which is a technique some languages rely on and Zuri does not offer.

Inheritance is single: a class has at most one parent, written with <. There are no interfaces, no mixins and no abstract keyword. The sections that follow show what you write instead.

Sealed does not mean unchangeable forever, though. A separate declaration written with > instead of < — a class extension — can add methods to a class after the fact, including one you did not write. Class Extensions covers it.