Classes in Fancy are first-class values (they are instances of the
Class class, just as in Ruby or Smalltalk).
There's several ways to define Classes in Fancy, but the common one is to use the built-in syntax for it. Compared to Smalltalk, where class definitions are just message sends as well, Fancy also provides syntax for doing so, just as Ruby and most other object-oriented programming languages out there.
class Person {
}Defines an empty class called Person which implicitly inherits from
Object, the root class of Fancy's class hierarchy.
To define a class that inherits from another class we specify the super class after a colon, like so:
class ImaginaryPerson : Person {
# Person is the super class of ImaginaryPerson
# ...
}Let's define a name method for Person which returns a String
representing the name of a Person (for now we'll just use the same
name for all instances of Person):
class Person {
def name {
"John Doe"
}
}If we create a new Person now and send it the name message, we'll
get "John Doe" as a response:
p = Person new
p name # => "John Doe"Let's refactor this code a little and define a constructor that takes
a name and returns that when sent the name message:
class Person {
def initialize: name {
@name = name
}
def name {
@name
}
}As you can see, constructor methods start with initialize. In our
case the constructor method just takes one argument, a name, and
stores it into the instance variable @name. Instance variables
start with @ and class variables start with @@, just as in Ruby.
Now we can create persons and pass in their names:
john = Person new: "John Doe"
betty = Person new: "Betty Boop"
john name # => "John Doe"
betty name # => "Betty Boop"Constructor methods like initialize: are just normal instance
methods, like name. They're called with the appropriate arguments
when calling the Class##new: class method. For constructors with
multiple arguments, the necessary class constructor methods are
created for you implicitly. Let me show you what I mean:
class Person {
def initialize: name age: age city: city {
@name = name
@age = age
@city = city
}
def age {
@age
}
def city {
@city
}
}
p = Person new: "John Doe" age: 42 city: "New York"
p name # => "John Doe"
p age # => 42
p city # => "New York"When defining a method starting with initialize: it will create a
corresponding class method replacing initialize: with new: and
keeping the rest of the name. E.g. for an instance method called
initialize:age:city: a class method new:age:city: is defined which
passes all the arguments on to the initialize method.
There's a simpler way to define so-called getter- and setter-methods in Fancy:
class Person {
# define getter-methods:
read_slots: ['name, 'age, 'city]
# or if, you'd want them all to be writable:
write_slots: ['name, 'age, 'city]
# or define both getter & setter methods:
read_write_slots: ['name, 'age, 'city]
}We usually refer to instance variables as slots within Fancy. It's just a different name, but they refer to the exact same thing.
read_slots: defines reader methods for all the slotnames within a
given Array. write_slots: defines setter methods for the slotnames
in a given Array. read_write_slots: does both at once. These methods
are completely defined in Fancy and can be found in the Class class
definition in Fancy's standard library.
OK, let's have a look at some more nice syntactic features Fancy
provides for common idioms in method definitions.
First, let's have a look at all the code for the Person class we've
written so far. We'll modify it a little to have multiple constructor
methods to emulate default arguments for the missing arguments:
class Person {
def initialize: name {
initialize: name age: 42 # default is 42
}
def initialize: name age: age {
initialize: name age: age city: "New York" # default is "New York"
}
def initialize: name age: age city: city {
@name = name
@age = age
@city = city
}
# slot readers
def name {
@name
}
def age {
@age
}
def city {
@city
}
# let's also define slot writer methods:
def name: new_name {
@name = new_name
}
def age: new_age {
@age = new_age
}
def city: new_city {
@city = new_city
}
}OK, and now let's define the same stuff with some standard helper methods and syntax sugar:
class Person {
read_write_slots: ['name, 'age, 'city]
def initialize: @name age: @age (42) city: @city ("New York") {
}
}Yup, that's it. =)
What's happening here is we're defining getter & setter methods for
the slots @name, @age and @city and use Fancy's auto slot assignment
syntax in the constructor method, which sets the slot with the given
name to the value passed in as the argument. Finally, we're also using
default argument values (any expression within ( and )). Note,
that you're allowed to refer to arguments passed in further left as
default arguments further right.
For example:
def hello: name1 and: name2 (name1) {
"Hello, #{name1} and #{name2}!" println
}
# call with both arguments:
hello: "Robert" and: "Meggie" # prints "Hello, Robert and Meggie!"
# call with one argument:
hello: "Max" # prints "Hello, Max and Max!"