Constructors

Constructors are messages that are sent to a type, not an instance of a type.

constructor Float PI = 3.14
Float PI echo

PI here is a message for type Float

Default parameters mimicry

It's impossible to create default field values in niva, but you can replace them with constructors:

type Rectangle 
    x: Int
    y: Int
    w: Int
    h: Int
    
constructor Rectangle w::Int h::Int = Rectangle 
    x: 0 // default x and y
    y: 0 
    w: w // send remaining args
    h: h
    
// default constructor
rectangle1 = Rectangle x: 0 y: 0 w: 5 h: 10
// our custom constructor
rectangle2 = Rectangle w: 5 h: 10

No global state

Constructors are very similar to static methods, but note that there are no static fields, it is impossible to create a global state.

Default arguments pattern

Since there is no default argument, you can simulate them with additional methods:

type Point
    x: Int
    y: Int
    
p = Point x: 0 y: 0
// but I dont want to set the fields each time
// we can make a custom constructor

constructor Point new = Point x: 0 y: 0
// now we can create it just with single new message
p = Point new

constructor Point x::Int = Point x: x y: 0
// or with only one argument set to 0
p = Point x: 0 // y is 0 here

Many constructors sugar

You can define many constructors for the same type:

constructor Point [
    on new = Point x: 0 y: 0
    on x::Int = Point x: x y: 0
]

Last modified: 22 October 2024