Nullary constructor

In computer programming, a nullary constructor is a constructor that takes no arguments.

Object-oriented constructors

In object-oriented programming, a constructor is code that is run when an object is created. A constructor that takes no arguments is usually called a default constructor.

Java example

 public class Example 
 {
    /* '''nullary constructor''' */
    public Example ()
    {
       this(1);
    }
 
    /* non-nullary constructor */
    public Example (int data)
    {
       this.data = data;
    }
 
    protected int data;
 }

Algebraic data types

In algebraic data types, a constructor is one of many tags that wrap data. If a constructor does not take any data arguments, it is nullary.

Haskell example

 -- '''nullary type constructor''' with two '''nullary data constructors'''
 data Bool = False
           | True

 -- non-nullary type constructor with one non-nullary data constructor
 data Point a = Point a a

 -- nun-nullary type constructor with...
 data Maybe a = Nothing -- ...'''nullary data constructor'''
              | Just a  -- ...unary data constructor