Nullary constructor

The term constructor has different meanings depending on the context.

OO constructors

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