Nullary constructor

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The term constructor has different meanings depending on the context.

1 OO constructors
- 1.1 Java example
2 Algebraic data types
- 2.1 Haskell example

[edit] 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.

[edit] Java example

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

[edit] 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.

[edit] 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