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Arity

A function takes one or many values as parameters and returns another value. The arity of a function is the number of parameters it takes. For instance:

• A nullary (arity 0) function does not take any parameter
• A unary (arity 1) function takes only one parameter
• A binary (arity 2) function takes two parameters

A type constructor is a type that accepts parameters. It is called type constructor, because it constructs a concrete type when we pass a concrete type to it. For instance, Option[A] is a type constructor. When we pass a concrete type Int to it, we obtain a concrete type Option[Int].

As type constructors can accept 0 to n arguments, the concept or arity also applies here:

• A nullary type does not take any parameter. It is a concrete ...