Newtype declarations

A special case of a new type declaration newtype can be used if the type has a single constructor with a single argument. For example, a type for representing an IP address could be declared this way:

newtype IPAddress = IP String

The newtype is different from type because it defines a completely new type rather than just a synonym for an existing type such as:

type IPAddress = String

So any function that requires an IPAddress declared as a newtype will only work with that type, and not simply a String type.

When compared to data, the newtype has some efficiency benefits because it states that IPAddress is simply a wrapper for the value of an existing type String, whereas the data declaration:

data IPAddress = IP String

states that IPAddress is a completely new data type.