Chapter 5

Types and Classes

Singletons

Singleton types are used to indicate individual objects. When determining whether a singleton specializer matches a given object, the object must be == to the object used to create the singleton.

A singleton for an object is created by passing the object to the function singleton, or by calling the function make on the class <singleton>.

Singleton methods are considered more specific than methods defined on an object's class. Singletons are the most specific specializer.

define method double (thing :: singleton(#"cup"))
  #"pint"
end method
double (#"cup")
 ⇒  #"pint"

Dylan provides a concise syntax for singletons used as method specializers. The following definition is equivalent to the previous one; it generates a call to the binding of singleton in the current lexical environment.

define method double (thing == #"cup")
  #"pint"
end method
double (#"cup")
 ⇒  #"pint"