Overview of Polymorphism -多态的分类

多态有类型系统衍生。

有限类型、无限类型、确定类型。

Classifications

Christopher Strachey (1967) introduced the concept of polymorphism informally into procedural programming languages by distinguishing functions

• that work differently on different argument types
• that work uniformly on a range of argument types 

He defined the former as ad-hoc polymorphism and the latter as parametric polymorphism:

"Ad-Hoc polymorphism is obtained when a function works, or appears to work, on several different types (which may not exhibit a common structure) and may behave in unrelated ways for each type.  Parametric polymorphism is obtained when a function works uniformly on a range of types; these types normally exhibit some common structure." (Strachey, 1967)

Cardelli and Wegner (1985) expanded Strachey's distinction to accommodate object-oriented languages.  They distinguished functions

• that work on a finite set of different and potentially unrelated types
  • coercion
  • overloading
• that work on a potentially infinite number of types across some common structure
  • inclusion
  • parametric

Inclusion polymorphism is specific to object-oriented languages.

https://scs.senecac.on.ca/~oop244/pages/content/adhoc.html

Level 1 - universal vs, ad hoc polymorphism

• functions that are universally polymorphic work uniformly for an infinite set of types
  all of which have some common structure
   
  • execute the same code for all admissible types
     
• An ad hoc polymorphic function is just a syntactic abbreviation for small set of
  different monomorphic functions.
   

http://www.cs.kent.edu/~durand/CS43101Fall2004/DT-TypeSystems.html