In computer programming, duck typing is an application of the duck test in type safety. It requires that type checking be deferred to runtime, and is implemented by means of dynamic typing or reflection .
Duck typing is concerned with establishing the suitability of an object for some purpose, using the principle, "If it walks like a duck and it quacks like a duck, then it must be a duck." With normal typing, suitability is assumed to be determined by an object's type only. In duck typing, an object's suitability is determined by the presence of certain methods and properties (with appropriate meaning), rather than the actual type of the object.
Video Duck typing
Example
This is a simple example in Python 3 that demonstrates how any object may be used in any context, up until it is used in a way that it does not support.
Maps Duck typing
In statically typed languages
Certain usually statically typed languages such as Boo and the version 4 release of C# have extra type annotations that instruct the compiler to arrange for type checking of classes to occur at run-time rather than compile time, and include run-time type checking code in the compiled output.
Comparison with other type systems
Structural type systems
Duck typing is similar to, but distinct from structural typing. Structural typing is a static typing system that determines type compatibility and equivalence by a type's structure, whereas duck typing is dynamic and determines type compatibility by only that part of a type's structure that is accessed during run time.
The OCaml, Scala, Go, Elm, and Gosu languages support structural typing to varying degrees.
Protocols and Interfaces
Protocols and interfaces can provide some of the benefits of duck typing, but duck typing is distinct in that no explicit interface is defined. For example, if a third party library implements a class that cannot be modified, a client cannot use an instance of it with an interface unknown to that library even if the class does in fact satisfy the interface requirements. (A common solution to this problem is the Adapter pattern.) Duck typing would allow this. Again, all of an interface must be satisfied for compatibility.
Templates or generic types
Template, or generic functions or methods apply the duck test in a static typing context; this brings all the advantages and disadvantages of static versus dynamic type checking in general. Duck typing can also be more flexible in that only the methods actually called at run time need to be implemented, while templates require implementation of all methods that cannot be proven unreachable at compile time.
Examples include the languages C++ and D with templates, which developed from Ada generics.
Criticism
Criticism of the term itself
Use of the term "duck typing" has been considered superfluous in light of the fact that other terms, such as dynamic binding, express the concept more clearly. To proponents of static type checking, duck typing suggests the absence of typing, making its incorporation of the term typing appear incoherent.
History
Alex Martelli made an early (2000) use of the term in a message to the comp.lang.python newsgroup:
In other words, don't check whether it IS-a duck: check whether it QUACKS-like-a duck, WALKS-like-a duck, etc, etc, depending on exactly what subset of duck-like behaviour you need to play your language-games with.
See also
- Extension methods
- UFCS
- Loose coupling
- Monkey patch
References
External links
- Apache Software Foundation "commons" proxy project provides DuckTyping implementation in Java
- Dr. Dobbs June 01 2005: "Templates and Duck Typing"
- Duck Typing: Ruby
- How to duck type? - the psychology of static typing in Ruby
- Javascript 'typeof' limitations and duck typing
- Python documentation glossary entry on duck-typing
- Python from a Java perspective - Part 2 - How duck typing influences class design and design principles
Source of the article : Wikipedia
0 Comments