Dynamic programming language
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In computer science, a dynamic programming language is a class of high-level programming languages which at runtime execute many common programming behaviours that static programming languages perform during compilation. These behaviors could include an extension of the program, by adding new code, by extending objects and definitions, or by modifying the type system. Although similar behaviors can be emulated in nearly any language, with varying degrees of difficulty, complexity and performance costs, dynamic languages provide direct tools to make use of them. Many of these features were first implemented as native features in the Lisp programming language.
Most dynamic languages are also dynamically typed, but not all are. Dynamic languages are frequently (but not always) referred to as scripting languages, although that term in its narrowest sense refers to languages specific to a given run-time environment.
Implementation
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Eval
Some dynamic languages offer an
Object runtime alteration
A type or object system can typically be modified during runtime in a dynamic language. This can mean generating new objects from a runtime definition or based on
Type inference
As a lot of dynamic languages come with a dynamic type system, runtime inference of types based on values for internal interpretation marks a common task. As value types may change throughout interpretation, it is regularly used upon performing atomic operations.
Variable memory allocation
Static programming languages (possibly indirectly) require developers to define the size of utilized memory before compilation (unless working around with pointer logic). Consistent with object runtime alteration, dynamic languages implicitly need to (re-)allocate memory based on program individual operations.
Reflection
Macros
A limited number of dynamic programming languages provide features which combine
Assembly, C, C++, early Java, and Fortran do not generally fit into this category.[clarification needed]
Example code
The following examples show dynamic features using the language Common Lisp and its Common Lisp Object System (CLOS).
Computation of code at runtime and late binding
The example shows how a function can be modified at runtime from computed source code
; the source code is stored as data in a variable
CL-USER > (defparameter *best-guess-formula* '(lambda (x) (* x x 2.5)))
*BEST-GUESS-FORMULA*
; a function is created from the code and compiled at runtime, the function is available under the name best-guess
CL-USER > (compile 'best-guess *best-guess-formula*)
#<Function 15 40600152F4>
; the function can be called
CL-USER > (best-guess 10.3)
265.225
; the source code might be improved at runtime
CL-USER > (setf *best-guess-formula* `(lambda (x) ,(list 'sqrt (third *best-guess-formula*))))
(LAMBDA (X) (SQRT (* X X 2.5)))
; a new version of the function is being compiled
CL-USER > (compile 'best-guess *best-guess-formula*)
#<Function 16 406000085C>
; the next call will call the new function, a feature of late binding
CL-USER > (best-guess 10.3)
16.28573
Object runtime alteration
This example shows how an existing instance can be changed to include a new slot when its class changes and that an existing method can be replaced with a new version.
; a person class. The person has a name.
CL-USER > (defclass person () ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4020081FB3>
; a custom printing method for the objects of class person
CL-USER > (defmethod print-object ((p person) stream)
(print-unreadable-object (p stream :type t)
(format stream "~a" (slot-value p 'name))))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 4020066E5B>
; one example person instance
CL-USER > (setf *person-1* (make-instance 'person :name "Eva Luator"))
#<PERSON Eva Luator>
; the class person gets a second slot. It then has the slots name and age.
CL-USER > (defclass person () ((name :initarg :name) (age :initarg :age :initform :unknown)))
#<STANDARD-CLASS PERSON 4220333E23>
; updating the method to print the object
CL-USER > (defmethod print-object ((p person) stream)
(print-unreadable-object (p stream :type t)
(format stream "~a age: ~" (slot-value p 'name) (slot-value p 'age))))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 402022ADE3>
; the existing object has now changed, it has an additional slot and a new print method
CL-USER > *person-1*
#<PERSON Eva Luator age: UNKNOWN>
; we can set the new age slot of instance
CL-USER > (setf (slot-value *person-1* 'age) 25)
25
; the object has been updated
CL-USER > *person-1*
#<PERSON Eva Luator age: 25>
Assembling of code at runtime based on the class of instances
In the next example, the class person gets a new superclass. The print method gets redefined such that it assembles several methods into the effective method. The effective method gets assembled based on the class of the argument and the at runtime available and applicable methods.
; the class person
CL-USER > (defclass person () ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4220333E23>
; a person just prints its name
CL-USER > (defmethod print-object ((p person) stream)
(print-unreadable-object (p stream :type t)
(format stream "~a" (slot-value p 'name))))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 40200605AB>
; a person instance
CL-USER > (defparameter *person-1* (make-instance 'person :name "Eva Luator"))
*PERSON-1*
; displaying a person instance
CL-USER > *person-1*
#<PERSON Eva Luator>
; now redefining the print method to be extensible
; the around method creates the context for the print method and it calls the next method
CL-USER > (defmethod print-object :around ((p person) stream)
(print-unreadable-object (p stream :type t)
(call-next-method)))
#<STANDARD-METHOD PRINT-OBJECT (:AROUND) (PERSON T) 4020263743>
; the primary method prints the name
CL-USER > (defmethod print-object ((p person) stream)
(format stream "~a" (slot-value p 'name)))
#<STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 40202646BB>
; a new class id-mixin provides an id
CL-USER > (defclass id-mixin () ((id :initarg :id)))
#<STANDARD-CLASS ID-MIXIN 422034A7AB>
; the print method just prints the value of the id slot
CL-USER > (defmethod print-object :after ((object id-mixin) stream)
(format stream " ID: ~a" (slot-value object 'id)))
#<STANDARD-METHOD PRINT-OBJECT (:AFTER) (ID-MIXIN T) 4020278E33>
; now we redefine the class person to include the mixin id-mixin
CL-USER 241 > (defclass person (id-mixin) ((name :initarg :name)))
#<STANDARD-CLASS PERSON 4220333E23>
; the existing instance *person-1* now has a new slot and we set it to 42
CL-USER 242 > (setf (slot-value *person-1* 'id) 42)
42
; displaying the object again. The print-object function now has an effective method, which calls three methods: an around method, the primary method and the after method.
CL-USER 243 > *person-1*
#<PERSON Eva Luator ID: 42>
Examples
Popular dynamic programming languages include JavaScript, Python, Ruby, PHP, Lua and Perl. The following are generally considered dynamic languages:
- ActionScript
- BeanShell[2]
- C# (using Reflection)
- Clojure
- CobolScript
- ColdFusion Markup Language
- Common Lisp and most other Lisps
- Dylan
- E
- Elixir
- Erlang
- FORTH
- Gambas
- GDScript
- Groovy[3]
- Java (using Reflection)
- JavaScript
- Julia
- Lua
- MATLAB / Octave
- Objective-C
- Perl
- PHP
- PowerShell
- Prolog
- Python
- R
- Raku
- Rebol
- Ruby
- Smalltalk
- SuperCollider
- Tcl
- VBScript
- Wolfram Language
See also
References
- CiteSeerX 10.1.1.69.5966
- ^ Chapter 24. Dynamic language support. Static.springsource.org. Retrieved on 2013-07-17.
- ^ < "Groovy - Home". Archived from the original on 2014-03-02. Retrieved 2014-03-02.
Further reading
- Tratt, Laurence (2009). Dynamically Typed Languages. Advances in Computers. Vol. 77. pp. 149–184. ISBN 9780123748126.
External links
(Many use the term "scripting languages".)
- Prechelt, Lutz (August 18, 2002). "Are Scripting Languages Any Good? A Validation of Perl, Python, Rexx, and Tcl against C, C++, and Java" (PDF). Advances in Computers. 57: 205–270. ISSN 0065-2458. Retrieved 2020-07-27.
- Bezroukov, Nikolai (2013). "A Slightly Skeptical View on Scripting Languages". Softpanorama (2.1 ed.). Retrieved 2020-07-27.
- Wall, Larry (December 6, 2007). Programming is Hard, Let's Go Scripting... (Speech). State of the Onion 11. Perl.com. Retrieved 2020-07-27.
- Roth, Gregor (November 20, 2007). "Scripting on the Java platform". JavaWorld. Retrieved 2020-07-27.
- ISSN 0018-9162. Archived from the original(PDF) on 2020-07-27. Retrieved 2020-07-27.
- "ActiveState Announces Focus on Dynamic Languages". ActiveState. July 26, 2004. Retrieved 2020-07-27.
- Ascher, David (July 27, 2004). "Dynamic Languages — ready for the next challenges, by design" (PDF). Whitepapers. ActiveState. Archived from the original (PDF) on 2008-11-18.
- Ascher, David (July 27, 2004). "Dynamic Languages — ready for the next challenges, by design". Whitepapers. ActiveState. Archived from the original on 2008-12-08.