Dynamic programming language

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This article is about a class of programming languages. For the method for reducing the running time of algorithms, see dynamic programming.

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Dynamic programming language in computer science 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 the term "scripting language" 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 eval function. This function takes a string parameter containing code in the language and executes it. If this code stands for an expression, the resulting value is returned. However, Erik Meijer and Peter Drayton suggests that programmers "use eval as a poor man's substitute for higher-order functions."[1]

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 mixins of existing types or objects. This can also refer to changing the inheritance or type tree, and thus altering the way that existing types behave (especially with respect to the invocation of methods).

Reflection

Reflection is common in many dynamic languages, and typically involves analysis of the types and metadata of generic or polymorphic data. It can, however, also include full evaluation and modification of a program's code as data, such as the features that Lisp provides in analyzing S-expressions.

Macros

A limited number of dynamic programming languages provide features which combine code introspection (the ability to examine classes, functions, and keywords to know what they are, what they do and what they know) and eval in a feature called macros. Most programmers today who are aware of the term macro have encountered them in C or C++, where they are a static feature which is built in a small subset of the language, and are capable only of string substitutions on the text of the program. In dynamic languages, however, they provide access to the inner workings of the compiler, and full access to the interpreter, virtual machine, or runtime, allowing the definition of language-like constructs which can optimize code or modify the syntax or grammar of the language.

Assembly, C, C++, early Java, and Fortran do not generally fit into this category.Template:Clarify

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

<source lang="lisp">

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*)

  1. <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*)

  1. <Function 16 406000085C>
the next call will call the new function, a feature of late binding

CL-USER > (best-guess 10.3) 16.28573 </source>

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.

<source lang="lisp">

a person class. The person has a name.

CL-USER > (defclass person () ((name :initarg :name)))

  1. <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))))
  1. <STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 4020066E5B>
one example person instance

CL-USER > (setf *person-1* (make-instance 'person :name "Eva Luator"))

  1. <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)))

  1. <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))))
  1. <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*

  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*

  1. <PERSON Eva Luator age: 25>

</source>

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.

<source lang="lisp">

the class person

CL-USER > (defclass person () ((name :initarg :name)))

  1. <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))))
  1. <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*

  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)))
  1. <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)))
  1. <STANDARD-METHOD PRINT-OBJECT NIL (PERSON T) 40202646BB>
a new class id-mixin provides an id

CL-USER > (defclass id-mixin () ((id :initarg :id)))

  1. <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)))
  1. <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)))

  1. <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*

  1. <PERSON Eva Luator ID: 42>

</source>

Examples

Popular dynamic programming languages include JavaScript, Python, Ruby, PHP, Lua and Perl. The following are generally considered dynamic languages:

See also

References

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  1. ↑ Template:Citation
  2. ↑ Chapter 24. Dynamic language support. Static.springsource.org. Retrieved on 2013-07-17.
  3. ↑ < Template:Cite web

Further reading

External links

(Many use the term "scripting languages".)


Template:Programming language

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