Dynamic programming language

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

Eval

Some dynamic languages offer an '' eval'' function. This function takes a string or abstract syntax tree containing code in the language and executes it. If this code stands for an expression, the resulting value is returned. Erik Meijer and Peter Drayton distinguish the runtime code generation offered by eval from the dynamic loading offered by shared libraries, and warn that in many cases eval is used merely to implement higher-order functions (by passing functions as strings) or deserialization.

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).

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

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.

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*)
#

; 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*)
#

; 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)))
#

; 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))))
#

; one example person instance
CL-USER > (setf *person-1* (make-instance 'person :name "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)))
#

; 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))))
#

; the existing object has now changed, it has an additional slot and a new print method
CL-USER > *person-1*
#

; 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*
#

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)))
#

; 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))))
#

; a person instance
CL-USER > (defparameter *person-1* (make-instance 'person :name "Eva Luator"))
*PERSON-1*

; displaying a person instance
CL-USER > *person-1*
#

; 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)))
#

; the primary method prints the name
CL-USER > (defmethod print-object ((p person) stream)
(format stream "~a" (slot-value p 'name)))
#

; a new class id-mixin provides an id
CL-USER > (defclass id-mixin () ((id :initarg :id)))
#

; 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)))
#

; now we redefine the class person to include the mixin id-mixin
CL-USER 241 > (defclass person (id-mixin) ((name :initarg :name)))
#

; 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*
#

Examples

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

* ActionScript
* BeanShell
* C# (using Reflection)
* Clojure
* CobolScript
* ColdFusion Markup Language
* Common Lisp and most other Lisps
* Dylan
* E
* Elixir
* Erlang
* FORTH
* Gambas
* GDScript
* Groovy<
* 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

* Comparison of programming languages
* Name binding
* Von Neumann architecture

References

Further reading

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External links

''(Many use the term "scripting languages".)''
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Evaluation strategy