Dynamic programming language: Difference between revisions

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{{Short description|Programming languages with runtime extensibility}}

In [[computer science]], a '''dynamic programming language''' is a kind of [[programming language]] in which programs can change their structure as they run: functions may be introduced or removed, new classes of objects may be created, new modules may appear. As a side effect of this dynamism, most dynamic programming languages are [[dynamic typing|dynamically typed]], which static typing advocates consider a drawback (see also [[static typing]]). According to advocates of dynamic programming languages, however, the flexibility of dynamic languages offsets these drawbacks, and even provides advantages so considerable as to make this an essential feature, for example for [[Interactive programming]]. More recent researchers claims that with slightly more overhead and syntax design, it's possible and beneficial to combine static typing with dyanmic features to achieve interactivity while also providing the safety and performance benifits of a stronly typed language.

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{{disputed|date=March 2012}}

{{confusing|date=October 2009}}

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A '''dynamic programming language''' is a type of programming language that allows various operations to be determined and executed at runtime. This is different from the compilation phase. Key decisions about variables, method calls, or data types are made when the program is running, unlike in [[Static program analysis|static languages]], where the structure and types are fixed during compilation. Dynamic languages provide flexibility. This allows developers to write more adaptable and concise code.

For instance, in a dynamic language, a variable can start as an integer. It can later be reassigned to hold a string without explicit type declarations. This feature of dynamic typing enables more fluid and less restrictive coding. Developers can focus on the logic and functionality rather than the constraints of the language.

Generally programming consists of writing together bits of computer code known as ''functions'' which operate on data. These functions are physically represented by computer code at some ___location in memory. In most programming languages function calls in the [[source code]] are replaced with instructions to run the code at that physical ___location (the exact ___location being defined by the [[linker]]). One problem with this approach is that it does not allow for modification of the code once it is compiled. For instance, if a bug is found in the code, the only solution is to correct the original source code and recompile the application.

==Implementation==

Dynamic languages rely on the addresses for functions being looked up at runtime, instead of being [[compiler|compiled]] into addesses at [[compile time]]. This allows the symbols to be modified to point to new functions, allowing the definitions to change. Many dynamic languages also look up data in the same fashion, allowing "static" objects such as classes to be modified.

{{Expand section|date=October 2009}}

===Eval===

This introduces a lookup during runtime however, as each function call requires the symbol to be looked up, and then the function pointer followed. For this reason dynamic languages often run slower than non-dynamic ones, another "kiss of death" during the 1990s. This delay can be dramatically reduced almost to zero in practice however, for instance, the [[Objective-C]] programming language used a pre-loaded [[cache]] and a small bit of [[assembler]] code in order to reduce this overhead to a single operation.

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 (computer scientist)|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 function]]s (by passing functions as strings) or [[deserialization]].<ref>{{Citation | citeseerx = 10.1.1.69.5966 | title=Static Typing Where Possible, Dynamic Typing When Needed: The End of the Cold War Between Programming Languages | author=[[Erik Meijer (computer scientist)|Meijer, Erik]] and Peter Drayton | year=2005 | publisher=[[Microsoft]] Corporation|url=https://people.dsv.su.se/~beatrice/DYPL/meijer_drayton.pdf}}</ref>

===Object runtime alteration===

The degree of dynamism varies between languages. Objective-C was based on the [[GNU/GCC]] compiler and allowed dynamism of the function calls only (called ''categorization'' for unrelated reasons) by re-writing the method dispatch code. [[TOM programming language|TOM]] is a development of Objective-C in nature, but using a more complex runtime it allows considerably more flexibility at the cost of performance. Most dynamic languages tend toward this end of the spectrum, allowing classes to be re-defined at runtime by collecting code from several modules. The copy-and-modify ([[prototype based]]) languages such as [[Self programming language|Self]] and [[NewtonScript programming language|NewtonScript]] tend to be fully dynamic as a matter of course.

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 [[mixin]]s of existing types or objects. This can also refer to changing the [[Inheritance (object-oriented programming)|inheritance]] or type tree, and thus altering the way that existing types behave (especially with respect to the invocation of [[Method (computer science)|methods]]).

===Type inference===

Dynamic languages include:

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.

*'''[[ChucK]]'''

*'''[[Dylan programming language|Dylan]]'''

*'''[[HyperTalk]]'''

*'''[[JavaScript]]'''

*'''[[Lisp programming language|Lisp]]'''

*'''[[Perl]]'''

*'''[[PHP]]'''

*'''[[Pike programming language|Pike]]'''

*'''[[Python programming language|Python]]'''

*'''[[Revolution (development environment)|Revolution]]'''

*'''[[Ruby programming language|Ruby]]'''

*'''[[Scheme programming language|Scheme]]'''

*'''[[Self programming language|Self]]'''

*'''[[Smalltalk]]'''

*'''[[SuperCard]]'''

*'''[[SuperCollider]]'''

*'''[[Tcl]]'''

*'''[[xHarbour]]'''

===Variable memory allocation===

[[C programming language|C]], [[C Plus Plus|C++]], [[Java programming language|Java]], and [[FORTRAN]] do not generally fit into this category, though it is often possible to build layers on top of a language.

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 (computer science)|Reflection]] is common in many dynamic languages, and typically involves [[Introspection (computer science)|analysis]] of the types and metadata of generic or [[Type polymorphism|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-expression]]s.

===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 [[Macro (computer science)|macros]]. Most programmers today who are aware of the term ''macro'' have encountered them in [[C (programming language)|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 language|Assembly]], [[C (programming language)|C]], [[C++]], early [[Java (programming language)|Java]], and [[Fortran]] do not generally fit into this category.{{clarify|date=September 2016}}

The earliest dynamic programming language is considered to be Lisp (McCarthy, 1965) which continued to influence the design of programming languages to the present day.<ref>{{cite book| last=Harper| first=Robert | title=Practical Foundations for Programming languages | page=195 | year=2016 |publisher=Cambridge University Press| ___location=New York| isbn=9-781107-150300}}</ref>

==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

<syntaxhighlight 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*)

#<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

</syntaxhighlight>

===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.

<syntaxhighlight lang="lisp">

; 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>

</syntaxhighlight>

let foo = 42; // foo is now a number

foo = "bar"; // foo is now a string

foo = true; // foo is now a boolean

===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.

<syntaxhighlight lang="lisp">

; 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>

</syntaxhighlight>

==Examples==

Popular dynamic programming languages include [[JavaScript]], [[Python (programming language)|Python]], [[Ruby (programming language)|Ruby]], [[PHP]], [[Lua (programming language)|Lua]] and [[Perl]]. The following are generally considered dynamic languages:

* [[ActionScript]]

* [[BeanShell]]<ref>[http://static.springsource.org/spring/docs/2.0.x/reference/dynamic-language.html Chapter 24. Dynamic language support]. Static.springsource.org. Retrieved on 2013-07-17.</ref>

* [[C Sharp (programming language)|C#]] (using reflection)

* [[Clojure]]

* [[CobolScript]]

* [[ColdFusion Markup Language]]

* [[Common Lisp]] and most other [[Lisp (programming language)|Lisps]]

* [[Dylan (programming language)|Dylan]]

* [[E programming language|E]]

* [[Elixir (programming language)|Elixir]]

* [[Erlang (programming language)|Erlang]]

* [[Forth (programming language)|Forth]]

* [[Gambas]]

* [[GDScript]]

* [[Groovy (programming language)|Groovy]]<ref>< {{cite web |url=http://groovy.codehaus.org/ |title=Groovy - Home |access-date=2014-03-02 |url-status=dead |archive-url=https://web.archive.org/web/20140302111159/http://groovy.codehaus.org/ |archive-date=2014-03-02 }}</ref>

* [[Java (programming language)|Java (using Reflection)]]

* [[JavaScript]]

* [[Julia (programming language)|Julia]]

* [[Lua (programming language)|Lua]]

* [[MATLAB]] / [[GNU Octave|Octave]]

* [[Objective-C]]

* [[Object REXX|ooRexx]]

* [[Perl]]

* [[PHP]]

* [[PowerShell]]

* [[Prolog]]

* [[Python (programming language)|Python]]

* [[R (programming language)|R]]

* [[Raku (programming language)|Raku]]

* [[Rebol]]

* [[Ring (programming language)|Ring]]

* [[Ruby (programming language)|Ruby]]

* [[Smalltalk]]

* [[SuperCollider]]

* [[Tcl]]

* [[VBScript]]

* [[Wolfram Language]]

==See also==

* [[Comparison of programming languages]]

* [[Name binding]]

* [[Von Neumann architecture]]

==References==

{{Reflist}}

==Further reading==

* {{cite book|doi=10.1016/s0065-2458(09)01205-4|url=https://tratt.net/laurie/research/pubs/html/tratt__dynamically_typed_languages/|title=Dynamically Typed Languages|volume=77|pages=149–184|series=Advances in Computers|year=2009|last1=Tratt|first1=Laurence|isbn=9780123748126}}

==External links==

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

* {{cite journal |last1=Prechelt |first1=Lutz |date=2002-08-18 |df=mdy |title=Are Scripting Languages Any Good? A Validation of Perl, Python, Rexx, and Tcl against C, C++, and Java |journal=Advances in Computers |volume=57 |pages=205–270 |issn=0065-2458 |doi=10.1016/S0065-2458(03)57005-X |isbn=9780120121571 |url=https://page.mi.fu-berlin.de/prechelt/Biblio/jccpprt2_advances2003.pdf |access-date=2020-07-27}}

* {{cite web |last1=Bezroukov |first1=Nikolai |year=2013 |url=http://www.softpanorama.org/Articles/a_slightly_skeptical_view_on_scripting_languages.shtml |title=A Slightly Skeptical View on Scripting Languages |edition=2.1 |work=Softpanorama |access-date=2020-07-27}}

* {{cite speech |author-link1=Larry Wall |last1=Wall |first1=Larry |date=2007-12-06 |df=mdy |url=https://www.perl.com/pub/2007/12/06/soto-11.html/ |title=Programming is Hard, Let's Go Scripting... |event=[[Perl#State of the Onion|State of the Onion]] 11 |work=Perl.com |access-date=2020-07-27}}

* {{cite web |last1=Roth |first1=Gregor |date=2007-11-20 |df=mdy |url=https://www.infoworld.com/article/2077792/scripting-on-the-java-platform.html |title=Scripting on the Java platform |work=[[JavaWorld]] |access-date=2020-07-27}}

* {{cite magazine |author-link1=John Ousterhout |last1=Ousterhout |first1=John K. |date=March 1998 |df=mdy |url=http://www.stanfordlibrary.us/~ouster/cgi-bin/papers/scripting.pdf |title=Scripting: Higher-Level Programming for the 21st Century |magazine=[[Computer (magazine)|Computer]] |volume=31 |issue=3 |pages=23–30 |issn=0018-9162 |doi=10.1109/2.660187 |access-date=2020-07-27 |archive-date=2020-07-27 |archive-url=https://web.archive.org/web/20200727185732/http://www.stanfordlibrary.us/~ouster/cgi-bin/papers/scripting.pdf |url-status=dead }}

* {{cite news |date=2004-07-26 |df=mdy |url=https://www.activestate.com/company/press/press-releases/activestate-announces-focus-dynamic-languages/ |title=ActiveState Announces Focus on Dynamic Languages |publisher=[[ActiveState]] |access-date=2020-07-27}}

** {{cite web |last1=Ascher |first1=David |date=2004-07-27 |df=mdy |url=https://www.activestate.com/Corporate/Publications/ActiveState_Dynamic_Languages.pdf |title=Dynamic Languages — ready for the next challenges, by design |department=Whitepapers |publisher=[[ActiveState]] |archive-url=https://web.archive.org/web/20081118035341/https://www.activestate.com/Corporate/Publications/ActiveState_Dynamic_Languages.pdf |archive-date=2008-11-18}}

** {{cite web |last1=Ascher |first1=David |date=2004-07-27 |df=mdy |url=http://www.activestate.com/company/newsroom/whitepapers_ADL.plex |title=Dynamic Languages — ready for the next challenges, by design |department=Whitepapers |publisher=[[ActiveState]] |archive-url=https://web.archive.org/web/20081208121835/http://www.activestate.com/company/newsroom/whitepapers_ADL.plex |archive-date=2008-12-08}}

{{Types of programming languages}}

{{DEFAULTSORT:Dynamic Programming Language}}

[[Category:Evaluation strategy]]

[[Category:Dynamic programming languages| ]]