Dynamic programming language

{{Expand section|date=October 2009}}

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.{{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=Meijer, Erik and Peter Drayton | year=2005 | publisher=Microsoft Corporation|url=https://people.dsv.su.se/~beatrice/DYPL/meijer_drayton.pdf}}

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).

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.

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 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.

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.{{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.{{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}}

The following examples show dynamic features using the language Common Lisp and its Common Lisp Object System (CLOS).

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

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*

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

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

foo = true; // foo is now a boolean

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*

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

• ActionScript
• BeanShell[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.
• C# (using Reflection)
• Clojure
• CobolScript
• ColdFusion Markup Language
• Common Lisp and most other Lisps
• Dylan
• E
• Elixir
• Erlang
• FORTH
• Gambas
• GDScript
• Groovy< {{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 }}
• Java (using Reflection)
• JavaScript
• Julia
• Lua
• MATLAB / Octave
• Objective-C
• ooRexx
• Perl
• PHP
• PowerShell
• Prolog
• Python
• R
• Raku
• Rebol
• Ring
• Ruby
• Smalltalk
• SuperCollider
• Tcl
• VBScript
• Wolfram Language

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

{{Reflist}}

• {{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}}

(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=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 |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}}

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Category:Evaluation strategy