Thursday, August 23, 2007

This article is about a class of programming languages, for the method for reducing the runtime of algorithms, see Dynamic programming.
Dynamic programming language is a term used broadly in computer science to describe a class of high level programming languages that execute at runtime many common behaviors that other languages might perform during compilation, if at all. These behaviors could include extension of the program, by adding new code, by extending objects and definitions, or by modifying the type system, all during program execution. These behaviors can be emulated in nearly any language of sufficient complexity, but dynamic languages provide direct tools to make use of them.
Dynamic languages and dynamic typing are not identical concepts, and a dynamic language need not be dynamically typed, though many dynamic languages are dynamically typed.

Limitations and ambiguity in the definition
There are several mechanisms closely associated with the concept of dynamic programming. None are essential to the classification of a language as dynamic, but most can be found in a wide variety of such languages.

Eval is a term which was introduced in Lisp, and refers to the process of executing instructions which are represented by data structures called S-expressions. In its modern sense, eval or evalling refers to the mechanism or process of executing any sort of instructions that are available to the program as text or non-machine code data. The evaluation of new program text is a common aspect of many languages that, unlike Lisp, do not make a distinction between reading text and transforming it into an internal form and further transforming that internal form into actions to be taken. These languages are often called interpreted languages when the process of normal program execution is an eval.

Dynamic programming language Eval
However, Erik Meijer and Peter Drayton caution that any language, capable of loading executable code at runtime, is capable of eval in some respect, even when that code is in the form of dynamically linked shared libraries of machine code. They suggest that higher-order functions are the true measure of dynamic programming, and some languages "use eval as a poor man's substitute for higher-order functions."

Higher-order functions
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).

Runtime alteration of object or type system
Functional programming concepts are a feature of many dynamic languages, and also derive from Lisp.

Another feature of some dynamic languages is the continuation. Continuations represent execution state that can be re-invoked. For example, a parser might return an intermediate result and a continuation that, when invoked, will continue to parse the input. Continuations interact in very complex ways with scoping, especially with respect to closures. For this reason, many dynamic languages do not provide continuations.

Introspection is common in many dynamic languages, and typically involves a program analyzing its own structure, code, types or data. This can be as simple as being able to determine the type of a generic or polymorphic value. It can also include full analysis 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 and eval in a feature called macros. Most programmers today who are aware of macros have encountered them in C or C++, where they are a static feature which are 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 runtime, allowing the definition of language-like constructs which can optimize code or modify the syntax or grammar of the language.


Comparison of programming languages
Name binding