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An interpreted language is a programming language for which most of its implementations execute instructions directly and freely, without previously compiling a program into machine-language instructions. The interpreter executes the program directly, translating each statement into a sequence of one or more subroutines already compiled into machine code.
The terms interpreted language and compiled language are not well defined because, in theory, any programming language can be either interpreted or compiled. In modern programming language implementation it is increasingly popular for a platform to provide both options.
Interpreted languages can also be contrasted with machine languages. Functionally, both execution and interpretation mean the same thing -- fetching the next instruction/statement from the program and executing it. Although interpreted byte code is additionally identical to machine code in form and has an assembler representation, the term "interpreted" is practically reserved for "software processed" languages (by virtual machine or emulator) on top of the native (i.e. hardware) processor.
In principle, programs in many languages may be compiled or interpreted, emulated or executed natively, so this designation is applied solely based on common implementation practice, rather than representing an essential property of a language.
Many languages have been implemented using both compilers and interpreters, including BASIC, C, Lisp, Pascal, and Python. Java and C# are compiled into bytecode, the virtual machine-friendly interpreted language. Lisp implementations can freely mix interpreted and compiled code.
In the early days of computing, language design was heavily influenced by the decision to use compiling or interpreting as a mode of execution. For example, Smalltalk (1980), which was designed to be interpreted at run-time, allows generic objects to dynamically interact with each other.
Initially, interpreted languages were compiled line-by-line; that is, each line was compiled as it was about to be executed, and if a loop or subroutine caused certain lines to be executed multiple times, they would be recompiled every time. This has become much less common. Most so-called interpreted languages use an intermediate representation, which combines compiling and interpreting.
The intermediate representation can be compiled once and for all (as in Java), each time before execution (as in Perl or Ruby), or each time a change in the source is detected before execution (as in Python).
Interpreting a language gives implementations some additional flexibility over compiled implementations. Features that are often easier to implement in interpreters than in compilers include:
Furthermore, source code can be read and copied, giving users more freedom.
Disadvantages of interpreted languages are:
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