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In computer science, an interpreter is a computer program that executes, or performs, instructions written in a computer programming language. It is not to be confused with a command line interpreter.

Interpretation is one of the two major ways in which a programming language can be implemented, the other being compilation. The term interpreter may refer to the program that executes source code that has already been translated to some intermediate form, or it may refer to the program that performs both the translation and execution.

The Java virtual machine is an example of the former: Java source programs are compiled ahead of time and stored as machine-independent bytecode, which is then linked at run-time and executed by the virtual machine. Perl is an example of the latter, where the one interpreter translates Perl source into an intermediate in-memory bytecode to be executed in-place.

A language is an abstraction that is independent of its implementation in an interpreter or a compiler; the terms interpreted language or compiled language merely mean that the canonical implementation of that language is an interpreter or a compiler.

Many "interpreters" today include some element of compilation as well, such as a bytecode compiler, as found in Perl and Python. In such an arrangement, source code is compiled to a more efficient intermediate form, which is then interpreted.

The main disadvantage of interpreters is that when a program is interpreted, it runs slower than if it had been compiled. The difference in speeds could be tiny or great; often an order of magnitude and sometimes more. It generally takes longer to run a program under an interpreter than to run the compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle.

Interpreting code is slower than running the compiled code because the interpreter must analyze each statement in the program each time it is executed and then perform the desired action whereas the compiled code just performs the action. This run-time analysis is known as "interpretive overhead". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time.

There are various compromises between the development speed when using an interpreter and the execution speed when using a compiler. Some systems (e.g., some LISPs) allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute the source code as it stands but convert it into some more compact internal form. For example, some BASIC interpreters replace keywords with single byte tokens which can be used to find the instruction in a jump table. An interpreter might well use the same lexical analyzer and parser as the compiler and then interpret the resulting abstract syntax tree.

There is a spectrum of possibilities between interpreting and compiling, depending on the amount of analysis performed before the program is executed. For example, Emacs Lisp is compiled to bytecode, which is a highly compressed and optimized representation of the Lisp source, but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then interpreted by a bytecode interpreter (itself written in C). The compiled code in this case is machine code for a virtual machine, which is implemented not in hardware, but in the bytecode interpreter. The same approach is used with the Forth code used in Open Firmware systems: the source language is compiled into "F code" (a bytecode), which is then interpreted by a virtual machine.

Further blurring the distinction between interpreters, byte-code interpreters and compilation is just-in-time compilation (or JIT), a technique in which bytecode is compiled to native machine code at runtime. This confers the efficiency of running native code, at the cost of startup time and increased memory use when the bytecode is first compiled. Adaptive optimization is a complementary technique in which the interpreter profiles the running program and compiles its most frequently-executed parts into native code. Both techniques are a few decades old, appearing in languages such as Smalltalk in the 1980s.

Just-in-time compilation has gained mainstream attention amongst language implementors in recent years, with Java, Python and the .NET Framework all now including JITs.

The term "interpreter" often referred to a piece of unit record equipment that could read punched cards and print the characters in human-readable form on the card. The IBM 550 Numeric Interpreter and IBM 557 Alphabetic Interpreter are typical examples from 1930 and 1954, respectively.

• partial evaluation
• interpreted languages
• compiled languages
• dynamic compilation including the section on incremental compilation.
• Metacircular Interpreter

• IBM Card Interpreters page at Columbia University

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.