interpreter

A high-level programming language translator that translates and runs the program at the same time. It translates one program statement into machine language, executes it, and then proceeds to the next statement. This differs from regular executable programs that are presented to the computer as binary-coded instructions. Interpreted programs remain in the source language the programmer wrote in, which is human readable text.

Slower, But Easier to Test
Interpreted programs run slower than their compiler counterparts. Whereas the compiler translates the entire program before it is run, interpreters translate a line at a time while the program is being run. However, it is very convenient to write an interpreted program, since a single line of code can be tested interactively.

Some languages can be both interpreted and compiled, in which case a program may be developed with the interpreter for ease of testing and debugging and later compiled for production use. See JIT compiler.

The Runtime Interpreter Must Be Present
Interpreted programs must always be run with the interpreter, commonly called a runtime module. For example, in order to run a BASIC or Foxbase program, the BASIC or Foxbase interpreter must be running as well.

Interpreters and Compilers

Unlike compiled languages which are translated into machine language ahead of time (right), interpreted languages are translated at runtime. dBASE and BASIC interpreters (middle) translate the original source code. Java and Visual Basic (left) interpreters translate "bytecode," which is an intermediate language compiled from the original source code.

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(programming)

interpreter - A program which executes other programs. This is in contrast to a compiler which does not execute its input program (the "source code") but translates it into executable "machine code" (also called "object code") which is output to a file for later execution. It may be possible to execute the same source code either directly by an interpreter or by compiling it and then executing the machine code produced. It 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 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 analyse 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 index into a jump table. An interpreter might well use the same lexical analyser and parser as the compiler and then interpret the resulting abstract syntax tree. There is thus 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 "byte-code" which is a highly compressed and optimised representation of the Lisp source but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then executed (interpreted) by a byte code 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 byte-code interpreter. See also partial evaluation.