An interpreter is a computer program that 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 analyzer 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 bytecode 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 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 byte-code interpreter.
A relatively recent technique called Just In Time compilation is further blurring the distinction between interpreters, byte-code interpreters and compilation.
The following simple interpreter program is written using BASIC. When compiled using the QuickBASIC compiler it is a straightforward interpreter but when run on the QBASIC interpreter, it is an example of an interpreted interpreter.
InterpretKeyword:
LET J = INSTR(aRest + " ", " ")
LET aFirst = LTRIM$(LEFT$(aRest, J - 1))
LET aRest = LTRIM$(MID$(aRest, J))
There is a program to try with this interpreter, on the Literate programming article. If you want to do so you must save the interpreter as INTERP.BAS and then save the Literate programming article as TESTPROG.TXT in the same folder.
See also: partial evaluationExample of a simple interpreter
DECLARE SUB Split (aFirst AS STRING, aRest AS STRING)
LET Q$ = "TESTPROG.TXT"
LET F = FREEFILE
OPEN Q$ FOR INPUT AS #F
DO WHILE NOT EOF(F)
LINE INPUT #F, FileInput$
LET FileInput$ = LTRIM$(FileInput$)
Split KeyWord$, FileInput$
SELECT CASE KeyWord$
CASE "-"
Split KeyWord$, FileInput$
GOSUB InterpretKeyword
END SELECT
LOOP
CLOSE #F
SYSTEM SELECT CASE UCASE$(KeyWord$)
CASE "STORE"
GOSUB AssignToValue
CASE "ADD"
GOSUB AddToValue
CASE "MULTIPLYBY"
GOSUB MultiplyWithValue
CASE "PRINT"
GOSUB PutOutput
CASE "CLS"
GOSUB ClearScreen
CASE "NEWLINE"
PRINT
CASE ELSE
PRINT
PRINT "I don't know what "; KeyWord$; " "; FileInput$; " means."
END SELECT
RETURN
AssignToValue:
GOSUB GetArg
LET Value$ = Arg$
RETURN
AddToValue:
GOSUB GetArg
LET Value$ = LTRIM$(STR$(VAL(Value$) + VAL(Arg$)))
RETURN
MultiplyWithValue:
GOSUB GetArg
LET Value$ = LTRIM$(STR$(VAL(Value$) * VAL(Arg$)))
RETURN
GetArg:
Split KeyWord$, FileInput$
SELECT CASE UCASE$(KeyWord$)
CASE "INPUT"
GOSUB GetInput
LET Arg$ = UserInput$
CASE "VALUE"
LET Arg$ = Value$
CASE "TEXT"
LET Arg$ = FileInput$
CASE ELSE
LET Arg$ = KeyWord$ + " " + FileInput$
END SELECT
RETURN
GetInput:
LINE INPUT "", UserInput$
RETURN
PutOutput:
GOSUB GetArg
PRINT Arg$; " ";
RETURN
ClearScreen:
CLS
RETURN
NewLine:
PRINT
RETURN
SUB Split (aFirst AS STRING, aRest AS STRING) DIM J AS INTEGER
END SUB