Pure and impure interpreters     

In a pure interpreter, the source program is maintained in the source form throughout its interpretation. This arrangement acquires substantial analysis overheads when interpreting a statement.

An impure interpreter carries out some preliminary processing of the source program to decrease the analysis overheads during interpretation. The preprocessor transforms the program to an intermediate representation (IR) that is used during interpretation. This speeds up interpretation since the code component of the IR that is the IC, can be analyzed more resourcefully than the source form of the program.