3. The Target Language The "target language" is a highly restricted subset of MacLISP, rather than any particular machine language for an actual hardware machine such as the PDP-10. RABBIT produces MacLISP function definitions which are then compiled by the standard MacLISP compiler. In this way we do not need to deal with the uninteresting vagaries of a particular piece of hardware, nor with the peculiarities of the many and various data-manipulation primitives (CAR, RPLACA, +, etc.). We allow the MacLISP compiler to deal with them, and concentrate on the issues of environment and control which are unique to SCHEME. While for production use this is mildly inconvenient (since the code must be passed through two compilers before use), for research purposes it has saved the wasteful' reimplementation of much knowledge already contained in the MacLISP compiler.
On the other hand, the use of MacLISP as a target language does not by any means trivialize the task of RABBIT. The MacLISP function-calling mechanism cannot be used as a target construct for the SCHEME function call, because MacLISP's function calls are not guaranteed to behave tail-recursively. Since tail-recursion is a most crucial characteristic distinguishing SCHEME from most LISP systems, we must implement SCHEME function calls by more primitive methods.
Similarly, since SCHEME is a full-funarg dialect of LISP while MacLISP is not, we cannot in general use MacLISP's variable-binding mechanisms to implement those of SCHEME. On the other hand, it is a perfectly legitimate optimization to use MacLISP mechanisms in those limited situations where they are applicable.
Aside from ordinary MacLISP data-manipulation primitives, the only MacLISP constructs used in the target language are FROG, GO, RETURN, PROGN, COND, SETQ, and ((LAMBDA ...) ...). PROG is never nested; there is only a single, outer PROG. RETURN is used only in the form (RETURN NIL) to exit this outer
