Page:Scheme - An interpreter for extended lambda calculus.djvu/31

Sussman and Steele

December 22, 1975

30

Implementation of the Interpreter

The following declarations and macros are present only to make the compiler happy, and to make the version number of the SCHEME implementation available in the global variable VERSION.

(DECLARE (SPECIAL **EXP** **UNEVLIS** **ENV** **EVLIS** **PC** **CLINK** **VAL** **TEM**
                     **TOP** **QUEUE** **TICK** **PROCESS** **QUANTUM**
                     VERSION LISPVERSION))

(DEFUN VERSION MACRO (X)
       (COND (COMPILER-STATE (LIST 'QUOTE (STATUS UREAD)))
             (T (RPLACA X 'QUOTE)
                (RPLACD X (LIST VERSION))
                (LIST 'QUOTE VERSION))))

(DECLARE (READ))

(SETQ VERSION ((LAMBDA (COMPILER-STATE) (VERSION)) T))

The function SCHEME initializes the system driver. The two SETQ's merely set up version numbers. The top level loop itself is written in SCHEME, and is a LABELS which binds the function **TOP** to be a read-eval-print loop. The LISP global variable **TOP** is initialized to the closure of the **TOP** function for convenience and accessibility to user-defined functions.

(DEFUN SCHEME ()
       (SETQ VERSION (VERSION)  LISPVERSION (STATUS LISPVERSION))
       (TERPRI)
       (PRINC '|This is SCHEME |)
       (PRINC VERSION)
       (PRINC '| running in LISP |)
       (PRINC LISPVERSION)
       (SETQ **ENV** NIL  **QUEUE** NIL
             **PROCESS** (CREATE!PROCESS '(**TOP** '|SCHEME -- Toplevel|)))
       (SWAPINPROCESS)
       (ALARMCLOCK 'RUNTIME **QUANTUM**)
       (MLOOP))

(SETQ **TOP**
      '(BETA (LAMBDA (**MESSAGE**)
                (LABELS ((**TOP1**
                          (LAMBDA (**IGNORE1** **IGNORE2** **IGNORE3**)
                             (**TOP1** (TERPRI) (PRINC '|==> |)
                                       (PRINT (SET '* (EVALUATE (READ))))))))
                    (**TOP1** (TERPRI) (PRINC **MESSAGE**) NIL)))
             NIL))

When the LISP alarmclock tick occurs, the global register **TICK** is set to T. **QUANTUM**, the amount of runtime between ticks, is measured in