Page:Scheme - An interpreter for extended lambda calculus.djvu/36

Sussman and Steele

December 22, 1975

35

Implementation of the Interpreter

so that execution can be resumed at EVLIS1 when the evaluation of the component returns with a value. It then sets up **EXP** to point to the component to be evaluated and dispatches to AEVAL.

(DEFUN EVLIS ()
       (COND ((NULL **UNEVLIS**)
              (SETQ **EVLIS** (REVERSE **EVLIS**))
              (COND ((ATOM (CAR **EVLIS**))
                     (SETQ **VAL** (APPLY (CAR **EVLIS**) (CDR **EVLIS**)))
                     (RESTORE))
                    ((EQ (CAAR **EVLIS**) 'LAMBDA)
                     (SETQ **ENV** (PAIRLIS (CADAR **EVLIS**) (CDR **EVLIS**) **ENV**)
                           **EXP** (CADDAR **EVLIS**)
                           **PC** 'AEVAL))
                    ((EQ (CAAR **EVLIS**) 'BETA)
                     (SETQ **ENV** (PAIRLIS (CADR (CADAR **EVLIS**))
                                            (CDR **EVLIS**)
                                            (CADDAR **EVLIS**))
                           **EXP** (CADDR (CADAR **EVLIS**))
                           **PC** 'AEVAL))
                    ((EQ (CAAR **EVLIS**) 'DELTA)
                     (SETQ **CLINK** (CADAR **EVLIS**))
                     (RESTORE))
                    (T (ERROR '|BAD FUNCTION - EVARGLIST| **EXP** 'FAIL-ACT))))
             (T (SAVEUP 'EVLIS1)
                (SETQ **EXP** (CAR **UNEVLIS**)
                      **PC** 'AEVAL))))

The purpose of EVLIS1 is to gobble up the value, passed in the **VAL** register, of the subexpression just evaluated. It saves this value on the list in the **EVLIS** register, pops off the unevaluated subexpression from the **UNEVLIS** register, and dispatches back to EVLIS.

(DEFUN EVLIS1 ()
       (SETQ **EVLIS** (CONS **VAL** **EVLIS**)
             **UNEVLIS** (CDR **UNEVLIS**)
             **PC** 'EVLIS))

Here is the code for the various AINTs. On arrival at the instruction for an AINT, **EXP** contains the expression whose functional position contains the name of the AINT. None of the arguments have been evaluated, and no new control frame has been created. Note that each AINT is defined by the presence of an AINT property on the property list of the LISP atom which is its name. The value of this property is the LISP function which is the first "instruction" of the AINT.

EVALUATE is similar to the LISP function EVAL; it evaluates its argument, which should result in a s-expression, which is then fed back into the SCHEME expression evaluator (AEVAL).