Page:Scheme - An interpreter for extended lambda calculus.djvu/37

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Sussman and Steele December 22, 1975 36 Implementation of the Interpreter
(DEFPROP EVALUATE EVALUATE AINT)
 
(DEFUN EVALUATE ()
       (SAVEUP 'EVALUATE1)
       (SETQ **EXP** (CADR **EXP**)
             **PC** 'AEVAL))
 
(DEFUN EVALUATE1 ()
       (SETQ **EXP** **VAL**
             **PC** 'AEVAL))

IF evaluates its first argument, with a return address of IF1. IF1 examines the resulting **VAL**, and gives either the second or third argument to AEVAL depending on whether the **VAL** was non-NIL or NIL.

(DEFPROP IF IF AINT)
 
(DEFUN IF ()
       (SAVEUP 'IF1)
       (SETQ **EXP** (CADR **EXP**)
             **PC** 'AEVAL))
 
(DEFUN IF1 ()
       (COND (**VAL** (SETQ **EXP** (CADDR **EXP**)))
             (T (SETQ **EXP** (CADDDR **EXP**))))
       (SETQ **PC** 'AEVAL))

As it was in the beginning, is now, and ever shall be: QUOTE without end. (Amen, amen.)

(DEFPROP QUOTE AQUOTE AINT)
 
(DEFUN AQUOTE ()
       (SETQ **VAL** (CADR **EXP**))
       (RESTORE))

LABELS merely feeds its second argument to AEVAL after constructing a fiendishly clever environment structure. This is done in two stages: first the skeleton of the structure is created, with null environments in the closures of the bound functions; next the created environment is clobbered into each of the closures.